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| 24 | </div> |
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| 25 | <div class="section" lang="en"> |
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| 26 | <div class="titlepage"><div><div><h3 class="title"> |
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| 27 | <a name="id2713384"></a>Relation to other Boost libraries</h3></div></div></div> |
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| 28 | <div class="toc"><dl> |
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| 29 | <dt><span class="section"><a href="s08.html#id2713388">Boost Function</a></span></dt> |
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| 30 | <dt><span class="section"><a href="s08.html#id2713474">Boost Bind</a></span></dt> |
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| 31 | </dl></div> |
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| 32 | <div class="section" lang="en"> |
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| 33 | <div class="titlepage"><div><div><h4 class="title"> |
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| 34 | <a name="id2713388"></a>Boost Function</h4></div></div></div> |
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| 35 | <p>Sometimes it is convenient to store lambda functors in variables. |
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| 36 | 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. |
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| 37 | <span class="emphasis"><em>The Boost Function library</em></span>[<a href="../lambda.html#cit:boost::function" title="[function]"><span class="abbrev">function</span></a>] defines wrappers for arbitrary function objects, for example |
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| 38 | lambda functors; and these wrappers have types that are easy to type out. |
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| 39 | |
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| 40 | For example: |
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| 41 | |
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| 42 | </p> |
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| 43 | <pre class="programlisting"> |
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| 44 | boost::function<int(int, int)> f = _1 + _2; |
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| 45 | boost::function<int&(int&)> g = (_1 += 10); |
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| 46 | int i = 1, j = 2; |
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| 47 | f(i, j); // returns 3 |
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| 48 | g(i); // sets i to = 11; |
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| 49 | </pre> |
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| 50 | <p> |
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| 51 | |
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| 52 | 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. |
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| 53 | 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. |
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| 54 | |
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| 55 | Note that storing lambda functors inside <code class="literal">boost::function</code> |
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| 56 | introduces a danger. |
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| 57 | Certain types of lambda functors may store references to the bound |
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| 58 | arguments, instead as taking copies of the arguments of the lambda expression. |
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| 59 | When temporary lambda functor objects are used |
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| 60 | in STL algorithm invocations this is always safe, as the lambda functor gets |
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| 61 | destructed immediately after the STL algortihm invocation is completed. |
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| 62 | |
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| 63 | However, a lambda functor wrapped inside <code class="literal">boost::function</code> |
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| 64 | may continue to exist longer, creating the possibility of dangling references. |
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| 65 | For example: |
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| 66 | |
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| 67 | </p> |
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| 68 | <pre class="programlisting"> |
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| 69 | int* sum = new int(); |
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| 70 | *sum = 0; |
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| 71 | boost::function<int&(int)> counter = *sum += _1; |
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| 72 | counter(5); // ok, *sum = 5; |
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| 73 | delete sum; |
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| 74 | counter(3); // error, *sum does not exist anymore |
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| 75 | </pre> |
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| 76 | </div> |
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| 77 | <div class="section" lang="en"> |
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| 78 | <div class="titlepage"><div><div><h4 class="title"> |
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| 79 | <a name="id2713474"></a>Boost Bind</h4></div></div></div> |
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| 80 | <div class="toc"><dl><dt><span class="section"><a href="s08.html#id2713521">First argument of bind expression</a></span></dt></dl></div> |
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| 81 | <p><span class="emphasis"><em>The Boost Bind</em></span>[<a href="../lambda.html#cit:boost::bind" title="[bind]"><span class="abbrev">bind</span></a>] library has partially overlapping functionality with the BLL. |
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| 82 | Basically, the Boost Bind library (BB in the sequel) implements the bind expression part of BLL. |
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| 83 | There are, however, some semantical differerences. |
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| 84 | </p> |
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| 85 | <p> |
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| 86 | The BLL and BB evolved separately, and have different implementations. |
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| 87 | This means that the bind expressions from the BB cannot be used within |
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| 88 | bind expressions, or within other type of lambda expressions, of the BLL. |
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| 89 | The same holds for using BLL bind expressions in the BB. |
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| 90 | The libraries can coexist, however, as |
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| 91 | the names of the BB library are in <code class="literal">boost</code> namespace, |
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| 92 | whereas the BLL names are in <code class="literal">boost::lambda</code> namespace. |
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| 93 | </p> |
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| 94 | <p> |
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| 95 | The BLL requires a compiler that is reasonably conformant to the |
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| 96 | C++ standard, whereas the BB library is more portable, and works with |
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| 97 | a larger set of compilers. |
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| 98 | </p> |
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| 99 | <p> |
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| 100 | The following two sections describe what are the semantic differences |
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| 101 | between the bind expressions in BB and BLL. |
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| 102 | </p> |
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| 103 | <div class="section" lang="en"> |
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| 104 | <div class="titlepage"><div><div><h5 class="title"> |
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| 105 | <a name="id2713521"></a>First argument of bind expression</h5></div></div></div> |
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| 106 | |
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| 107 | In BB the first argument of the bind expression, the target function, |
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| 108 | is treated differently from the other arguments, |
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| 109 | as no argument substitution takes place within that argument. |
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| 110 | In BLL the first argument is not a special case in this respect. |
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| 111 | |
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| 112 | For example: |
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| 113 | |
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| 114 | <pre class="programlisting"> |
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| 115 | template<class F> |
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| 116 | int foo(const F& f) { |
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| 117 | int x; |
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| 118 | .. |
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| 119 | bind(f, _1)(x); |
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| 120 | ... |
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| 121 | } |
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| 122 | </pre> |
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| 123 | <pre class="programlisting"> |
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| 124 | int bar(int, int); |
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| 125 | nested(bind(bar, 1, _1)); |
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| 126 | </pre> |
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| 127 | |
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| 128 | The bind expression inside <code class="literal">foo</code> becomes: |
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| 129 | <pre class="programlisting"> |
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| 130 | bind(bind(bar, 1, _1), _1)(x) |
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| 131 | </pre> |
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| 132 | |
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| 133 | The BLL interpretes this as: |
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| 134 | <pre class="programlisting"> |
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| 135 | bar(1, x)(x) |
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| 136 | </pre> |
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| 137 | whereas the BB library as |
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| 138 | <pre class="programlisting"> |
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| 139 | bar(1, x) |
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| 140 | </pre> |
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| 141 | |
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| 142 | To get this functionality in BLL, the bind expression inside the <code class="literal">foo</code> function can be written as: |
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| 143 | <pre class="programlisting"> |
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| 144 | bind(unlambda(f), _1)(x); |
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| 145 | </pre> |
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| 146 | as explained in <a href="le_in_details.html#lambda.unlambda" title="Unlambda">the section called “Unlambda”</a>. |
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| 147 | |
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| 148 | </div> |
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| 149 | <p> |
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| 150 | The BB library supports up to nine placeholders, while the BLL |
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| 151 | defines only three placeholders. |
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| 152 | The rationale for not providing more, is that the highest arity of the |
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| 153 | function objects accepted by any STL algorithm is two. |
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| 154 | The placeholder count is easy to increase in the BB library. |
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| 155 | In BLL it is possible, but more laborous. |
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| 156 | The BLL currently passes the actual arguments to the lambda functors |
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| 157 | internally just as they are and does not wrap them inside a tuple object. |
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| 158 | The reason for this is that some widely used compilers are not capable |
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| 159 | of optimizing the intermediate tuple objects away. |
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| 160 | The creation of the intermediate tuples would cause a significant |
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| 161 | performance hit, particularly for the simplest (and thus the most common) |
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| 162 | lambda functors. |
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| 163 | We are working on a hybrid approach, which will allow more placeholders |
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| 164 | but not compromise the performance of simple lambda functors. |
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| 165 | </p> |
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| 166 | </div> |
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| 167 | </div> |
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| 168 | <table width="100%"><tr> |
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| 169 | <td align="left"></td> |
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| 170 | <td align="right"><small>Copyright © 1999-2004 Jaakko Järvi, Gary Powell</small></td> |
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