feedc0de/boost_bind
1
0
Fork 0
forked from boostorg/bind
Code Pull Requests Activity

Fix #1642.

Browse Source 
[SVN r43321]
This commit is contained in:
Peter Dimov
2008-02-19 15:09:10 +00:00
parent ef993c777a
commit aef08dd0cd
1 changed files with 21 additions and 18 deletions
Download Patch File Download Diff File Expand all files Collapse all files

39
bind.html
View File

@@ -307,21 +307,22 @@ std::for_each(v.begin(), v.end(), bind(apply<void>(), _1, 5));
<h3><a name="operators">Overloaded operators</a> (new in Boost 1.33)</h3>
<p>For convenience, the function objects produced by <tt>bind</tt> overload the
logical not operator <code>!</code> and the relational and logical operators <code>==</code>,
<code>!=</code>, <code>&lt;</code>, <code>&lt;=</code>, <code>&gt;</code>,
<code>&gt;=</code>, <code>&amp;&amp;</code>, <code>||</code>.</p>
<code>!=</code>, <code>&lt;</code>, <code>&lt;=</code>, <code>&gt;</code>, <code>&gt;=</code>,
<code>&amp;&amp;</code>, <code>||</code>.</p>
<P><tt>!bind(f, ...)</tt> is equivalent to <tt>bind( <EM>logical_not</EM>(), bind(f,
...) )</tt>, where <tt><EM>logical_not</EM></tt> is a function object that
takes one argument <tt>x</tt> and returns <tt>!x</tt>.</P>
<P><tt>bind(f, ...) <EM>op</EM> x</tt>, where <EM>op</EM> is a relational or logical operator,
is equivalent to <tt>bind( <EM>relation</EM>(), bind(f, ...), x )</tt>, where <em>relation</em>
is a function object that takes two arguments <tt>a</tt> and <tt>b</tt> and
returns <tt>a <EM>op</EM> b</tt>.</P>
<P><tt>bind(f, ...) <EM>op</EM> x</tt>, where <EM>op</EM> is a relational or
logical operator, is equivalent to <tt>bind( <EM>relation</EM>(), bind(f, ...), x )</tt>,
where <em>relation</em> is a function object that takes two arguments <tt>a</tt>
and <tt>b</tt> and returns <tt>a <EM>op</EM> b</tt>.</P>
<P>What this means in practice is that you can conveniently negate the result of <tt>bind</tt>:</P>
<P><tt>std::remove_if( first, last, !bind( &amp;X::visible, _1 ) ); // remove invisible
objects</tt></P>
<P>and compare the result of <tt>bind</tt> against a value:</P>
<P><tt>std::find_if( first, last, bind( &amp;X::name, _1 ) == "Peter" );</tt></P>
<P><tt>std::find_if( first, last, bind( &amp;X::name, _1 ) == "Peter" || bind( &amp;X::name, _1 ) == "Paul" );</tt></P>
<P><tt>std::find_if( first, last, bind( &amp;X::name, _1 ) == "Peter" || bind(
&amp;X::name, _1 ) == "Paul" );</tt></P>
<P>against a placeholder:</P>
<P><tt>bind( &amp;X::name, _1 ) == _2</tt></P>
<P>or against another <tt>bind</tt> expression:</P>
@@ -384,10 +385,12 @@ void connect()
}
</pre>
<h2><a name="Limitations">Limitations</a></h2>
<p>The function objects generated by <b>bind</b> take their arguments by reference
and cannot, therefore, accept non-const temporaries or literal constants. This
is an inherent limitation of the C++ language, known as <A href="http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1385.htm">
the forwarding problem</A>.</p>
<p>As a general rule, the function objects generated by <b>bind</b> take their
arguments by reference and cannot, therefore, accept non-const temporaries or
literal constants. This is an inherent limitation of the C++ language in its
current (2003) incarnation, known as <A href="http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1385.htm">
the forwarding problem</A>. (It will be fixed in the next standard, usually
called C++0x.)</p>
<p>The library uses signatures of the form
</p>
<pre>template&lt;class T&gt; void f(T &amp; t);
@@ -395,17 +398,17 @@ void connect()
<p>to accept arguments of arbitrary types and pass them on unmodified. As noted,
this does not work with non-const r-values.
</p>
<p>An oft-proposed "solution" to this problem is to add an overload:
<p>On compilers that support partial ordering of function templates, a possible
solution is to add an overload:
</p>
<pre>template&lt;class T&gt; void f(T &amp; t);
template&lt;class T&gt; void f(T const &amp; t);
</pre>
<p>Unfortunately, this (a) requires providing 512 overloads for nine arguments and
(b) does not actually work for const arguments, both l- and r-values, since the
two templates produce the exact same signature and cannot be partially ordered.
</p>
<p>[Note: this is a dark corner of the language, and the <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#214">
corresponding issue</a> has only recently been resolved.]
<p>Unfortunately, this requires providing 512 overloads for nine arguments, which
is impractical. The library chooses a small subset: for up to two arguments, it
provides the const overloads in full, for arities of three and more it provides
a single additional overload with all of the arguments taken by const
reference. This covers a reasonable portion of the use cases.
</p>
<h2><a name="FAQ">Frequently Asked Questions</a></h2>
<h3><a name="Q_doesnt_compile">Why doesn't this compile?</a></h3>