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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<int(int, int)> f = _1 + _2; |
| boost::function<int&(int&)> 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<int&(int)> 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<class F> |
| int foo(const F& 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 “Unlambda”</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> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 1999-2004 Jaakko Järvi, 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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