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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id1999132"></a>Relation to other Boost libraries</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="s08.html#id1999138">Boost Function</a></span></dt>
<dt><span class="section"><a href="s08.html#id1999231">Boost Bind</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id1999138"></a>Boost Function</h3></div></div></div>
<p>Sometimes it is convenient to store lambda functors in variables.
However, the types of even the simplest lambda functors are long and unwieldy, and it is in general unfeasible to declare variables with lambda functor types.
<span class="emphasis"><em>The Boost Function library</em></span> <a class="xref" href="../lambda.html#cit:boost::function" title="Boost Function Library">[<abbr class="abbrev">function</abbr>]</a> defines wrappers for arbitrary function objects, for example 
lambda functors; and these wrappers have types that are easy to type out.

For example:

</p>
<pre class="programlisting">
boost::function&lt;int(int, int)&gt; f = _1 + _2;
boost::function&lt;int&amp;(int&amp;)&gt; g = (_1 += 10);
int i = 1, j = 2;
f(i, j); // returns 3
g(i);    // sets i to = 11;
</pre>
<p>

The return and parameter types of the wrapped function object must be written explicilty as the template argument to the wrapper template <code class="literal">boost::function</code>; even when lambda functors, which otherwise have generic parameters, are wrapped.
Wrapping a function object with <code class="literal">boost::function</code> introduces a performance cost comparable to virtual function dispatch, though virtual functions are not actually used.

Note that storing lambda functors inside <code class="literal">boost::function</code> 
introduces a danger.
Certain types of lambda functors may store references to the bound 
arguments, instead as taking copies of the arguments of the lambda expression.
When temporary lambda functor objects are used 
in STL algorithm invocations this is always safe, as the lambda functor gets 
destructed immediately after the STL algortihm invocation is completed.

However, a lambda functor wrapped inside <code class="literal">boost::function</code> 
may continue to exist longer, creating the possibility of dangling references.
For example:

</p>
<pre class="programlisting">
int* sum = new int();
*sum = 0;
boost::function&lt;int&amp;(int)&gt; counter = *sum += _1;
counter(5); // ok, *sum = 5;
delete sum; 
counter(3); // error, *sum does not exist anymore
</pre>
<p>

</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id1999231"></a>Boost Bind</h3></div></div></div>
<div class="toc"><dl><dt><span class="section"><a href="s08.html#id1999283">First argument of bind expression</a></span></dt></dl></div>
<p>
<span class="emphasis"><em>The Boost Bind</em></span> <a class="xref" href="../lambda.html#cit:boost::bind" title="Boost Bind Library">[<abbr class="abbrev">bind</abbr>]</a> library has partially overlapping functionality with the BLL. 
Basically, the Boost Bind library (BB in the sequel) implements the bind expression part of BLL.
There are, however, some semantical differerences.
</p>
<p>
The BLL and BB evolved separately, and have different implementations. 
This means that the bind expressions from the BB cannot be used within 
bind expressions, or within other type of lambda expressions, of the BLL.
The same holds for using BLL bind expressions in the BB.
The libraries can coexist, however, as
the names of the BB library are in <code class="literal">boost</code> namespace, 
whereas the BLL names are in <code class="literal">boost::lambda</code> namespace.
</p>
<p>
The BLL requires a compiler that is reasonably conformant to the 
C++ standard, whereas the BB library is more portable, and works with 
a larger set of compilers. 
</p>
<p>
The following two sections describe what are the semantic differences 
between the bind expressions in BB and BLL.
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="id1999283"></a>First argument of bind expression</h4></div></div></div>

In BB the first argument of the bind expression, the target function, 
is treated differently from the other arguments, 
as no argument substitution takes place within that argument.
In BLL the first argument is not a special case in this respect.

For example:

<pre class="programlisting">
template&lt;class F&gt;
int foo(const F&amp; f) {
  int x;
  ..
  bind(f, _1)(x);
  ...
}
</pre>
<pre class="programlisting">
int bar(int, int);
nested(bind(bar, 1, _1));
</pre>

The bind expression inside <code class="literal">foo</code> becomes:
<pre class="programlisting">
bind(bind(bar, 1, _1), _1)(x)
</pre>

The BLL interpretes this as:
<pre class="programlisting">
bar(1, x)(x)
</pre>
whereas the BB library as
<pre class="programlisting">
bar(1, x)
</pre>

To get this functionality in BLL, the bind expression inside the <code class="literal">foo</code> function can be written as:
<pre class="programlisting">
bind(unlambda(f), _1)(x);
</pre>
as explained in <a class="xref" href="le_in_details.html#lambda.unlambda" title="Unlambda">the section called &#8220;Unlambda&#8221;</a>. 

</div>
<p>
The BB library supports up to nine placeholders, while the BLL 
defines only three placeholders. 
The rationale for not providing more, is that the highest arity of the
function objects accepted by any STL algorithm is two. 
The placeholder count is easy to increase in the BB library.
In BLL it is possible, but more laborous.
The BLL currently passes the actual arguments to the lambda functors
internally just as they are and does not wrap them inside a tuple object.
The reason for this is that some widely used compilers are not capable
of optimizing the intermediate tuple objects away. 
The creation of the intermediate tuples would cause a significant
performance hit, particularly for the simplest (and thus the most common) 
lambda functors.  
We are working on a hybrid approach, which will allow more placeholders
but not compromise the performance of simple lambda functors.
</p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 1999-2004 Jaakko J&#228;rvi, Gary Powell<p>Use, modification and distribution is subject to the Boost
    Software License, Version 1.0. (See accompanying file
    <code class="filename">LICENSE_1_0.txt</code> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</p>
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