xpcom/string/doc/string-guide.html
author Bas Schouten <bschouten@mozilla.com>
Mon, 16 Aug 2010 09:15:03 +0200
changeset 50660 3fc41668b25c0c748671964328c5310df4377eda
parent 29681 9cc460ccf0252e0ae2242a135da422c9118c0061
permissions -rw-r--r--
Fix line-endings in gfxWindowsPlatform.

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<html>
  <head>
    <title>an incomplete guide to mozilla/string</title>

    <link rel="stylesheet" href="http://www.mozilla.org/projects/string/string-guide.css" title="remote stylesheet" type="text/css">
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<!-- ----|---------|---------|---------|---------|---------|---------|---------| -->
<!-- ...............................................................Front Matter -->
<h1>an incomplete guide to <a class="exact-uri" href="http://lxr.mozilla.org/seamonkey/source/string/">mozilla/string</a></h1>
    <h1><font color="red">This document is now deprecated in favor of <a href="http://www.mozilla.org/projects/xpcom/string-guide.html">The new string guide</a>.</font></h1>
<div class="author-note">
  <p>by <a href="http://ScottCollins.net/">Scott Collins</a><!-- /p -->
  <p>last modified 8 April 2001<!-- /p -->
</div>

<div class="abstract">
  <p>
    <h1>Abstract</h1>
    This document <span class="LXRSHORTDESC">provides
      an <a href="#users_guide">introduction</a> to the design and use of the string classes in mozilla,
      <a href="#implementors_guide">detailed information</a> on their implementation and how one may extend them,
      and <a href="#faq">answers</a> to frequently asked questions about strings</span>.
  </p>
</div>



<h2><a name="contents">contents</a></h2>

<div class="contents">
  <ul>
    <li><a href="#users_guide"       >user's guide</a></li>
    <li><a href="#implementors_guide">implementor's guide</a></li>
    <li><a href="#faq"               >frequently asked questions</a></li>
  </ul>
</div>

<p>
  Please direct all comments, requests, and contributions to,
      in order of preference,
    the tracking bug <a href="http://bugzilla.mozilla.org/show_bug.cgi?id=70076">#70076</a> for this document,
    the author <a class="exact-uri" href="mailto:scc@mozilla.org?subject=string-guide">scc@mozilla.org</a>, and/or
    the newsgroup <a class="exact-uri" href="news:netscape.public.mozilla.xpcom">news:netscape.public.mozilla.xpcom</a>
      (should there be a strings newsgroup?)
</p>

<div class="author-note">
  <p>
    A note to potential editors:
      don't even <strong>consider</strong> modifying this document with an HTML editor.
    That would destroy the internal formatting,
      and make patches unmanagable.
  </p>
</div>




<!-- ...............................................................User's Guide -->
<hr>
<h1><a name="users_guide">user's guide</a></h1>

<div class="author-note">
  <p>
    Strings in mozilla are a world apart from <span class="code">char*</span>s.
    If you don't know why they are different,
      this section is the place for you to start.
    If you're already familiar with the hierarchy of string classes in mozilla,
      then you might want to skip ahead to the <a href="#implementors_guide">implementor's guide</a>
      or the <a href="#faq">FAQ</a>.
  </p>
</div>

<div class="contents">
  <ul>
    <li><a href="#users_guide_introduction">introduction</a></li>
    <li><a href="#users_guide_how_to"      >using the string classes correctly; using the correct string class</a></li>
    <li><a href="#users_guide_iterators"   >using string iterators</a></li>
    <li><a href="#users_guide_summary"     >summary</a></li>
  </ul>
</div>

<h2><a name="users_guide_introduction">introduction</a></h2>
  <h3>what and what isn't a string?</h3>
<p>
  A string is an opaque container holding a, possibly zero length, linear sequence of characters.
  Understanding the implications of this statement is the foundation for understanding all mozilla's string classes.
</p>

  <h3>readable and writable</h3>
  <h3>dependent strings</h3>
  <h3>flat strings</h3>
  <h3>encoding</h3>
  <h3>sharing</h3>

<h2><a name="users_guide_how_to">using the string classes correctly; using the correct string class</a></h2>
  <h3>basic string operations</h3>
    <h4>comparison</h4>
    <h4>concatenation</h4>
    <h4>substrings</h4>
    <h4>find and replace</h4>
  <h3>conversions</h3>
    <h4>calling a function that expects a different kind of string</h4>
    <h4>converting between string classes</h4>
    <h4>converting between encodings</h4>
  <h3>selecting the right string class</h3>
    <h4>user string classes</h4>
    <h4>selecting the right string class for a parameter</h4>
    <h4>selecting the right string class for a local variable</h4>
    <h4>selecting the right string class for a member variable</h4>
    <h4>selecting the right string class for a return value</h4>
    <h4>selecting the right string class in IDL</h4>
  <h3>dont's</h3>

<h2><a name="users_guide_iterators">using string iterators</a></h2>
  <h3>what is an iterator?</h3>
  <h3>reading iterators and writing iterators</h3>
  <h3>`chunky' iterating for efficiency</h3>
  <h3><span class="code">copy_string</span>, character sources and sinks</h3>
  <h3>encoding conversion iterators</h3>

<h2><a name="users_guide_summary">summary</a></h2>


<!-- ........................................................Implementor's Guide -->
<hr>
<h1><a name="implementors_guide">implementor's guide</a></h1>

<div class="author-note">
  <p>
    
  </p>
</div>

<div class="contents">
  <ul>
    <!-- li></li -->
  </ul>
</div>



<!-- ........................................................................FAQ -->
<hr>
<h1><a name="faq">frequently asked questions</a></h1>

<div class="author-note">
</div>

<div class="contents">
  <ul>
<!--
    <li>
      I have a wide string, i.e., an instance of a class derived from <span class="code">nsAString</span>
      <ul>
        <li>I want a pointer to the characters</span>
        <li>I want a narrow string</li>
        <li>I want to <span class="code">printf</span> it</li>
      </ul>
    </li>
    <li>
      I have a <span class="code">PRUnichar*</span>
      <ul>
        <li>I want a wide string</span>
        <li>I want a narrow string</span>
        <li>I want to <span class="code">printf</span> it</li>
      </ul>
    </li>
    <li>
      I have a narrow string, i.e., an instance of a class derived from <span class="code">nsACString</span>
      <ul>
        <li>I want a pointer to the characters</span>
        <li>I want a narrow string</li>
        <li>I want to <span class="code">printf</span> it</li>
      </ul>
    </li>
    <li>
      I have a <span class="code">char*</span>
      <ul>
        <li>I want a wide string</span>
        <li>I want a narrow string</span>
      </ul>
    </li>
    <li>
      I have a literal character sequence, e.g., <span class="code">"Hello, World!\n"</span>
      <ul>
        <li>I want a wide string</span>
        <li>I want a narrow string</span>
      </ul>
    </li>
    <li>What's the best way to return a string?</li>
    <li>How can I get a pointer to the characters in a string?</li>
    <li>How can I <span class="code">printf</span> a string?</li>
  </ul>
-->
</div>


<table class="chart">
  <tr>
    <th></th>
    <th colspan="5">you have some <span class="code">char</span>s</th>
  </tr>
  <tr>
    <th>you want</th>
    <th><span class="code">'x'</span></th>
    <th><span class="code">char c</span></th>
    <th><span class="code">"foo"</span></th>
    <th><span class="code">char* cp</span></th>
    <th><span class="code">nsACString& cs</span></th>
  </tr>
  <tr>
    <th class="row-label"><span class="code">char</span></th>
    <td colspan="2">.</td>
<!-- "foo"          -->    <td><span class="code">[]</span></td>
<!-- char* cp       -->    <td><span class="code">[]</span></td>
<!-- nsACString& cs -->    <td><a href="#faq_how_to_extract_a_character">extract a character</a></td>
  </tr>
  <tr>
    <th class="row-label"><span class="code">PRUnichar</span></th>
<!-- 'x'            -->    <td><span class="code">PRUnichar('x')</span></td>
<!-- char c         -->    <td><span class="code">PRUnichar(c)</span></td>
    <td colspan="3"><a href="#faq_how_to_convert_encoding">convert encoding</a>, <a href="#faq_how_to_extract_a_character">extract a character</a></td>
  </tr>
  <tr>
    <th class="row-label"><span class="code">char*</span></th>
<!-- 'x'            -->    <td><span class="code">&amp;</span></td>
<!-- char c         -->    <td><span class="code">&amp;</span></td>
<!-- "foo"          -->    <td><span class="code">&amp;</span></td>
<!-- char* cp       -->    <td>.</td>
<!-- nsACString& cs -->    <td><a href="#faq_how_to_get_a_pointer">get a pointer</a></td>
  </tr>
  <tr>
    <th class="row-label"><span class="code">PRUnichar*</span></th>
    <td colspan="5"><a href="#faq_how_to_convert_encoding">convert encoding</a>, <a href="#faq_how_to_get_a_pointer">get a pointer</a></td>
  </tr>
  <tr>
    <th class="row-label"><span class="code">nsACString</span></th>
<!-- 'x'            -->    <td><span class="code">NS_LITERAL_CSTRING("x")</span></td>
<!-- char c         -->    <td><a href="#faq_how_to_make_a_string">make a string</a></td>
<!-- "foo"          -->    <td><span class="code">NS_LITERAL_CSTRING("foo")</td>
<!-- char* cp       -->    <td><a href="#faq_how_to_make_a_string">make a string</a></td>
<!-- nsACString& cs -->    <td>.</td>
  </tr>
  <tr>
    <th class="row-label"><span class="code">nsAString</span></th>
<!-- 'x'            -->    <td><span class="code">NS_LITERAL_STRING("x")</span></td>
<!-- char c         -->    <td><a href="#faq_how_to_convert_encoding">convert encoding</a></td>
<!-- "foo"          -->    <td><span class="code">NS_LITERAL_STRING("foo")</span></td>
    <td colspan="2"><a href="#faq_how_to_convert_encoding">convert encoding</a></td>
  </tr>
  <tr>
    <th class="row-label">to call <span class="code">printf</span></th>
    <td colspan="4">.</td>
<!-- nsACString& cs -->    <td><a href="#faq_how_to_call_printf">call <span class="code">printf</span></a></td>
  </tr>
</table>

<table class="chart">
  <tr>
    <th></th>
    <th colspan="3">you have some <span class="code">PRUnichar</span>s</th>
  </tr>
  <tr>
    <th>you want</th>
    <th><span class="code">PRUnichar w</span></th>
    <th><span class="code">PRUnichar* wp</span></th>
    <th><span class="code">nsAString& s</span></th>
  </tr>
  <tr>
    <th class="row-label"><span class="code">char</span></th>
<!-- PRUnichar w    -->    <td></td>
<!-- PRUnichar* wp  -->    <td></td>
<!-- nsAString& s   -->    <td></td>
  </tr>
  <tr>
    <th class="row-label"><span class="code">PRUnichar</span></th>
<!-- PRUnichar w    -->    <td></td>
<!-- PRUnichar* wp  -->    <td><span class="code">[]</span></td>
<!-- nsAString& s   -->    <td><a href="#faq_how_to_extract_a_character">extract a character</a></td>
  </tr>
  <tr>
    <th class="row-label"><span class="code">char*</span></th>
<!-- PRUnichar w    -->    <td></td>
<!-- PRUnichar* wp  -->    <td></td>
<!-- nsAString& s   -->    <td></td>
  </tr>
  <tr>
    <th class="row-label"><span class="code">PRUnichar*</span></th>
<!-- PRUnichar w    -->    <td><span class="code">&amp;</span></td>
<!-- PRUnichar* wp  -->    <td></td>
<!-- nsAString& s   -->    <td><a href="#faq_how_to_get_a_pointer">get a pointer</a></td>
  </tr>
  <tr>
    <th class="row-label"><span class="code">nsACString</span></th>
<!-- PRUnichar w    -->    <td></td>
<!-- PRUnichar* wp  -->    <td></td>
<!-- nsAString& s   -->    <td></td>
  </tr>
  <tr>
    <th class="row-label"><span class="code">nsAString</span></th>
<!-- PRUnichar w    -->    <td></td>
<!-- PRUnichar* wp  -->    <td></td>
<!-- nsAString& s   -->    <td></td>
  </tr>
  <tr>
    <th class="row-label">to call <span class="code">printf</span></th>
<!-- PRUnichar w    -->    <td></td>
<!-- PRUnichar* wp  -->    <td></td>
<!-- nsAString& s   -->    <td><a href="#faq_how_to_call_printf">call <span class="code">printf</span></a></td>
  </tr>
</table>

<div class="faq">
  <dl>
    <dt>
      is there any string doc?
    </dt>
    <dd>
      Yes, you're soaking in it!
    </dd>



<!-- getting a pointer -->
    <dt>
      <a name="faq_how_to_get_a_pointer">I have a string, how do I get a pointer to the characters?</a>
    </dt>
    <dd>
      You want to avoid this situation.
      In your own interfaces, prefer string types over raw pointers.
      Any interface that wants to process a string using a single pointer is making two expensive assumptions.
      First, that the string is stored in one contiguous hunk; and
        second, that the string is zero-terminated.
      If this isn't the case,
        then to get a pointer, storage must be allocated and the entire string must be copied to it and zero-terminated.
      You may not be able to avoid needing a pointer when interacting with system calls. 
    </dd>
    <dd>
      Some string classes guarantee that they are `flat'.
      That is, that their data is stored in one contiguous zero-terminated hunk.
      This <strong>does not</strong> imply that there are no embedded nulls.  Caveat emptor.
      All strings that explicitly promise flatness
        inherit from the class <span class="code">nsAFlatString</span>
          or <span class="code">nsAFlatCString</span>
        and can produce a constant pointer to their data with the <span class="code">get()</span> member function.
      Even strings that don't explicitly promise to be flat
        may happen to be flat.
      The helper function <span class="code">PromiseFlatString</span> will produce
        a <span class="code">const</span> dependent string that is guaranteed to be flat.
      If you use this on a string that already happens to be flat,
        the result is simply a reference through to that string.
      Otherwise,
        <span class="code">PromiseFlatString</span> does the work to allocate, copy, terminate, and manage
        a temporary flat string.
      Since the result of <span class="code">PromiseFlatString</span> is a temporary,
        you must be careful not to get and hold a pointer to its data for longer than the temporary itself lives.
    </dd>
    <dd>
<div class="source-code">
<pre>
  /* I have a string, how do I get a pointer to the characters? */

extern void EvilNarrowOSFunction( const char* );    // evil OS routines that want a pointers
extern void EvilWideOSFunction( const PRUnichar* );

void func( const nsAString&amp; aString, const nsACString&amp; aCString )
  {
    EvilWideOSFunction( NS_LITERAL_STRING("Hello, World!").<span class="notice">get()</span> );
      // literal strings are flat already (as are |nsString|s, et al), just use |.get()|

    EvilWideOSFunction( <span class="notice">PromiseFlatString(</span>aString<span class="notice">).get()</span> );
      // for strings that don't explicitly guarantee flatness, use |PromiseFlatString|


      // beware holding the pointer for longer than the life of the promise
    <span class="warning">const PRUnichar* wp = PromiseFlatString(aString).get(); // BAD! |wp| dangles
    EvilWideOSFunction(wp);</span>

      // if you really need to use the pointer from |PromiseFlatString| in more than one expression...
    const nsAFlatString&amp; flat = <span class="notice">PromiseFlatString(</span>aString<span class="notice">)</span>;
    EvilWideOSFunction(flat.<span class="notice">get()</span>);
    SomeOtherFunction(flat.<span class="notice">get()</span>);

      // similarly for |char| strings
    EvilNarrowOSFunction( <span class="notice">PromiseFlatCString(</span>aCString<span class="notice">).get()</span> );
  }
</pre>
</div>
    </dd>



<!-- extracting a character -->
    <dt>
      <a name="faq_how_to_extract_a_character">How do I get a particular character out of a string?</a>
    </dt>
    <dd>
      Flat strings provide <span class="code">operator[]</span> and <span class="code">CharAt()</span>.
      All strings provide <span class="code">First()</span>, <span class="code">Last()</span>, and access with iterators.
      <strong>Don't</strong> promise a string flat just to do character indexing.
      Prefer, instead, to get an iterator and <span class="code">advance</span> it to the position you care about.
    </dd>
    <dd>
<div class="source-code">
<pre>
  /* How do I get a particular character out of a string? */

PRUnichar Get5thCharacterOf( const nsAString& aString )
  {
    if ( aString.Length() >= 5 )
      {
        nsAString::const_iterator iter;
        aString.BeginReading(iter); // make |iter| point to the beginning of |aString|
        iter.advance(5);
        return *iter;
      }

    return PRUnichar(0);
  }
</pre>
</div>
    </dd>
    <dd>
      Using iterators isn't as bad as the example above makes it feel.
      The typical use is for advancing through a string, examining many characters.
    </dd>



<!-- how to convert encoding -->
    <dt>
      <a name="faq_how_to_convert_encoding">How do I convert from one encoding to another?</a>
    </dt>
    <dd>
    </dd>



<!-- how to make a string -->
    <dt>
      <a name="faq_how_to_make_a_string">How do I create a string?</a>
    </dt>
    <dd>
    </dd>


<!-- how to return a string -->
    <dt>
      What is the best way to return a string?
    </dt>
    <dd>
      <p>
        There are several reasonable ways to produce a string result from a function.
        If you are already holding the answer as a sharable string,
          you can simply return that string (pass-by-value).
        Otherwise,
          the most efficient and flexible way to return a string is
          to assign your result into a non-<span class="code">const</span> reference parameter.
        Don't bother to create a sharable string from scratch with your generated result.
      </p>
      <p>
        Why?
        The two things you want to minimize in string manipulation are,
          in order of importance,
            heap allocation, and
            moving characters around.
      </p>
    </dd>
    <dd>
<div class="source-code">
<pre>
  /* What is the best way to return a string? */

class foo
  {
    public:
      // ...
      void GetShortName( nsAString&amp; aResult ) const;
      nsCommonString GetFullName() const;
      
    private:
      nsCommonString    mFullName;

      const PRUnichar*  mShortName;
      PRUint32          mShortNameLength;
      
  };

nsCommonString
foo::GetFullName() const
  {
    return mFullName;
  }

void
foo::GetShortName( nsAString&amp; aResult ) const
  {
    aResult = DependentString(mShortName, mShortNameLength);
  }
</pre>
</div>
    </dd>


    <dt>
      <a name="faq_how_to_call_printf">How do I <span class="code">printf</span> a string, e.g., for debugging.</a>
    </dt>
    <dd>
      If your string is already narrow, you just have to worry about <a href="#faq_how_to_get_a_pointer">making it flat, and then getting a pointer</a>.
    </dd>
    <dd>
      If your string happens to be wide,
        you'll need to convert it before you can <span class="code">printf</span> something reasonable.
      If it's just for debugging,
        you probably wouldn't care if something odd was printed in the case of a Unicode character that didn't have
        an ASCII equivalent. (If you have a UTF-8 terminal, the result is 
       perfectly legible and nothing odd is printed.)
      The simplest thing in this case is to make a temporary conversion using <span class="code">NS_ConvertUTF16toUTF8</span>.
      The result is conveniently flat already, so getting the pointer is simple.
      Remember not to hold onto the pointer you get out of this beyond the lifetime of temporary.
    </dd>
    <dd>
<div class="source-code">
<pre>
  /* How do I |printf| a string? */


void PrintSomeStrings( const nsAString& aString, const PRUnichar* aKey, const nsACString& aCString )
  {
      // |printf|ing a narrow string is easy
    printf("%s\n", <span class="notice">PromiseFlatCString(</span>aCString<span class="notice">).get()</span>);     // GOOD

      // the simplest way to get a |printf|-able |const char*| out of a string
    printf("%s\n", <span class="notice">NS_ConvertUTF16toUTF8(</span>aKey<span class="notice">).get()</span>);       // GOOD

      // works just as well with an formal wide string type...
    printf("%s\n", <span class="notice">NS_ConvertUTF16toUTF8(</span>aString<span class="notice">).get()</span>);


      // But don't hold onto the pointer longer than the lifetime of the temporary!
    <span class="warning">const char* cstring = NS_ConvertUTF16toUTF8(aKey).get(); // BAD! |cstring| is dangling
    printf("%s\n", cstring);</span>
  }
</pre>
</div>
    </dd>

  </dl>

<p>
  Here are the email answers I have yet to format into the FAQ.
  Some of the URLs may be out-dated or moved.
  The messages are in order from oldest to newest.
</p>
<p class="editnote">[Note : In June, 2003, these emails were modified
to better reflect what is stored in 'wide' string
classes (UTF-16 string instead of UCS-2)  and what        
related methods do as a part of the patch for <a href=
"http://bugzilla.mozilla.org/show_bug.cgi?id=183156" 
title="replace UCS2 in function/class/method names with UTF16">bug 183156</a>.
Therefore, they're a little different from  the original emails
written by <a href="http://ScottCollins.net/">Scott Collins</a>]
</p>
<hr>
<pre>
Date: Thu, 13 Apr 2000 19:41:47 -0400
</pre>

<p>Encoding Wars

<p>This message is all about strings and the various encodings that might
be used to interpret their contents, the ramifications of that, and
where we're heading.  The point of this message is to say what we're
currently thinking, and get feedback.  I apologize in advance for the
rambling, and for the fact that this message may accidentally mix
discussion of how things <strong>are</strong> and how they will be.

<p>There are many different possible encodings.  Three in common use in
the Mozilla source base are: ASCII, UTF-16, and UTF-8.  In ASCII, every
<!--the Mozilla source base are: ASCII, UCS2, and UTF8.  In ASCII, every-->
character fits in 7-bits and is typically stored in an 8-bit byte.  We
usually represent ASCII strings with <span class="code">nsCString</span>s, <span class="code">nsXPIDLCString</span>s,
or <span class="code">char</span> string literals.  In UTF-16, characters occupy one 16-bit code unit (
<a href="http://www.unicode.org/glossary/index.html#BMP_character">
<abbr title="Basic Multilingual Plane">BMP</abbr>characters</a>) 
or two 16-bit code units 
(<a href="http://www.unicode.org/glossary/index.html#supplementary_character">
<abbr title="Supplementary Plane : Plane 1 through 16">non-BMP</abbr> characters</a>).
We usually represent UTF-16 strings as <span class="code">nsString</span>s, etc., i.e., two-byte
or `wide' strings.  UTF-8 is a multi-byte encoding.  A character might
occupy one, two, three, or four bytes.  It is easiest to store and
manipulate such a string within a single-byte or `narrow' string
implementation.

<p>None of our current string implementations know the encoding of the
data they hold at any given moment.  An <span class="code">nsCString</span> might legitimately
hold data encoded in ASCII, UTF-8 or even EBCDIC for that matter.

<p>Operations that convert from one encoding to another, or operations
that are encoding sensitive (e.g., <span class="code">to_upper</span>), rightly belong in
i18n.  The fact that our current string interfaces automatically and
implicitly convert between wide and narrow strings is actually the
source of many errors in two particular categories: (1) unintended
extra work, (2) mistaken re-encoding, e.g., accidentally `converting'
a UTF-8 string to UTF-16 by pretending the UTF-8 string is ASCII and then
padding with <span class="code">'\0'</span>s.

<p>We've known these were bad for a long time, and have been trying to
find the right way to fix them.  The current thinking is to just byte
the bullet and eliminate implicit conversions.  That has interesting
ramifications.

<div class="source-code">
<pre>
void foo( const nsString&amp;  aUTF16string );

foo("hello"); // works!  constructs a temporary |nsString| by
              // converting the ASCII literal with padding.
              // Note: this requires an allocation
</pre>
</div>

<p>Though we've always hated this form since it requires a heap
allocation.  In current code, we recommend

<div class="source-code">
<pre>
foo( nsAutoString("hello") );
</pre>
</div>

<p>which still copy/converts, but at least it probably doesn't need to do
a heap allocation.  In the best of all worlds, no conversion, copying,
or allocation would be necessary.  To do that, you would need to be
able to directly specify a UTF-16 string, e.g., with the <span class="code">L"hello"</span>
notation, and wrap that in an interface that just held a pointer. 
E.g., something like

<div class="source-code">
<pre>
void foo( const nsAReadableString&amp;  aUTF16string );

foo( nsLiteralString(L"hello") );
</pre>
</div>

<p>There are problems with this example, however.  The <span class="code">L</span> notation
specifically makes objects that are arrays of <span class="code">wchar_t</span>, which under
GCC is a 4-byte element.  This leads to incompatibility with JS, and
the annoyance of possibly bloated storage  (I'm sort of minimizing the
situation here.  It's worse that I make it sound).  More about tricks
to get around this in a bit, but first, let me talk about what to do
in the meantime while we're just getting rid of implicit constructors.
 Initially to get around this problem (what problem?  The problem that
<span class="code">foo("hello")</span> stopped compiling on my machine when I threw the
switch) I made a routine called <span class="code">NS_ConvertToString</span> which looked like
this

<div class="source-code">
<pre>
inline
nsAutoString
NS_ConvertToString( const char* anASCIIstring )
  {
    nsAutoString aUCS2string;
    aUCS2string.AssignWithConversion(anASCIIstring);
    return aUCS2string;
  }
</pre>
</div>

<p>Which lets me write

<div class="source-code">
<pre>
foo( NS_ConvertToString("hello") );
</pre>
</div>

<p>This was <strong>OK</strong>, but in discussion there were concerns about performance
on machines that didn't <span class="code">inline</span> well, and issues about naming.  In
that meeting we came up with an alternate naming strategy that we
think has room for growth and an implementation more likely to be
efficient on every platform.  The implementation is to define a new
class that derives from <span class="code">nsAutoString</span>, but allows construction from a
<span class="code">char*</span>

<div class="source-code">
<pre>
class NS_ConvertASCIItoUTF16 : public nsAutoString
  {
    public:
      NS_ConvertASCIItoUTF16( const char* );
      // ...
  };
</pre>
</div>

<p>Which gives identical (though renamed) notation for calling <span class="code">foo</span>:

<div class="source-code">
<pre>
foo( NS_ConvertASCIItoUTF16("hello") );
</pre>
</div>

<p>It looks like a function call to an explicit encoding conversion.  It
acts like a function call to an explicit encoding conversion.  It <strong>is</strong>
a function call to an explicit encoding conversion.  We think that
this naming pattern has room for growth.  In the meeting, we concluded
that the best representation for encoding conversions is a family of
functions, and <span class="code">NS_ConvertASCIItoUTF16</span> fits right in.  We think that
XPCOM probably can't live without the ASCII to UTF-16 conversion (though
as explicit as possible) but that all others rightly belong in i18n
land.

<p>You can probably deduce from the clues in <span class="code">NS_ConvertToString</span>, above,
that constructors weren't the only thing that became explicit. 
Assignment, appending, comparison, et al, got renamed so that when
assigning, appending, or comparing to a value in a different encoding
the `WithConversion' form must be used.  E.g.,

<div class="source-code">
<pre>
nsString aUTF16string;
nsCString anASCIIstring;
// ...

aUTF16string += anASCIIstring;  // Currently legal, but not for long
aUTF16string.Append(anASCIIstring); // same

aUTF16string.AppendWithConversion(anASCIIstring); // the new way

if ( aUTF16string == anASCIIstring ) // Sorry, this is going away too
  // ...

if ( aUTF16string.EqualsWithConversion(anASCIIstring) )
  // ...
</pre>
</div>

<p>Yes, it's long and annoying.  Just like the extra work you were
implicitly asking to have done, perhaps incorrectly.  There are other
reasons to rename these functions.  When <span class="code">nsString</span> and <span class="code">nsCString</span>
defined a ton of, e.g., <span class="code">Append</span>s each there was no problem, because
nobody wanted to override <span class="code">Append</span>.  Now, with strings inheriting from
abstract base classes we immediately run into the problem that
overriding and overloading don't mix very well in C++.  Because of a
feature of C++ called name hiding, it is problematic to override only
a single signature of a name overloaded in a base class.  The base
<span class="code">nsAWritableString</span> provides several <span class="code">Append</span>s, all for objects of
(hopefully) the same encoding.  <span class="code">nsString</span> can't easily add a bunch of
new <span class="code">Append</span>s (the converting ones) without running face first into
the name hiding problem.  The discussion of the fix for this is mostly
unrelated to encoding issues, so I'll defer it to another post.

<p>In hindsight, after the meeting, it seemed clear that all the
`WithConversion' forms would be better named

<div class="source-code">
<pre>
xxxConvertingASCIItoUTF16
xxxConvertingUTF16toASCII
</pre>
</div>

<p>however, the <strong>real</strong> goal (probably) is to move most such conversions
into i18n.  Just bringing attention to the previously implicit
conversions is a good first step.  Renaming these conversions as just
suggested is probably the right thing to do, though it sort of
validates them, which I'm not sure we really want.  This is a decision
we need to discuss further.

<p>Now, back to the string literal problem above.  One possible solution
is to use a macro.  Imagine

<div class="source-code">
<pre>
NS_LITERAL_STRING("Hello")
</pre>
</div>

<p>which on a machine where the <span class="code">L</span> trick works, turns into

<div class="source-code">
<pre>
nsLiteralString(L"Hello")
</pre>
</div>

<p>but on a machine where there is trouble, turns into something less
appealing, but more likely to work, like

<div class="source-code">
<pre>
NS_ConvertASCIItoUTF16("Hello")
</pre>
</div>

<p>Another solution is to add a compilation step that fixes <span class="code">L</span> strings
on bad platforms to be non-<span class="code">L</span> strings, but padded with <span class="code">\0</span>s.  E.g.,
<span class="code">L"Hello"</span> gets preprocessed into <span class="code">"\000H\000e\000l\000l\000o\000"</span>. 
This solution is more annoying to the developer, where the prior
solution is more annoying during the runtime.

<p>Before we go to too much trouble on this specific feature, we will
probably want to do more measurement to see just how much and how
often we are converting constant literal strings, and why.


<p>I'm currently ripping through the tree fixing things to use the
`WithConversion' forms where appropriate.  I was also converting
things to use <span class="code">NS_ConvertToString</span> where appropriate; unless I get
talked out of it, I want to switch midstream to
<span class="code">NS_ConvertASCIItoUTF16</span>, then go back and fix up the
<span class="code">NS_ConvertToString</span> instances later.  I've set things up so I can
check in as I go.  After all these conversions have been done, I'll be
able to throw the switch (what switch?  NEW_STRING_APIS) which will
make <span class="code">nsString</span> inherit from <span class="code">nsAWritableString</span>, etc. and allow us to
start exploiting these other opportunities (e.g., for literal strings,
shared strings, etc.  See
<a class="exact-uri" href="http://bugzilla.mozilla.org/show_bug.cgi?id=28221">http://bugzilla.mozilla.org/show_bug.cgi?id=28221</a> for details and
reasoning.)

<p>I guess I'm expecting comments on:

<ul>
  <li>how really annoying this whole topic is
  <li>how bad <span class="code">L"xxx"</span> is
  <li>whether to move forward with <span class="code">NS_ConvertASCIItoUTF16</span>
  <li>whether we should move to xxxConvertingASCIItoUTF16 etc instead
      of `WithConverting'
  <li>arguments about where encoding conversions should live
  <li>arguments about whether going between 1 and 2 byte storage is an
      encoding conversion
  <li>questions about stuff I didn't mention or didn't explain well
  <li>pointing out stuff I'm just plain wrong about, or things I forgot
  <li>etc
</ul>

<p>So as not to jumble the discussion, I'll be separately posting other
requests for comments about specific features of the design of the new
string hierarchy.

<p>I hope this helps keep everybody filled in on what we're thinking and
able to point out what we're forgetting or screwing up :-)





<hr>
<pre>
Date: Wed, 19 Apr 2000 21:12:47 -0400
Subject: more string info
</pre>

<p>  <a class="exact-uri" href="news://news.mozilla.org/scc-705460.16423913042000@news.mozilla.org">news://news.mozilla.org/scc-705460.16423913042000@news.mozilla.org</a>





<hr>
<pre>
Date: Fri, 26 May 2000 15:31:37 -0400
Subject: Re: Question on ==
</pre>

<p>I would prefer you compare with <span class="code">Equals</span> (which should really be named
<span class="code">IsEqualTo</span>) rather than <span class="code">operator==()</span> because of this:

<div class="source-code">
<pre>
char* a;
char* b;

// ...

if ( a == b )
  // ...
</pre>
</div>

<p>Comparing two raw `string' pointers doesn't compare the characters
they point to, but instead compares the bits of the pointers.  For
this reason, I may eventually make comparison of a string with a
pointer using operators just go away.





<hr>
<pre>
Date: Wed, 14 Jun 2000 14:38:55 -0400
Subject: Re: Fix to XprtDefs.h
</pre>

<p>Yes, we're aware that turning off <span class="code">wchar_t</span> support makes <span class="code">wchar_t</span> be
a synonym for <span class="code">unsigned short</span> under Metrowerks.  We know that the
current version of VC++ also makes these types equivalent.  In theory,
though, the types are distinct even when they are the same size and
shape.  By using real <span class="code">wchar_t</span> support, we are forced to recognize
the distinction and navigate it appropriately with <span class="code">reinterpret_cast</span>.
The win here is that we aren't caught by
compiler changes that suddenly make some set of compilers compliant
and therefore break our code.  We will add an autoconf test that lets
UNIX compilers opt in to our string scheme when they have an
appropriately shaped <span class="code">wchar_t</span>.  If these happen to be compliant
compilers, all will be well.  If they don't, the casts don't hurt,
because they are type correct.  We are writing our code to meet the
standard as we move forward.

<p>The win for us is realized by the following macros

<div class="source-code">
<pre>
#ifdef HAVE_CPP_2BYTE_WCHAR_T
  #define NS_LITERAL_STRING(s)  nsLiteralString(L##s, \
                      (sizeof(L##s)/sizeof(wchar_t))-1)
#else
  #define NS_LITERAL_STRING(s)  NS_ConvertASCIItoUTF16(s, \
                       sizeof(s)-1)
#endif
</pre>
</div>

<p>An <span class="code">nsLiteralString</span> points directly to the literal characters.  No
copying, no conversion, and the length calculation happens at compile
time.  This has turned out to be as large a savings as 15% of code
space and 8% of data space, net, in our string test harness  It's
faster as well, again by eliminating the copying, conversion, and
length calculation.  We don't know yet what those numbers translate
into in our real code base, but we have high hopes.

<p>I don't want to be in the position to ask you to change your code.  I
don't think it's appropriate for me to do so.  The AIM application
that is your client is our client as well.  They need to resolve this
difference between us in whatever way they think best.  That may mean
asking you if changing your apis is the right thing to do.  Or it may
mean applying the casts.  Our code-base and yours, Justin, are more
like cousins.  I don't think you should have to change just to conform
to us.  You may think my arguments for using real <span class="code">wchar_t</span> have
merit, and adopt similar usage just because you agree; but I think the
only obligation you have is to follow the technical solution you think
is right for your code.

<p>If you decide to make this api change, it will mean shipping a new
binary (on Mac) for your library to clients who want to switch over to
the new api (since the name mangling will be different, and therefore,
the link requirements will change).

<p>Hope this helps,





<hr>
<pre>
Date: Thu, 15 Jun 2000 19:36:55 -0400
Subject: Re: Checkin approval for bug 32336
</pre>

<div class="source-code">
<pre>
S.Equals(NS_LITERAL_STRING("bar"), PR_TRUE, 3)
</pre>
</div>

<p>doesn't compile because there is no three parameter form for <span class="code">Equals</span>.
 For all definitions of <span class="code">Equals</span> on strings, see "nsAReadableString.h"

<p><a class="exact-uri" href="http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsAReadableString.h">http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsAReadableString.h</a>

<p>There is an <span class="code">EqualsWithConversion</span> that takes three parameters.

<p>  <a class="exact-uri" href="http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsString2.h#731">http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsString2.h#731</a>

<p>It is ``EqualsWithConversion'' because it admits the possibility of an
encoding specific transformation, in this case to provide
case-insensitive comparison.  This also wouldn't compile, however,
since, at the moment, an <span class="code">nsLiteralString</span> doesn't provide an operator
to produce a <span class="code">const PRUnichar*</span> (though perhaps it should), and it
doesn't satisfy the other interfaces that match this call, e.g., a
<span class="code">const nsString&</span>.

<p>Perhaps I need to move case-insensitive comparison up out of
<span class="code">nsString</span> into a global encoding specific transformations and
algorithms file (which was on its way anyway as Waterson, knows); this
use is one bit of evidence to support this.  In the short term, this
can be fixed (if we think the current behavior is wrong) by providing
<span class="code">operator const CharT*() const</span> on literal string.

<p>If you can live with out case-folding, the earlier form is preferred

<div class="source-code">
<pre>
S == NS_LITERAL_STRING("bar")
</pre>
</div>

<p>if you can't, then one of the fixes I mentioned is in order.





<hr>
<pre>
Date: Thu, 15 Jun 2000 19:47:12 -0400
Subject: Re: [Fwd: how to use nsString ?]
</pre>

<pre class="email-quote">
  >I see these same examples time and again in the embedding
  >samples/docs, but I can't compile them.
</pre>

<p>Apologies.  Documentation mentioning strings is getting out of date. 
Here are some specific answers.


<pre class="email-quote">
  >nsString URLString("http://www.mozilla.org");
</pre>

<p>...is now perhaps best expressed as

  nsString URLString( NS_LITERAL_STRING("http://www.mozilla.org") );

<p>since an <span class="code">nsString</span> is a sequence of 2-byte wide characters, and the
routines that implicitly convert 1-byte sequences (like the literal
sequence you specified, "http:...") are now gone.

<p>Up until not too long ago, one would have had to say

<div class="source-code">
<pre>
nsString URLString;
URLString.AssignWithConversion("http://www.mozilla.org");
</pre>
</div>

<p>The <span class="code">NS_LITERAL_STRING</span> construction is new machinery that has the
potential to make many operations much more efficient.

<pre class="email-quote">
  >nsString URLString;
  >URLString.SetString("www.mozilla.org");
</pre>

<p><span class="code">SetString</span> was a synonym for <span class="code">Assign</span> or assignment with
<span class="code">operator=()</span>, it too went away.  The equivalent is the second
example I gave above, that is, the one with <span class="code">AssignWithConversion</span>. 

<p><span class="code">Assign</span> still exists.  <span class="code">AssignWithConversion</span> takes on that
functionality for assignments that require encoding transformations
(e.g., from ASCII to UTF16).  <span class="code">SetString</span> is gone, since it was always
a synonym for <span class="code">Assign</span>. 

<p>Learn more about the general APIs for strings that we are trying to
move to by examining

<a class="exact-uri" href="http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsAReadableString.h">http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsAReadableString.h</a>
<a class="exact-uri" href="http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsAWritableString.h">http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsAWritableString.h</a>

<p>Hope this helps,





<hr>
<pre>
Date: Thu, 15 Jun 2000 21:26:51 -0400
Subject: Re: Checkin approval for bug 32336
</pre>

<pre class="email-quote">
  >I *need* the count attribute, because I need to compare only the first 
  >chars (that's inherent to the logic).
</pre>

<p>This is what substrings are for.  In that case, you could use

<div class="source-code">
<pre>
Substring(S, 0, 3) == NS_LITERAL_STRING("bar")
</pre>
</div>

<p>As for case-folding, it's best if you can case-fold everything up
front, instead of doing it repeatedly.  I'll have to get back to you
on a general solution to that problem, or what my schedule for getting
it checked in would be.  I'm sorry, I know that's not what you needed
to hear.  If the source string is an <span class="code">nsString</span>, you can continue to
exploit its implementation of these routines, e.g., <span class="code">ToLower</span> all
up-front.

<p>Hope this helps,





<hr>
<pre>
Date: Mon, 19 Jun 2000 14:23:47 -0400
Subject: Re: string fu
</pre>

<pre class="email-quote">
  >It seems less convenient to have to first check path.IsEmpty, and
  >then if false get path.Last and test it.
</pre>

<p>What would you prefer?  That extracting a character not in the string
always return <span class="code">CharT(0)</span>?  Can't do it for two reasons: (1) <span class="code">0</span> may be
a valid character in a particular encoding, so it can't be used in
general as a ``no character at that position'' marker; and (2) I can't
control what an individual string implementation does when asked to
get an out-of-bounds fragment, it's explicitly undefined.  That means
the result of <span class="code">CharAt</span> is explicitly undefined for indexes outside the
defined contents of the string.  As a debugging convenience, I have
made this assert, but it has always been the case that retrieving such
a character had undefined results ... even in [the old] code.

<p>OK, you might say, well at least let me ask for a character that is
only off the end by one.  E.g., <span class="code">Last</span> of an empty string.  Reason (1)
from above still applies.  How bad is it to say, for the case you gave

<div class="source-code">
<pre>
PRBool needsDelim = PR_FALSE;
if ( !path.IsEmpty() )
  {
    PRUnichar last = path.Last();
    needsDelim = !(last == '/' || last == '\\');
  }
</pre>
</div>

<p>In general, you probably want to opt out of a whole lot of work when
the source string is empty.  It is slightly less convenient, but it
doesn't tie us to a bunch of implementation specific mojo.


<pre class="email-quote">
  >Can we fix GetUnicode in this case?
</pre>

<p>This is an annoying property of auto strings, e.g., that they always
have an allocated buffer.  I'm happy to fix this bug, however, be
aware that <span class="code">GetUnicode</span> and <span class="code">GetBuffer</span> are artifacts of [the old]
implementation that we don't want to support.  They are not part of
the abstract interface.  We will keep them no longer than we have to. 
They don't support our multi-fragment paradigm.  People who require a
contiguous hunk of characters in the future, and are unwilling to
switch over to chunky-iterators, may be forced to copy the string to
their own buffer.  There will be an implementation of narrow character
string that guarantees contiguous allocation and a zero-terminator,
much as <span class="code">nsCString</span> does now, for compatibility with platform uses,
but this won't be the default string class.





<hr>
<pre>
Date: Mon, 19 Jun 2000 17:22:31 -0400
</pre>

<p>Clarifying String Sematics

<p>Recently, I added an assert to the string operations that extract
characters, namely <span class="code">First()</span>, <span class="code">Last()</span>, <span class="code">CharAt()</span>, and
<span class="code">operator[]()</span>.  This assert fires when any of these routines are used
to access a character outside the defined contents of the string.  For
<span class="code">First()</span> and <span class="code">Last()</span> that means whenever they are applied to an
empty string.  For <span class="code">CharAt()</span> and <span class="code">operator[]()</span>, that means whenever
they are used to access an index outside the range of
<span class="code">0</span>..<span class="code">Length()-1</span>.  There have been some complaints, however, the
result was always undefined.  What follows is extracted from an email
exchange between me and warren on this topic.  I hope it clarifies
strings semantics

<p>Warren writes:
<pre class="email-quote">
  >I hit your funky CharAt assertion tonight in this piece of code:

  >NS_IMETHODIMP
  >nsIOService::ResolveRelativePath(
  >    const char *relativePath,
  >    const char* basePath,
  >    char **result )
  >  {
  >    nsCAutoString name;
  >    nsCAutoString path(basePath);
  >    
  >    PRUnichar last = path.Last();
  >    PRBool needsDelim = !(last == '/' || last == '\\' || last ==
  >    '\0');
  >    ...

  >where basePath is null. It seems less convenient to have to first
  >check path.IsEmpty, and then if false get path.Last and test it.
</pre>

<p>I replied:
<pre class="email-quote">
  >What would you prefer?  That extracting a character not in the
  >string always return <span class="code">CharT(0)</span>?  Can't do it for two reasons:
  >(1) <span class="code">0</span> may be a valid character in a particular encoding, so it
  >can't be used in general as a ``no character at that position''
  >marker; and (2) I can't control what an individual string
  >implementation does when asked to get an out-of-bounds fragment,
  >it's explicitly undefined.  That means the result of <span class="code">CharAt</span> is
  >explicitly undefined for indexes outside the defined contents of
  >the string.  As a debugging convenience, I have made this assert,
  >but it has always been the case that retrieving such a character
  >had undefined results ... even in [the old] code.

  >OK, you might say, well at least let me ask for a character that
  >is only off the end by one.  E.g., <span class="code">Last</span> of an empty string.
  >Reason (1) from above still applies.  How bad is it to say, for the
  >case you gave

  >  PRBool needsDelim = PR_FALSE;
  >  if ( !path.IsEmpty() )
  >    {
  >      PRUnichar last = path.Last();
  >      needsDelim = !(last == '/' || last == '\\');
  >    }

  >In general, you probably want to opt out of a whole lot of work
  >when the source string is empty.  It is slightly less convenient,
  >but it doesn't tie us to a bunch of implementation specific mojo.
</pre>

<p>Warren also asks:
<pre class="email-quote">
  >Here's another issue, perhaps more serious. If I say this:

  >  foo(const PRUnichar* s) {
  >    nsAutoString str(s);
  >    bar(str.get());
  >  }

  >where s is null, bar will get passed a zero-length PRUnichar
  >sequence instead of null. This makes it so that you can't just
  >test for the argument == null. You have to nsCRT::strlen(arg) == 0
  >which is much less efficient. Can we fix GetUnicode in this case?
</pre>

<p>And I reply:
<pre class="email-quote">
  >This is an annoying property of auto strings, e.g., that they
  >always have an allocated buffer.  I'm happy to fix this bug,
  >however, be aware that <span class="code">GetUnicode</span> and <span class="code">GetBuffer</span> are artifacts
  >of [the old] implementation that we don't want to support.  They
  >are not part of the abstract interface.  We will keep them no
  >longer than we have to.  They don't support our multi-fragment
  >paradigm.  People who require a contiguous hunk of characters in
  >the future, and are unwilling to switch over to chunky-iterators,
  >may be forced to copy the string to their own buffer.  There will
  >be an implementation of narrow character string that guarantees
  >contiguous allocation and a zero-terminator, much as <span class="code">nsCString</span>
  >does now, for compatibility with platform uses, but this won't be
  >the default string class.
</pre>

<p>In a later message, Chris Waterson asks a related question
<pre class="email-quote">
  >scc: should we add <span class="code">operator PRUnichar*()</span> to
  >NS_ConvertASCIItoUTF16?
</pre>

<p>And I reply:
<pre class="email-quote">
  >It seems reasonable.  A lot more reasonable that forcing people to
  >call <span class="code">GetUnicode()</span>.  I alluded to platform specific classes in an
  >earlier message to warren that you were cc'd on, Chris.  I imagine
  >that the <span class="code">...Convert...</span> routines would be required to produce
  >contiguous allocation 0-terminated strings (though the as yet
  >unimplemented <span class="code">...Copy...</span> forms, of course wouldn't.  So <span class="code">operator
  >const PRUnichar*() const</span> makes perfect sense to me here.
</pre>

<p>Hope this makes sense,




<hr>
<pre>
Date: Tue, 20 Jun 2000 04:05:31 -0400
Subject: Re: NS_LITERAL_STRING is broken
</pre>

<p>The behavior you describe sounds exactly like when you say

<div class="source-code">
<pre>
const char* foobar = "foobar";

... NS_LITERAL_STRING(foobar).get() ...
</pre>
</div>

<p>because in this case, the thing passed in is a <span class="code">const char*</span>. 
<span class="code">NS_LITERAL_STRING</span> is not meant to be used in this way.  It is only
meant to be used around a <span class="code">"</span> delimited string.  The type of such is
<span class="code">const char[N]</span> where N is the number of characters in the string + 1
for the zero terminator it helpfully adds.  <span class="code">sizeof</span> such a type is
<span class="code">N</span>.

<p>Are you sure you had the actual string as an argument, as in your
example to me?  Or could the actual code have been like my sample,
above?





<hr>
<pre>
Date: Thu, 29 Jun 2000 13:35:10 -0400
Subject: Re: a fix
</pre>

<pre class="email-quote">
  > +       if (Length() == 0) { return nsnull; }
</pre>


<p>Dave,

<p>please read

  <a class="exact-uri" href="news://news.mozilla.org/scc-314ABF.14261619062000@news.mozilla.org">news://news.mozilla.org/scc-314ABF.14261619062000@news.mozilla.org</a>

<p>It's just plain wrong to let people try to index into a string outside
its defined contents.  I can't just return <span class="code">'\0'</span> or <span class="code">PRUnichar('\0')</span>
there as that <strong>could</strong> be a legal value to have somewhere in your
string for some encodings ... and the encoding is not specified.  So
your patch has the basic problem of defeating my plan to stop people
from doing this bad thing.

<p>The second problem with your patch is that you use the symbolic
constant <span class="code">nsnull</span>, which is ostensibly a pointer value; <span class="code">Last</span> returns
a character.  <span class="code">nsnull</span> is not appropriate for that purpose.  In fact,
C++ gurus pretty much eschew the use of symbolic constants for <span class="code">0</span>. 
<span class="code">NULL</span> is to be avoided.  <span class="code">nsnull</span> is wrong-headed in that it presumes
we could have some <strong>other</strong> application specific value for <span class="code">NULL</span>.  We
can't, it would never work.  It's just wasted brain-print.  Always use
<span class="code">0</span> for these situations, and if you want to communicate the fact that
something is a pointer type, either use a comment or a
(construction-style) cast, like so (graded examples from worst to
best:)

<ul>
  <li>F: FindChildByNameWithHint("Chuck", nsnull);

  <li>D: FindChildByNameWithHint("Chuck", NULL);

  <li>C: FindChildByNameWithHint("Chuck", /* Child* */ 0);

  <li>B: typedef Child* Child_ptr;
     FindChildByNameWithHint("Chuck", Child_ptr(0));

  <li>A: FindChildByNameWithHint("Chuck", 0);
</ul>

<p>Don't let this discourage you; keep up the good work :-)





<hr>
<pre>
Date: Tue, 8 Aug 2000 23:47:16 -0400
Subject: Re: nsWritingIterator?
</pre>

<pre class="email-quote">
  >Can you give me any pointers to examples, or docs, or just some
  >general advice?
</pre>

  <a class="exact-uri" href="http://ScottCollins.net/Journal/discussion/string_iterators.html">http://ScottCollins.net/Journal/discussion/string_iterators.html</a>

<p>does this help?

<p>I can personally walk you through any specific scenario you need.





<hr>
<pre>
Date: Wed, 9 Aug 2000 02:35:03 -0400
Subject: Re: nsWritingIterator?
</pre>

<p>You got it right... it's <span class="code">nsWritingIterator<CharT></span> for whichever
character type you care about, either <span class="code">char</span> or <span class="code">PRUnichar</span>.  You
_can_ use this iterator like a character pointer ... that is, you can
dereference it, assign into its dereference, etc.  It is more
efficient, though, to directly address a particular range of
characters around where it points by asking it for its actual
character pointer with <span class="code">get</span>, and knowing that there are
<span class="code">size_forward()</span> characters available ahead of that pointer and
<span class="code">size_backward()</span> characters available behind it.  After examining
those characters by hand, you can advance the iterator beyond the
characters you have examined (and possibly into the next chunk, should
one exist) by adding into it (with +=) the count of the characters you
have processed.

<p>Here are three examples of running through a string and modifying some
of the characters in it.  All use <span class="code">nsWritingIterator</span>s.


<div class="source-code">
<pre>
  // inefficient, but works in a pinch:
  //  iterators can hide all details of chunks by acting like
  //  a raw character pointer

nsWritingIterator&lt;PRUnichar&gt; s = S.BeginWriting();
nsWritingIterator&lt;PRUnichar&gt; done_with_string = S.EndWriting();

  // for each character in the string |S|
while ( s != done_with_string )
  {
      // if the character is lower case, capitalize it
    if ( 'a' &lt;= *s &amp;&amp; *s &lt;= 'z' )
      *s = *s -'a' + 'A';
  }




  // efficient
  //  iterators provide a mechanism by which you can process
  //  a chunk-at-a-time

nsWritingIterator&lt;PRUnichar&gt; iter = S.BeginWriting();
nsWritingIterator&lt;PRUnichar&gt; done_with_string = S.EndWriting();

  // for each chunk of the string
while ( iter != done_with_string )
  {
    size_t N = iter.size_forward();  // # of chars in this chunk
    PRUnichar* s = iter.get();
    PRUnichar* done_with_chunk = s + N;

      // for each character in this chunk
    for ( ; s &lt; done_with_chunk; ++s )
      {
         // if the character is lower case, capitalize it
       if ( 'a' &lt;= *s &amp;&amp; *s &lt;= 'z' )
          *s = *s - 'a' + 'A';
      } 

      // advance the iterator past characters
      //  we examined (and into the next chunk, if any)
    s += N;
  }



  // elegant
  //  pull your transformation into a `sink', and |copy_string|
  //  will efficiently pump any kind of string into it

struct Capitalize
  {
      // inline
    PRUint32
    write( PRUnichar* s, PRUint32 N )
        // processes one chunk, called repeatedly by |copy_string|
      {
        PRUnichar* done_with_chunk = s + N;

         // for each character in this chunk
        for ( ; s &lt; done_with_chunk; ++s )
          {
              // if the character is lower case, capitalize it
            if ( 'a' &lt;= *s &amp;&amp; *s &lt;= 'z' )
              *s = *s - 'a' + 'A';
          }
      }
  };

copy_string(S.BeginWriting(), S.EndWriting(), Capitalize());
</pre>
</div>



<p>Does this show it better?





<hr>
<pre>
Date: Thu, 17 Aug 2000 18:23:22 -0400
</pre>

<pre class="email-quote">
  >I tried looking at the string header files but they
  >are awfully complicated.
</pre>

<p>I'll explain things in a little <strong>more</strong> detail than you need, then so
that some of the stuff you see in these headers will make more sense. 
I'll also answer your questions out of order.

<p>First: the string hierarchy looks like this

<a class="exact-uri" href="http://ScottCollins.net/Journal/discussion/string_hierarchy.gif">http://ScottCollins.net/Journal/discussion/string_hierarchy.gif</a>

<p>The two most important headers are:

<a class="exact-uri" href="http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsAReadableString.h">http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsAReadableString.h</a>
<a class="exact-uri" href="http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsAWritableString.h">http://lxr.mozilla.org/seamonkey/source/xpcom/ds/nsAWritableString.h</a>

<p>These abstract classes, <span class="code">nsAReadable[C]String</span>, and
<span class="code">nsAWritable[C]String</span> are typically what you will want to use in the
interfaces of new code.  If you write a piece of code that takes a
string for input, consider, e.g.,

<div class="source-code">
<pre>
void consumes_a_string( const nsAReadableString&amp;  aInput );
</pre>
</div>

<p>If you write a piece of code that modifies a string, consider

<div class="source-code">
<pre>
void modifies_a_string( nsAWritableString&amp;  aResult );
</pre>
</div>


<p>When creating your own classes, member strings will typically be
<span class="code">nsString</span>s.  When you can't avoid creating a short string that you
need only temporarily during a function, you will typically use
<span class="code">nsAutoString</span>.  When someone passes you a raw pointer, or a raw
pointer and a length, representing a buffer of characters that you may
examine, but won't own, you can treat it like a string by wrapping it
in an <span class="code">nsLiteralString</span>, e.g.,

<div class="source-code">
<pre>
void
reads_a_buffer( const PRUnichar* aInput, PRUint32 aInputLength )
  {
    nsLiteralString input(aInput, aInputLength);
      // doesn't allocate or copy

    // ...
  }
</pre>
</div>

<p>You will use <span class="code">nsLiteralString</span> around quoted constant strings as well,
though typically through the <span class="code">NS_LITERAL_STRING</span> macro, to avoid doing
a length calculation 

<div class="source-code">
<pre>
NS_LITERAL_STRING("x")
</pre>
</div>

<p>expands to

<div class="source-code">
<pre>
nsLiteralString(L"x", (sizeof(L"x")/sizeof(PRUnichar) - 1))
</pre>
</div>

<p>if <span class="code">L</span> notation works as needed on your platform.

Those are the basics.  Now onto your questions:


<pre class="email-quote">
  >For example this won't compile. [...]

  >str1 += L"abc " + str2 + L"def";
</pre>


<p><span class="code">L"abc "</span> makes a an object that is a <span class="code">const wchar_t[5]</span>, and none of
the string code knows about <span class="code">wchar_t</span>.  The main reason is that
<span class="code">wchar_t</span> is not necessarily the right size (it can be 4 bytes under
gcc).  If you wrap these constant expressions in <span class="code">NS_LITERAL_STRING</span>,
as described above, you should get the right thing, e.g., 

<div class="source-code">
<pre>
str1 += NS_LITERAL_STRING("abc ") + str2 + NS_LITERAL_STRING("def");
</pre>
</div>


<pre class="email-quote">
  >Another one is:
  >function(const PRUnichar *foo);
  >call function(L"abc " + str2);

  >It won't create a temporary nsString.
</pre>

<p>This one, I have a quick and easy explanation for.  If <span class="code">function</span> was
declared like this

<div class="source-code">
<pre>
function( const nsAReadableString&amp;  )
</pre>
</div>

<p>then, no problem, since a <span class="code">nsPromiseConcatenation</span> (which was the
result of adding those two things together) <strong>is</strong> a readable string. 
No other objects need to be created; no copying needs to be performed.

<p>In all cases, we want the creation of <span class="code">nsString</span>s et al, to be
<span class="code">explicit</span>, since creation is unbelievably expensive, requiring heap
allocation, locks, copying, etc.

<p>I hope this answers both your posts,





<hr>
<pre>
Date: Thu, 17 Aug 2000 20:57:08 -0400
Subject: re our conversation
</pre>

  return ToNewUnicode( nsLiteralCString(buffer) );






<hr>
<pre>
Date: Fri, 18 Aug 2000 02:52:45 -0400
Subject: Re: More questions and new string API
</pre>

<pre class="email-quote">
  >1) How do I return a static string?

  >const nsAReadableString&amp;  foo() {return NS_LITERAL_STRING("x");}
  >errors on taking the address of a temporary variable.
</pre>

<p>Unfortunately, <span class="code">NS_LITERAL_STRING</span>s definition is not particularly
amenable to this use.  Instead, you would have to say something like
this:

<div class="source-code">
<pre>
const nsAReadableString&
foo()
  {
#ifdef HAVE_CPP_2BYTE_WCHAR_T
    static nsLiteralString static_foo(L"x", 1);
#else
    static nsLiteralString static_foo;
    static PRBool initialized = PR_FALSE;
    if ( !initialized )
      {
        static_foo.AssignWithConversion("x", 1);
        initialized = PR_TRUE;
      }
#endif
    return static_foo;
  }
</pre>
</div>


<pre class="email-quote">
  >2) I'm using these with the STL library in an XPCOM component.
  >What type should I use with map?  This doesn't work...

  >typedef map<const nsAReadableString&, myType*> mapStringMyType;
  >mapStringMyType foo;
  >foo.find(nsAReadableString);  - I want to find on a ReadableString
</pre>

<p>I don't know what errors you are getting; but it probably doesn't work
because a reference isn't an assignable type.  This is just a guess. 
You may need to use

<div class="source-code">
<pre>
map<const nsAReadableString*, myType*>
</pre>
</div>

<p>If you actually want the map to manage ownership of the keys, then
you'll want to use a concrete type, e.g.,

<div class="source-code">
<pre>
map<nsString, myType*>
</pre>
</div>

<p>or perhaps

<div class="source-code">
<pre>
map<nsSharedStringPtr, myType*>
</pre>
</div>

<p>Or maybe there's something else wrong.  Send me the error messages. 
If you end up using a pointer, then of course you'll have to supply a
comparison function to the <span class="code">map</span> template.  You won't be satisfied
with the default comparison of pointers :-)  Sorry I couldn't answer
this one more completely.


<pre class="email-quote">
  >3) How do a get a raw PRUnichar pointer out of nsAReadableString
  >when I need to call something that wants 'unsigned short *'?
</pre>

<p>The problem with this scenario is that an <span class="code">nsAReadableString</span> doesn't
promise that all its data is contiguous, nor that it is
zero-terminated, which is what I suspect you want in this case.  If
the function you want to call can take {pointer, length} tuples, and
can consume the string in hunks without zero termination ... then you
can use <span class="code">copy_string</span> to pump the string into your function, see

  <a class="exact-uri" href="http://ScottCollins.net/Journal/discussion/string_iterators.html">http://ScottCollins.net/Journal/discussion/string_iterators.html</a>

<p>If not, and you absolutely have to have a contiguous zero-terminated
buffer, then there is a new facility (part of the DOMAPI branch) that
does what you need.  It's not checked in on the trunk; it should
be in early next week.  It is <span class="code">nsPromiseFlatString</span>.  This class
promises a contiguous zero-terminated buffer; and has an <span class="code">operator
PRUnichar*</span> to produce a pointer to that buffer automatically.  If the
underlying class <strong>is</strong> one that happens to be a single fragment and
zero-terminated, then, like <span class="code">nsPromiseSubstring</span> and
<span class="code">nsPromiseConcatenation</span>, this class merely holds a reference into the
original data.  If, however, the underlying string is multi-fragment
or not zero-terminated, then <span class="code">nsPromiseFlatString</span> allocates a
contiguous buffer of appropriate size and copies the fragmented string
data to it.  So given

<div class="source-code">
<pre>
void ReadBuffer( PRUnichar* );
</pre>
</div>

<p>You can call this as efficiently as possible with an arbitrary string
like so

<div class="source-code">
<pre>
ReadBuffer( nsPromiseFlatString(aString) );
</pre>
</div>


<p>If the function you are calling needs to take ownership of the buffer
you hand it, then you will probably call <span class="code">ToNewUnicode</span> like so

<div class="source-code">
<pre>
void ConsumeBuffer( PRUnichar* );

ConsumeBuffer( ToNewUnicode(aString) );
</pre>
</div>

<p>The global function <span class="code">ToNewUnicode</span> is declared in "nsReadableUtils.h",
and was only recently added to the build.  It is currently being used
in the DOMAPI branch.  It is part of the build, but the file
"dlldeps.c" in XPCOM may need to be modified to ensure it is exported
on your platform if you are building the tip.

Needless to say, you want to avoid functions that require bare
pointers for several reasons: (a) they typically assume
zero-termination, which is not guaranteed by the normal encodings; (b)
they require contiguous allocation, which may not be possible; (c)
they scan for the end of the string, at linear cost (if the encoding
makes it possible at all), when the length could be known in advance. 
If you have to do it, the above mechanisms work, but be aware of the
cost and the potential need to copy.


<pre class="email-quote">
  >4) How do I declare a local variable to hold a nsAReadableString?
  >and a member variable?
</pre>

<p><span class="code">nsAReadableString</span> is an abstract type.  So you can't have a concrete
instance of it.  All strings in the hierarchy are readable strings. 
If you just want a reference to a readable string, you can say, e.g.,

<div class="source-code">
<pre>
struct foo
  {
    const nsAReadableString&amp;  mString;
    // ...

    foo( const nsAReadableString&amp;  aString ) : mString(aString) { }
  };
</pre>
</div>

<p>...similarly with pointers; but I suspect you are looking for
something more concrete.  An <span class="code">nsString</span> is a <span class="code">nsAReadableString</span>, and
is the typical thing you want as a member variable.  An <span class="code">nsAutoString</span>
is also an <span class="code">nsAReadableString</span> and is typically what you would use for
a short (in length) temporary (in lifetime) local variable, as I
mentioned in my previous post.


<pre class="email-quote">
  >5) If I call a function that returns a PRUnichar* and I want t
  >use it as a nsAReadableString should I wrap it in a
  >nsLiteralString?
</pre>

<p>Yes, though remember, an <span class="code">nsLiteralString</span> assumes the lifetime of the
underlying data is under someone else's control.  If the called
function gives you a buffer that you need to <span class="code">delete</span>, you will have
to manage that yourself.  Currently, people often use <span class="code">nsXPIDLString</span>
to handle that.  XPIDL strings are <strong>not</strong> part of the hierarchy.  They
are only used as a sort of string-<span class="code">auto_ptr</span>.  However, I'm
integrating their functionality into <span class="code">nsString</span>.  There is no problem
in wrapping the same pointer in both as two separate local variables,
one to give you the readable interface, and one to manage the
lifetime.

<p>If it's OK with you, I'd like to post this reply (including your
quoted questions) to n.p.m.xpcom and also put a copy near the string
iterator discussion I provided a link to above, so that other people
with similar questions can see these answers.

<p>Hope this helps,





<hr>
<pre>
Date: Sun, 3 Sep 2000 03:52:17 -0400
</pre>

<p>In article <8nu9m2$eo14@secnews.netscape.com>, "Jon Smirl" 
<jonsmirl@mediaone.com> wrote:

> I have the new strings up and running in my app. They work as
> advertised  and
> I haven't found any bugs. Thanks for the good job in designing and
> implementing them.  Here's are a summary of issues I've encountered
> so far...

<p>Thanks, and I appreciate your comments and insights.


> 
> 1) Should there be a nsSegmentedString derived from nsString instead
> of building segment support into nsString? None of my strings are
> segmented  but
> I keep executing code that is supports it. nsPromiseFlatString would
> be trivial in the non-segmented case.

<p>The general case is that a string does not promise to have contiguous
data.  A specific case is that, for some implementations, it does. 
You couldn't do it the other way around, because a segmented string
couldn't satisfy all the promises of a flat string.  However, through
the use of chunky iterators, operating on strings that happen to be
flat is very efficient.  In fact, <span class="code">nsPromiseFlatString</span> is trivial in
the non-segmented case.  In addition, I'll be adding an abstract flat
class into the hierarchy, which will present additional interface ...
in your local routines where you actually have declared a concrete
string instance that happens to be flat, the compiler will give you
the benefit of using the flat specific routines (e.g., a substring
object over a flat string is simpler than the general purpose
substring).  I need to be cautious about this, though, since I don't
automatically want people propagating the flat type through their
interfaces.  That would put us in the same boat we're in right now ...
where routines only work on a specific kind of string, which denies
other parts of the code the opportunity to use an implementation
beneficial to its specific needs, and typically for no good reason.

> 
> 2) Should nsAWritableString have a way to get the buffer and then
> return  it?
> I need to get the buffer to pass it to OS calls. I'm doing this now
> by passing around nsStrings instead of the interface.  If I just use
> the interface I encur an extra copy since I have to use a temporary
> buffer. 

<p>A specific string implementation could promise this, but in general, a
writable could not.  After all, a writable doesn't even guarantee
contiguous storage.  To some degree, this is what
<span class="code">nsPromiseFlatString</span> is for.  However, this is a readable promise
only.  It will also be the case that <span class="code">ns[C]String</span>s, in the very near
future will be able to just assume ownership of an arbitrary buffer
allocated on the free store with the XPCOM allocators ... getting one
to give up its buffer, on the other hand, presents some problems.  Do
you have a lot of places where the system writes into your string
buffer space?  Or do you have a lot of system routines that return you
new buffers?  I can imagine using <span class="code">nsPromiseFlatString</span> for this, but
what happens when the OS alters the underlying data?  If the promise
had generated that flat data on behalf of a multi-fragment string,
should it now put the changes back?  It's possible to do, I just want
to know if it's correct to allow this situation to happen.



> 
> 3) There needs to be a NS_LITERAL_CHAR() to go along with
> NS_LITERAL_STRING().

<p>OK.



> Having NS_LITERAL_STRING() all over the code  clutters
> it up and makes it hard to tell what the code is doing, could we
> have a standard short alias for this?

<p>Yes, I'll try to think of something ... perhaps <span class="code">NS_LSTR</span>?


> 4) nsLiteralString should support n.ToInteger(&error);

<p><span class="code">ToInteger</span> is actually a bad interface.  It's only good if your
entire string is the number; this encourages you to edit your string
until it is one, or perhaps copy the numeric part to another string. 
Better if you just <span class="code">sscanf</span> a string (don't know if I can provide
that in the general case, but I'm thinking about it), or else use
regular C++ extractors (which wouldn't be too hard for me to
provide), or else I could give you a <span class="code">ToInteger</span> that works on a pair
of iterators, extracting the integer from the digits between them. 

> 
> 5) There should be a global define for an interface to a readonly
> empty string.

<p>Yes, there will be.


> 
> 6) Something is wrong with concatenation....

<p>Hopefully I've fixed this now.



> 8) A forward definition is missing in the h files

<p>I'll check it out.



<p>My understanding is that you have already found the answers to your
other questions.

<p>I hope this helps,




<hr>
<pre>
Date: Wed, 20 Sep 2000 17:32:13 -0400
Subject: Re: how to free an nsString::ToNewCString
</pre>

<pre class="email-quote">
  >What's the current approved way to free an nsString::ToNewCString? 
</pre>

<p><span class="code">nsMemory::Free</span>





<hr>

<p>You use several <span class="code">NS_ConvertASCIItoUTF16("...").get()</span>, these should be

  NS_LITERAL_STRING("...").get()

<p>Don't do this to the very first case where you aren't wrapping an actual literal string.
The first instance would should exploit <span class="code">NS_LITERAL_STRING</span> technology as well,
around the initial declarations of the strings ... probably want to do this with
<span class="code">NS_NAMED_LITERAL_STRING</span>.



<hr>
<pre>
Date: Thu, 12 Oct 2000 00:57:28 -0400
Subject: string answers
</pre>

<div class="source-code">
<pre>
nsresult
DoSomething( nsAWritableString&amp;  answer )
  {
    nsresult rv;

    nsXPIDLString registry_data;
    Fetch("key", getter_Shares(registry_data));

    nsLiteralString path(not_my_string);

    PRInt32 first_colon = path.FindChar(PRUnichar(':'));
    if ( first_colon != -1 )
      {
        // convert ... extract path from |path|
        nsCOMPtr<nsILocalFile> localFile( do_CreateInstance(CID, &rv)
);
        if ( localFile )
          {
           
localFile->SetPersistentDescriptor(NS_ConvertUTF16toUTF8(path));

            nsXPIDLString converted_path;
            localFile->GetUnicodePath(getter_Copies(converted_path));
            answer = converted_path.get();
          }
      }
    else
      {
        answer = path;
      }


    return rv;
  }
</pre>
</div>





<hr>
<pre>
Date: Thu, 12 Oct 2000 02:03:49 -0400
Subject: Re: and the answer is ...
</pre>

<p>You can see from the line of code that you're on, that this should
have been fine.  <span class="code">nsMemory::Alloc</span> would be asked to allocate a 1 byte
object.  But it failed trying to allocate that.  Which suggests that
the allocator was busy and non-reentrant and the debugger tried to
misuse it.  Yes?

<p>Of course, this doesn't solve your problem.  Perhaps we need to go
back to the idea of a function that returns a pointer to the first
hunk of the string.

<div class="source-code">
<pre>
const char*
debug_string( const nsAReadableCString& aCString )
  {
    nsReadingIterator&lt;char&gt; iter;
    aCString.BeginReading(iter);
    return aCString.IsEmpty() ? "" : iter.get();
  }
</pre>
</div>

<p>This code should work regardless of what the allocator is doing.  The
downsides are (a) it only returns the first hunk of the string, in the
case of a multi-fragment string; and (b) that hunk <strong>might</strong> not be
zero-terminated.

<p>Hope this helps,





<hr>
<pre>
Date: Thu, 12 Oct 2000 08:30:32 -0400
Subject: Re: Self healing the cache :-)
</pre>

<p>At 3:04 PM -0400 10/11/00, Mike Shaver wrote:
<pre class="email-quote">
  >NS_LITERAL_STRING(NS_XPCOM_SHUTDOWN_OBSERVER_ID);
</pre>

<p>Macro ugliness makes <span class="code">NS_LITERAL_STRING</span> inappropriate for use over
other macros.  In other words:

<div class="source-code">
<pre>
NS_LITERAL_STRING("foo")
</pre>
</div>

<p>is <strong>good</strong>.

<div class="source-code">
<pre>
#define FOO "foo"
NS_LITERAL_STRING(FOO)
</pre>
</div>

<p>is <strong>bad</strong>.  Why?  Because it turns into

<div class="source-code">
<pre>
nsLiteralString(LFOO, sizeof(LFOO)...
</pre>
</div>

<p>and there is no <span class="code">LFOO</span>.  Sorry.  If you have to do this to a
macro-ized string, do the magic by hand, e.g.,

<div class="source-code">
<pre>
nsLiteralString(FOO, sizeof(FOO)/sizeof(PRUnichar)
                                          + sizeof(PRUnichar('\0')))
</pre>
</div>

<p>or else if you don't care that <span class="code">nsLiteralString</span> will scan for the
length, just say

<div class="source-code">
<pre>
nsLiteralString(FOO)
</pre>
</div>

<p>Hope this helps,





<hr>
<pre>
Date: Thu, 12 Oct 2000 08:36:14 -0400
Subject: Re: Self healing the cache :-)
</pre>

<p>Actually, I'm not even sure you can do it by hand, since you didn't

<div class="source-code">
<pre>
#define FOO L"foo"
</pre>
</div>

<p>and <strong>can't</strong> do that cross-platform.  The other way around this is to
define a global instead of a macro, that is, instead of saying

<div class="source-code">
<pre>
#define FOO "foo"
</pre>
</div>

<p>at the top of your file, say

<div class="source-code">
<pre>
NS_NAMED_LITERAL_STRING(FOO, "foo")
</pre>
</div>

<p>or else, if the macro was used only in one spot ... perhaps you could
just eliminate the macro in favor of <span class="code">NS_NAMED_LITERAL</span> in situ.

<p>Arghh.  In this case, you may be stuck with the extra work of
<span class="code">AssignWithConversion</span>.





<hr>
<pre>
Date: Sun, 3 Dec 2000 16:38:07 -0400
Subject: Re: another copy_string question
</pre>

<pre class="email-quote">
  >Is there a way to tell, inside the write() sink, if one is in the
  >final hunk?  I need to do some special processing at the end.
</pre>

<p>No, there isn't.  But you could move such special processing into the
destructor of the sink.  Remember, the sink is passed by reference, so
you can exactly control its lifetime.

<div class="source-code">
<pre>
{
  MySink sink;
  nsReadingIterator&lt;PRUnichar&gt; sourceStart = aStr.BeginReading();
  nsReadingIterator&lt;PRUnichar&gt; sourceEnd = aStr.EndReading();
  copy_string(sourceStart, sourceEnd, sink);
    // |sink| destructor executed here
}
</pre>
</div>

<p>Hope this helps,





<hr>
<pre>
Date: Fri, 15 Dec 2000 20:02:08 -0400
Subject: fragment of code
</pre>

<div class="source-code">
<pre>
nsPromiseFlatString flatKey(aReadable);

flatKey.get()
</pre>
</div>






<hr>
<pre>
Date: Tue, 16 Jan 2001 16:47:37 -0400
Subject: Re: a few string questions...
</pre>

>I've accumulated a few questions I've been wanting to ask you, mostly
>about string stuff.  Nothing urgent, but I want to ask them before I
>forget.  So here goes...:
>
>1) Is it acceptable to use nsLiteralCString or nsLiteralString on 
>something that's not a literal?  This can be useful in some places,
>for example, to convert a char* to PRUnichar*:
>
>PRUnichar* new = ToNewUnicode(nsLiteralCString(myCharPtr));

<p>This is explicitly allowed.  That's why I'm proposing to change the
names of those classes to <span class="code">nsLocal[C]String</span>.


>2) Should nsString2x.h and nsString2x.cpp go away?  They look like a
>never-completed rewrite or something...

<p>Yes.  They should go away.  They are uncompleted [old] bullshit,
exactly as you diagnosed.

<p>I'll look into the other two questions.





<hr>
<pre>
Date: Thu, 1 Feb 2001 15:12:41 -0400
Subject: Re: [Fwd: bad string, bad string]
</pre>

<p>We've been removing implicit conversion operators because they
_always_ lead to trouble.  Usually they make it harder to pick the
right function when overloading is involved and in the past they have
led to huge performance suckage because we ended up doing conversions
when we didn't need to because the implicit operator made us pick the
wrong function.

<p>It's borderline when the class implements something that is <strong>so</strong>
close, as with a guaranteed flat string or an <span class="code">nsCOMPtr</span> ... but the
general recommendation is to avoid implicit conversions.

<p>See bug #53057.





<hr>
<pre>
Date: Tue, 6 Feb 2001 18:52:23 -0400
Subject: seeking review for bug #57087
</pre>

<p>  bug:
    <a class="exact-uri" href="http://bugzilla.mozilla.org/show_bug.cgi?id=57087">http://bugzilla.mozilla.org/show_bug.cgi?id=57087</a>

  patch:
    <a class="exact-uri" href="http://bugzilla.mozilla.org/showattachment.cgi?attach_id=24576">http://bugzilla.mozilla.org/showattachment.cgi?attach_id=24576</a>

<p>This patch is supposed to add the ability to define very long literal
strings more easily by breaking lines, e.g.,

<div class="source-code">
<pre>
NS_MULTILINE_LITERAL( NS_L("This is the start of a very long line")
                      NS_L(" which actually continues across")
                      NS_L(" a couple more.") )
</pre>
</div>

<p>The main danger in this scheme is callers who omit the inner <span class="code">NS_L</span>
wrapping.  Though I believe this will be caught at compile time as the
wrong type initializer.

<p>Seeking input from everybody, and waterson in particular.





<hr>
<pre>
Date: Wed, 14 Feb 2001 16:09:10 -0400
Subject: Re: Question...
</pre>

<p>There are some utilities in "xpcom/ds/nsReadableUtils.h".  In
particular, if you want to get back a new heap-allocated ASCII string
with the minimal work, you would say

<div class="source-code">
<pre>
PRUnichar* sourceChars = ...;

char* destChars = ToNewCString(nsLiteralString(sourceChars));
</pre>
</div>


<p>It's more efficient if you happen to already know the length.  If you
don't, don't bother counting, that's what I'll do in the constructor
for <span class="code">nsLiteralString</span>.  If you do, then call like this

<div class="source-code">
<pre>
destChars = ToNewCString( nsLiteralString(sourceChars, length) );
</pre>
</div>

<p>Other routines in that file will help you if, for instance, you wanted
to translate into a buffer you had already allocated.

<p>Hope this helps,





<hr>
<pre>
Date: Fri, 23 Feb 2001 03:12:58 -0400
Subject: string snippet
</pre>

<div class="source-code">
<pre>
nsCString aInput;



nsReadingIterator&lt;char&gt; search_start;
aInput.BeginReading(search_start);

nsReadingIterator&lt;char&gt; search_end;
aInput.EndReading(search_end);

if ( FindCharInReadable(':', search_start, search_end) )
  {
    ++search_start;
    return ToNewCString( Substring(aInput, search_start, search_end)
);
  }
</pre>
</div>






<hr>
<pre>
Date: Wed, 7 Mar 2001 19:44:08 -0400
Subject: string help
</pre>

<p>Here you go, Mike:

  http://scottcollins.net/journal/discussion/mjudge-scratch.cpp






<hr>
<pre>
Date: Fri, 9 Mar 2001 20:56:07 -0400
Subject: Re: string assertions
</pre>

<p>If you get an iterator into a string and you advance it all the way to
the end of the string, and then <strong>keep</strong> trying to advance it, you hit
this assert.  This could happen, for example if you tried to copy 10
characters out of a 9 character string.  I've tried to make this
impossible to get to.  As far as I know, all my routines trim requests
in advance of manipulating iterators.  When you see this, you should
get the stack.  That will take you right to the bad spot.





<hr>
<pre>
Date: Sat, 31 Mar 2001 11:04:03 -0400
Subject: Re: Sun bustage and string advice
</pre>

<p>You do know you are comparing two pointers now?  It seems unlikely
those two pointers would ever be the same pointer.  You probably want
to say something like

<div class="source-code">
<pre>
NS_LITERAL_STRING("foo").Equals(aTopic) // or

NS_LITERAL_STRING("foo") == nsLiteralString(aTopic)
</pre>
</div>

<p>...so that you compare the <strong>contents</strong> of two strings.  Right now,
you're just testing to see if two pointers both point to the same
location in memory.  A lot of people make this mistake.  I would like
to make it obvious to people that comparing two pointers does not
compare strings.  Can you tell me what gave you that impression so
that I can figure out how to better educate people not to do this?  By
the way, it's not that I don't <strong>want</strong> to make this compare two
strings; it's that in C++, you can't override operations for built-in
types.  And pointers are built-in types.  So I can't make
<span class="code">operator==(const PRUnichar*, const PRUnichar*)</span> do anything different
than it already does, which is the same thing it does for any other
pointer.






</div>



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