1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2 3 <html> 4 <head> 5 <meta http-equiv="Content-Language" content="en-us"> 6 <meta http-equiv="Content-Type" content="text/html; charset=us-ascii"> 7 8 <title>Boost Function Object Adapter Library</title> 9 </head> 10 11 <body bgcolor="#FFFFFF" text="#000000"> 12 <table border="1" bgcolor="#007F7F" cellpadding="2" summary=""> 13 <tr> 14 <td bgcolor="#FFFFFF"><img src="../../boost.png" alt= 15 "boost.png (6897 bytes)" width="277" height="86"></td> 16 17 <td><a href="../../index.htm"><font face="Arial" color= 18 "#FFFFFF"><big>Home</big></font></a></td> 19 20 <td><a href="../libraries.htm"><font face="Arial" color= 21 "#FFFFFF"><big>Libraries</big></font></a></td> 22 23 <td><a href="http://www.boost.org/people/people.htm"><font face="Arial" color= 24 "#FFFFFF"><big>People</big></font></a></td> 25 26 <td><a href="http://www.boost.org/more/faq.htm"><font face="Arial" color= 27 "#FFFFFF"><big>FAQ</big></font></a></td> 28 29 <td><a href="../../more/index.htm"><font face="Arial" color= 30 "#FFFFFF"><big>More</big></font></a></td> 31 </tr> 32 </table> 33 34 <h1>Function Object Traits</h1> 35 36 <p>The header <a href="../../boost/functional.hpp">functional.hpp</a> 37 provides two traits class templates for functions and function objects:</p> 38 39 <table border="1" summary=""> 40 <tr> 41 <th>Type</th> 42 43 <th>Contents</th> 44 45 <th>Description</th> 46 </tr> 47 48 <tr> 49 <td valign="top" rowspan="4"> 50 <tt>template <typename T><br> 51 struct unary_traits</tt></td> 52 53 <td valign="top"><tt>function_type</tt></td> 54 55 <td valign="top">The type of the function or function object itself 56 (i.e., <tt>T</tt>).</td> 57 </tr> 58 59 <tr> 60 <td valign="top"><tt>param_type</tt></td> 61 62 <td valign="top">The type that should be used to pass the function or 63 function object as a parameter.</td> 64 </tr> 65 66 <tr> 67 <td valign="top"><tt>result_type</tt></td> 68 69 <td valign="top">The type returned by the function or function 70 object.</td> 71 </tr> 72 73 <tr> 74 <td valign="top"><tt>argument_type</tt></td> 75 76 <td valign="top">The type of the argument to the function or function 77 object.</td> 78 </tr> 79 80 <tr> 81 <td valign="top" rowspan="5"> 82 <tt>template <typename T><br> 83 struct binary_traits</tt></td> 84 85 <td valign="top"><tt>function_type</tt></td> 86 87 <td valign="top">The type of the function or function object itself 88 (i.e., <tt>T</tt>).</td> 89 </tr> 90 91 <tr> 92 <td valign="top"><tt>param_type</tt></td> 93 94 <td valign="top">The type that should be used to pass the function or 95 function object as a parameter.</td> 96 </tr> 97 98 <tr> 99 <td valign="top"><tt>result_type</tt></td> 100 101 <td valign="top">The type returned by the function or function 102 object.</td> 103 </tr> 104 105 <tr> 106 <td valign="top"><tt>first_argument_type</tt></td> 107 108 <td valign="top">The type of the first argument to the function or 109 function object.</td> 110 </tr> 111 112 <tr> 113 <td valign="top"><tt>second_argument_type</tt></td> 114 115 <td valign="top">The type of the second argument to the function or 116 function object.</td> 117 </tr> 118 </table> 119 120 <h3>Usage</h3> 121 122 <p><tt>unary_traits</tt> should be instantiated with either a function 123 taking a single parameter, or an adaptable unary function object (i.e., a 124 class derived from <tt>std::unary_function</tt> or one which provides the 125 same typedefs). (See §20.3.1 in the C++ Standard.)</p> 126 127 <p><tt>binary_traits</tt> should be instantiated with either a function 128 taking two parameters, or an adaptable binary function object (i.e., a 129 class derived from <tt>std::binary_function</tt> or one which provides the 130 same typedefs). (See §20.3.1 in the C++ Standard.)</p> 131 132 <p>The most common usage of these templates is in function object adapters, 133 thus allowing them to adapt plain functions as well as function objects. 134 You can do this by wherever you would normally write, for example,</p> 135 136 <blockquote> 137 <pre> 138 typename Operation::argument_type 139 </pre> 140 </blockquote> 141 142 <p>simply writing</p> 143 144 <blockquote> 145 <pre> 146 typename boost::unary_traits<Operation>::argument_type 147 </pre> 148 </blockquote> 149 150 <p>instead.</p> 151 152 <h3>Additional Types Defined</h3> 153 154 <p>In addition to the standard result and argument typedefs, these traits 155 templates define two additional types.</p> 156 157 <h4><tt>function_type</tt></h4> 158 159 <p>This is the type of the function or function object, and can be used in 160 declarations such as</p> 161 162 <blockquote> 163 <pre> 164 template <class Predicate> 165 class unary_negate : // ... 166 { 167 // ... 168 private: 169 <strong>typename unary_traits<Predicate>::function_type</strong> pred; 170 }; 171 </pre> 172 </blockquote> 173 174 <p>If this typedef were not provided, it would not be possible to declare 175 <tt>pred</tt> in a way that would allow <tt>unary_negate</tt> to be 176 instantiated with a function type (see the C++ Standard §14.3.1 177 ¶3).</p> 178 179 <h4><tt>param_type</tt></h4> 180 181 <p>This is a type suitable for passing the function or function object as a 182 parameter to another function. For example,</p> 183 184 <blockquote> 185 <pre> 186 template <class Predicate> 187 class unary_negate : // ... 188 { 189 public: 190 explicit unary_negate(<strong>typename unary_traits<Predicate>::param_type</strong> x) 191 : 192 pred(x) 193 {} 194 // ... 195 }; 196 </pre> 197 </blockquote> 198 199 <p>Function objects are passed by reference to const; function pointers are 200 passed by value.</p> 201 202 <h3>Limitations</h3> 203 204 <p>This library uses these traits within all function object adapters, 205 theoretically rendering <tt>ptr_fun</tt> obsolete. However, third party 206 adapters probably won't take advantage of this mechanism, and so 207 <tt>ptr_fun</tt> may still be required. Accordingly, this library also 208 provides <a href="ptr_fun.html">improved versions of the standard function 209 pointer adapters</a>.</p> 210 211 <p>These traits templates will also not work with compilers that fail to 212 support partial specialisation of templates. With these compilers, the 213 traits templates can only be instantiated with adaptable function objects, 214 thus requiring <tt>ptr_fun</tt> to be used, even with the function object 215 adapters in this library.</p> 216 <hr> 217 218 <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src= 219 "../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional" 220 height="31" width="88"></a></p> 221 222 <p>Revised 223 <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02 224 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p> 225 226 <p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p> 227 228 <p><i>Distributed under the Boost Software License, Version 1.0. (See 229 accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or 230 copy at <a href= 231 "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p> 232 </body> 233 </html> 234
