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24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="boost_typetraits.reference.common_type"></a><a class="link" href="common_type.html" title="common_type">common_type</a> | |
28 | </h3></div></div></div> | |
29 | <p> | |
30 | <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span> | |
31 | <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">common_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> | |
32 | or <code class="computeroutput"> <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> | |
33 | </p> | |
34 | <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> | |
35 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special">;</span> | |
36 | <span class="special">}</span> | |
37 | </pre> | |
38 | <p> | |
39 | <code class="computeroutput"><span class="identifier">common_type</span></code> is a traits class | |
40 | used to deduce a type common to a several types, useful as the return type | |
41 | of functions operating on multiple input types such as in mixed-mode arithmetic.. | |
42 | </p> | |
43 | <p> | |
44 | The nested typedef <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code> | |
45 | could be defined as follows: | |
46 | </p> | |
47 | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">T</span><span class="special">></span> | |
48 | <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special">;</span> | |
49 | ||
50 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">,</span> <span class="keyword">class</span><span class="special">...</span> <span class="identifier">V</span><span class="special">></span> | |
51 | <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">,</span> <span class="identifier">V</span><span class="special">...></span> <span class="special">{</span> | |
52 | <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">V</span><span class="special">...>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span> | |
53 | <span class="special">};</span> | |
54 | ||
55 | <span class="keyword">template</span> <span class="special"><></span> | |
56 | <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><></span> <span class="special">{</span> | |
57 | <span class="special">};</span> | |
58 | ||
59 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> | |
60 | <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">{</span> | |
61 | <span class="keyword">typedef</span> <span class="keyword">typename</span> <a class="link" href="decay.html" title="decay">decay</a><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span> | |
62 | <span class="special">};</span> | |
63 | ||
64 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> | |
65 | <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">></span> <span class="special">{</span> | |
66 | <span class="keyword">typedef</span> <span class="keyword">typename</span> <a class="link" href="decay.html" title="decay">decay</a><span class="special"><</span> | |
67 | <span class="keyword">decltype</span><span class="special">(</span> <a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="keyword">bool</span><span class="special">>()?</span> | |
68 | <a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="keyword">typename</span> <a class="link" href="decay.html" title="decay">decay</a><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>():</span> | |
69 | <a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="keyword">typename</span> <a class="link" href="decay.html" title="decay">decay</a><span class="special"><</span><span class="identifier">U</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>()</span> <span class="special">)</span> | |
70 | <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span> | |
71 | <span class="special">};</span> | |
72 | </pre> | |
73 | <p> | |
74 | All parameter types must be complete. This trait is permitted to be specialized | |
75 | by a user if at least one template parameter is a user-defined type. <span class="bold"><strong>Note:</strong></span> Such specializations are required when only | |
76 | explicit conversions are desired among the <code class="computeroutput"><span class="identifier">common_type</span></code> | |
77 | arguments. | |
78 | </p> | |
79 | <p> | |
80 | Note that when the compiler does not support variadic templates (and the | |
81 | macro <code class="computeroutput"><span class="identifier">BOOST_NO_CXX11_VARIADIC_TEMPLATES</span></code> | |
82 | is defined) then the maximum number of template arguments is 9. | |
83 | </p> | |
84 | <h5> | |
85 | <a name="boost_typetraits.reference.common_type.h0"></a> | |
86 | <span class="phrase"><a name="boost_typetraits.reference.common_type.tutorial"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.tutorial">Tutorial</a> | |
87 | </h5> | |
88 | <p> | |
89 | In a nutshell, <code class="computeroutput"><span class="identifier">common_type</span></code> | |
90 | is a trait that takes 1 or more types, and returns a type which all of the | |
91 | types will convert to. The default definition demands this conversion be | |
92 | implicit. However the trait can be specialized for user-defined types which | |
93 | want to limit their inter-type conversions to explicit, and yet still want | |
94 | to interoperate with the <code class="computeroutput"><span class="identifier">common_type</span></code> | |
95 | facility. | |
96 | </p> | |
97 | <p> | |
98 | <span class="bold"><strong>Example:</strong></span> | |
99 | </p> | |
100 | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> | |
101 | <span class="identifier">complex</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span> | |
102 | <span class="keyword">operator</span><span class="special">+(</span><span class="identifier">complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span> <span class="identifier">complex</span><span class="special"><</span><span class="identifier">U</span><span class="special">>);</span> | |
103 | </pre> | |
104 | <p> | |
105 | In the above example, "mixed-mode" complex arithmetic is allowed. | |
106 | The return type is described by <code class="computeroutput"><span class="identifier">common_type</span></code>. | |
107 | For example the resulting type of adding a <code class="computeroutput"><span class="identifier">complex</span><span class="special"><</span><span class="keyword">float</span><span class="special">></span></code> and <code class="computeroutput"><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span></code> might be a <code class="computeroutput"><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span></code>. | |
108 | </p> | |
109 | <p> | |
110 | Here is how someone might produce a variadic comparison function: | |
111 | </p> | |
112 | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="special">...</span><span class="identifier">T</span><span class="special">></span> | |
113 | <span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">...>::</span><span class="identifier">type</span> | |
114 | <span class="identifier">min</span><span class="special">(</span><span class="identifier">T</span><span class="special">...</span> <span class="identifier">t</span><span class="special">);</span> | |
115 | </pre> | |
116 | <p> | |
117 | This is a very useful and broadly applicable utility. | |
118 | </p> | |
119 | <h5> | |
120 | <a name="boost_typetraits.reference.common_type.h1"></a> | |
121 | <span class="phrase"><a name="boost_typetraits.reference.common_type.how_to_get_the_common_type_of_types_with_explicit_conversions_"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.how_to_get_the_common_type_of_types_with_explicit_conversions_">How | |
122 | to get the common type of types with explicit conversions?</a> | |
123 | </h5> | |
124 | <p> | |
125 | Another choice for the author of the preceding operator could be | |
126 | </p> | |
127 | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> | |
128 | <span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span> <span class="identifier">complex</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span> | |
129 | <span class="keyword">operator</span><span class="special">+(</span><span class="identifier">complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span> <span class="identifier">complex</span><span class="special"><</span><span class="identifier">U</span><span class="special">>);</span> | |
130 | </pre> | |
131 | <p> | |
132 | As the default definition of <code class="computeroutput"><span class="identifier">common_type</span></code> | |
133 | demands the conversion be implicit, we need to specialize the trait for complex | |
134 | types as follows. | |
135 | </p> | |
136 | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> | |
137 | <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span> <span class="identifier">complex</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="special">></span> <span class="special">{</span> | |
138 | <span class="keyword">typedef</span> <span class="identifier">complex</span><span class="special"><</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">></span> <span class="special">></span> <span class="identifier">type</span><span class="special">;</span> | |
139 | <span class="special">};</span> | |
140 | </pre> | |
141 | <h5> | |
142 | <a name="boost_typetraits.reference.common_type.h2"></a> | |
143 | <span class="phrase"><a name="boost_typetraits.reference.common_type.how_important_is_the_order_of_the__code__phrase_role__identifier__common_type__phrase__phrase_role__special___lt__gt___phrase___code__template_arguments_"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.how_important_is_the_order_of_the__code__phrase_role__identifier__common_type__phrase__phrase_role__special___lt__gt___phrase___code__template_arguments_">How | |
144 | important is the order of the <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><></span></code> template arguments?</a> | |
145 | </h5> | |
146 | <p> | |
147 | The order of the template parameters is important. | |
148 | </p> | |
149 | <p> | |
150 | <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">,</span><span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span></code> is not equivalent to <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">C</span><span class="special">,</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">type</span></code>, but to <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span></code>. | |
151 | </p> | |
152 | <p> | |
153 | Consider | |
154 | </p> | |
155 | <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{};</span> | |
156 | <span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{};</span> | |
157 | <span class="keyword">struct</span> <span class="identifier">C</span> <span class="special">{</span> | |
158 | <span class="identifier">C</span><span class="special">()</span> <span class="special">{}</span> | |
159 | <span class="identifier">C</span><span class="special">(</span><span class="identifier">A</span> <span class="keyword">const</span><span class="special">&)</span> <span class="special">{}</span> | |
160 | <span class="identifier">C</span><span class="special">(</span><span class="identifier">B</span> <span class="keyword">const</span><span class="special">&)</span> <span class="special">{}</span> | |
161 | <span class="identifier">C</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">C</span> <span class="keyword">const</span><span class="special">&)</span> <span class="special">{</span> | |
162 | <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> | |
163 | <span class="special">}</span> | |
164 | <span class="special">};</span> | |
165 | </pre> | |
166 | <p> | |
167 | The following doesn't compile | |
168 | </p> | |
169 | <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">ABC</span><span class="special">;</span> <span class="comment">// Does not compile</span> | |
170 | </pre> | |
171 | <p> | |
172 | while | |
173 | </p> | |
174 | <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">C</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">ABC</span><span class="special">;</span> | |
175 | </pre> | |
176 | <p> | |
177 | compiles. | |
178 | </p> | |
179 | <p> | |
180 | Thus, as <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">type</span></code> | |
181 | is undefined, <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">,</span><span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span></code> | |
182 | is also undefined. | |
183 | </p> | |
184 | <p> | |
185 | It is intended that clients who wish for <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> | |
186 | <span class="identifier">B</span><span class="special">></span></code> | |
187 | to be well defined to define it themselves: | |
188 | </p> | |
189 | <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> | |
190 | <span class="special">{</span> | |
191 | ||
192 | <span class="keyword">template</span> <span class="special"><></span> | |
193 | <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span> <span class="special">{</span><span class="keyword">typedef</span> <span class="identifier">C</span> <span class="identifier">type</span><span class="special">;};</span> | |
194 | ||
195 | <span class="special">}</span> | |
196 | </pre> | |
197 | <p> | |
198 | Now this client can ask for <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> | |
199 | <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">></span></code> (and | |
200 | get the same answer). | |
201 | </p> | |
202 | <p> | |
203 | Clients wanting to ask <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> | |
204 | <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">></span></code> in | |
205 | any order and get the same result need to add in addition: | |
206 | </p> | |
207 | <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> | |
208 | <span class="special">{</span> | |
209 | ||
210 | <span class="keyword">template</span> <span class="special"><></span> <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">B</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> | |
211 | <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span> <span class="special">{};</span> | |
212 | ||
213 | <span class="special">}</span> | |
214 | </pre> | |
215 | <p> | |
216 | This is needed as the specialization of <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> | |
217 | <span class="identifier">B</span><span class="special">></span></code> | |
218 | is not be used implicitly for <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">B</span><span class="special">,</span> | |
219 | <span class="identifier">A</span><span class="special">></span></code>. | |
220 | </p> | |
221 | <h5> | |
222 | <a name="boost_typetraits.reference.common_type.h3"></a> | |
223 | <span class="phrase"><a name="boost_typetraits.reference.common_type.can_the__code__phrase_role__identifier__common_type__phrase___code__of_two_types_be_a_third_type_"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.can_the__code__phrase_role__identifier__common_type__phrase___code__of_two_types_be_a_third_type_">Can | |
224 | the <code class="computeroutput"><span class="identifier">common_type</span></code> of two types | |
225 | be a third type?</a> | |
226 | </h5> | |
227 | <p> | |
228 | Given the preceding example, one might expect <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">type</span></code> to be <code class="computeroutput"><span class="identifier">C</span></code> | |
229 | without any intervention from the user. But the default <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><></span></code> implementation doesn't grant that. | |
230 | It is intended that clients who wish for <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> | |
231 | <span class="identifier">B</span><span class="special">></span></code> | |
232 | to be well defined to define it themselves: | |
233 | </p> | |
234 | <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> | |
235 | <span class="special">{</span> | |
236 | ||
237 | <span class="keyword">template</span> <span class="special"><></span> | |
238 | <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span> <span class="special">{</span><span class="keyword">typedef</span> <span class="identifier">C</span> <span class="identifier">type</span><span class="special">;};</span> | |
239 | ||
240 | <span class="keyword">template</span> <span class="special"><></span> <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">B</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> | |
241 | <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span> <span class="special">{};</span> | |
242 | ||
243 | <span class="special">}</span> | |
244 | </pre> | |
245 | <p> | |
246 | Now this client can ask for <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> | |
247 | <span class="identifier">B</span><span class="special">></span></code>. | |
248 | </p> | |
249 | <h5> | |
250 | <a name="boost_typetraits.reference.common_type.h4"></a> | |
251 | <span class="phrase"><a name="boost_typetraits.reference.common_type.how_does__code__phrase_role__identifier__common_type__phrase___code__behave_with_pointers_"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.how_does__code__phrase_role__identifier__common_type__phrase___code__behave_with_pointers_">How | |
252 | does <code class="computeroutput"><span class="identifier">common_type</span></code> behave with | |
253 | pointers?</a> | |
254 | </h5> | |
255 | <p> | |
256 | Consider | |
257 | </p> | |
258 | <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">C</span> <span class="special">{</span> <span class="special">}:</span> | |
259 | <span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">:</span> <span class="identifier">C</span> <span class="special">{</span> <span class="special">};</span> | |
260 | <span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">:</span> <span class="identifier">C</span> <span class="special">{</span> <span class="special">};</span> | |
261 | </pre> | |
262 | <p> | |
263 | Shouldn't <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">*,</span><span class="identifier">B</span><span class="special">*>::</span><span class="identifier">type</span></code> | |
264 | be <code class="computeroutput"><span class="identifier">C</span><span class="special">*</span></code>? | |
265 | I would say yes, but the default implementation will make it ill-formed. | |
266 | </p> | |
267 | <p> | |
268 | The library could add a specialization for pointers, as | |
269 | </p> | |
270 | <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> | |
271 | <span class="special">{</span> | |
272 | ||
273 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">B</span><span class="special">></span> | |
274 | <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">*,</span> <span class="identifier">B</span><span class="special">*></span> <span class="special">{</span> | |
275 | <span class="keyword">typedef</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">>*</span> <span class="identifier">type</span><span class="special">;</span> | |
276 | <span class="special">};</span> | |
277 | <span class="special">}</span> | |
278 | </pre> | |
279 | <p> | |
280 | But in the absence of a motivating use cases, we prefer not to add more than | |
281 | the standard specifies. | |
282 | </p> | |
283 | <p> | |
284 | Of course the user can always make this specialization. | |
285 | </p> | |
286 | <h5> | |
287 | <a name="boost_typetraits.reference.common_type.h5"></a> | |
288 | <span class="phrase"><a name="boost_typetraits.reference.common_type.can_you_explain_the_pros_cons_of__code__phrase_role__identifier__common_type__phrase___code__against_boost_typeof_"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.can_you_explain_the_pros_cons_of__code__phrase_role__identifier__common_type__phrase___code__against_boost_typeof_">Can | |
289 | you explain the pros/cons of <code class="computeroutput"><span class="identifier">common_type</span></code> | |
290 | against Boost.Typeof?</a> | |
291 | </h5> | |
292 | <p> | |
293 | Even if they appear to be close, <code class="computeroutput"><span class="identifier">common_type</span></code> | |
294 | and <code class="computeroutput"><span class="identifier">typeof</span></code> have different | |
295 | purposes. You use <code class="computeroutput"><span class="identifier">typeof</span></code> | |
296 | to get the type of an expression, while you use <code class="computeroutput"><span class="identifier">common_type</span></code> | |
297 | to set explicitly the type returned of a template function. Both are complementary, | |
298 | and indeed <code class="computeroutput"><span class="identifier">common_type</span></code> is | |
299 | approximately equivalent to <code class="computeroutput"><span class="keyword">decltype</span><span class="special">(</span><a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="keyword">bool</span><span class="special">>()</span> | |
300 | <span class="special">?</span> <a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span> | |
301 | <span class="special">:</span> <a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="identifier">U</span><span class="special">>())</span></code>. | |
302 | </p> | |
303 | <p> | |
304 | <code class="computeroutput"><span class="identifier">common_type</span></code> is also similar | |
305 | to <code class="computeroutput"><span class="identifier">promote_args</span><span class="special"><</span><span class="keyword">class</span> <span class="special">...</span><span class="identifier">T</span><span class="special">></span></code> in | |
306 | <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">tools</span><span class="special">/</span><span class="identifier">promotion</span><span class="special">.</span><span class="identifier">hpp</span></code>, though | |
307 | it is not exactly the same as <code class="computeroutput"><span class="identifier">promote_args</span></code> | |
308 | either. <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T1</span><span class="special">,</span> <span class="identifier">T2</span><span class="special">>::</span><span class="identifier">type</span></code> | |
309 | simply represents the result of some operation on <code class="computeroutput"><span class="identifier">T1</span></code> | |
310 | and <code class="computeroutput"><span class="identifier">T2</span></code>, and defaults to the | |
311 | type obtained by putting <code class="computeroutput"><span class="identifier">T1</span></code> | |
312 | and <code class="computeroutput"><span class="identifier">T2</span></code> into a conditional | |
313 | statement. | |
314 | </p> | |
315 | <p> | |
316 | It is meant to be customizable (via specialization) if this default is not | |
317 | appropriate. | |
318 | </p> | |
319 | </div> | |
320 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
321 | <td align="left"></td> | |
322 | <td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams, | |
323 | Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, | |
324 | Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten | |
325 | Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven | |
326 | Watanabe<p> | |
327 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
328 | file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) | |
329 | </p> | |
330 | </div></td> | |
331 | </tr></table> | |
332 | <hr> | |
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