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<h1>Binders</h1>
<p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
provides enhanced versions of both the binder function object adapters from
the C++ Standard Library (§20.3.6):</p>
<ul>
<li><tt>binder1st</tt></li>
<li><tt>binder2nd</tt></li>
</ul>
<p>As well as the corresponding helper functions</p>
<ul>
<li><tt>bind1st</tt></li>
<li><tt>bind2nd</tt></li>
</ul>
<p>The key benefit of these adapters over those in the Standard Library is
they avoid the problem of <a href="#refref">references to
references.</a></p>
<h3>Usage</h3>
<p>Usage is identical to the standard binders. For example,</p>
<blockquote>
<pre>
class Foo {
public:
void bar(std::ostream &);
// ...
};
// ...
std::vector<Foo> c;
// ...
std::for_each(c.begin(), c.end(),
boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout));
</pre>
</blockquote>
<h3 id="refref">References to References</h3>
<p>Consider the usage example above</p>
<blockquote>
<pre>
class Foo {
public:
void bar(<strong>std::ostream &</strong>);
// ...
};
// ...
std::for_each(c.begin(), c.end(),
boost::bind2nd(boost::mem_fun_ref(&Foo::bar), std::cout));
</pre>
</blockquote>
<p>If this had been written using <tt>std::bind2nd</tt> and
<tt>std::mem_fun_ref</tt>, it would be unlikely to compile.</p>
<p>The problem arises because <tt>bar</tt> takes a reference argument. The
Standard defines <tt>std::mem_fun_ref</tt> such that it creates a function
object whose <tt>second_argument_type</tt> will be
<tt>std::ostream&</tt>.</p>
<p>The call to <tt>bind2nd</tt> creates a <tt>binder2nd</tt> which the
Standard defines as follows:</p>
<blockquote>
<pre>
template <class Operation>
class binder2nd
: public unary_function<typename Operation::first_argument_type,
typename Operation::result_type> {
...
public:
binder2nd(const Operation& x,
<strong>const typename Operation::second_argument_type& y</strong>);
...
</pre>
</blockquote>
<p>Since our operation's <tt>second_argument_type</tt> is
<tt>std::ostream&</tt>, the type of <tt>y</tt> in the constructor would
be <tt>std::ostream&&</tt>. Since you cannot have a reference to a
reference, at this point we should get a compilation error because
references to references are illegal in C++ (but see <a href=
"http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++
Standard core language active issues list</a>).</p>
<p>The binders in this library avoid this problem by using the Boost
<tt><a href="../utility/call_traits.htm">call_traits</a></tt>
templates.</p>
<p>Our constructor is declared</p>
<blockquote>
<pre>
binder2nd(const Operation& x,
<strong>typename call_traits<
typename binary_traits<Operation>::second_argument_type
>::param_type y</strong>)
</pre>
</blockquote>
<p>As a result, <tt>y</tt> has a type of <tt>std::ostream&</tt>, and
our example compiles.</p>
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<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02
December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p>
<p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p>
<p><i>Distributed under the Boost Software License, Version 1.0. (See
accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
copy at <a href=
"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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