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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="the_type_traits_introspection_library.tti_detail_has_function"></a><a class="link" href="tti_detail_has_function.html" title="Introspecting an inner function">Introspecting |
| an inner function</a> |
| </h2></div></div></div> |
| <p> |
| The TTI macro <code class="computeroutput"><a class="link" href="../BOOST_TTI_HAS_FUNCTION.html" title="Macro BOOST_TTI_HAS_FUNCTION">BOOST_TTI_HAS_FUNCTION</a></code> |
| introspects an inner function of a class. The function can be either a member |
| function or a static member function. |
| </p> |
| <p> |
| BOOST_TTI_HAS_FUNCTION takes a single parameter which is the name of an inner |
| function whose existence the programmer wants to check. The macro generates |
| a metafunction called 'has_function_'name_of_inner_function'. |
| </p> |
| <p> |
| The metafunction can be invoked by passing it the enclosing type to introspect |
| and a signature for the function as separate template arguments. The signature |
| for the function consists of a return type, optional parameter types in the |
| form of a boost::mpl forward sequence of types, and an optional Boost FunctionTypes |
| tag type. A typical boost::mpl forward sequence of types is a boost::mpl::vector<>. |
| </p> |
| <p> |
| The metafunction returns a single type called 'type', which is a boost::mpl::bool_. |
| As a convenience the metafunction returns the value of this type directly as |
| a compile time bool constant called 'value'. This is true or false depending |
| on whether the inner function, of the specified signature, exists or not. |
| </p> |
| <h4> |
| <a name="the_type_traits_introspection_library.tti_detail_has_function.h0"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_function.generating_the_metafunction"></a></span><a class="link" href="tti_detail_has_function.html#the_type_traits_introspection_library.tti_detail_has_function.generating_the_metafunction">Generating |
| the metafunction</a> |
| </h4> |
| <p> |
| You generate the metafunction by invoking the macro with the name of an inner |
| function: |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_TTI_HAS_FUNCTION</span><span class="special">(</span><span class="identifier">AnInnerFunction</span><span class="special">)</span> |
| </pre> |
| <p> |
| generates a metafunction called 'has_function_AnInnerFunction' in the current |
| scope. |
| </p> |
| <h4> |
| <a name="the_type_traits_introspection_library.tti_detail_has_function.h1"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_function.invoking_the_metafunction"></a></span><a class="link" href="tti_detail_has_function.html#the_type_traits_introspection_library.tti_detail_has_function.invoking_the_metafunction">Invoking |
| the metafunction</a> |
| </h4> |
| <p> |
| You invoke the metafunction by instantiating the template with an enclosing |
| type to introspect and the signature of the function as a series of template |
| parameters. |
| </p> |
| <p> |
| A return value called 'value' is a compile time bool constant. |
| </p> |
| <pre class="programlisting"><span class="identifier">has_function_AnInnerFunction</span> |
| <span class="special"><</span> |
| <span class="identifier">Enclosing_Type</span><span class="special">,</span> |
| <span class="identifier">Function_ReturnType</span><span class="special">,</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">Function_ParameterTypes</span><span class="special">>,</span> <span class="comment">// optional, can be any mpl forward sequence</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function_types</span><span class="special">::</span><span class="identifier">SomeTagType</span> <span class="comment">// optional, can be any FunctionTypes tag type</span> |
| <span class="special">>::</span><span class="identifier">value</span> |
| </pre> |
| <h4> |
| <a name="the_type_traits_introspection_library.tti_detail_has_function.h2"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_function.examples"></a></span><a class="link" href="tti_detail_has_function.html#the_type_traits_introspection_library.tti_detail_has_function.examples">Examples</a> |
| </h4> |
| <p> |
| First we generate metafunctions for various inner function names: |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">tti</span><span class="special">/</span><span class="identifier">has_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| |
| <span class="identifier">BOOST_TTI_HAS_FUNCTION</span><span class="special">(</span><span class="identifier">function1</span><span class="special">)</span> |
| <span class="identifier">BOOST_TTI_HAS_FUNCTION</span><span class="special">(</span><span class="identifier">function2</span><span class="special">)</span> |
| <span class="identifier">BOOST_TTI_HAS_FUNCTION</span><span class="special">(</span><span class="identifier">function3</span><span class="special">)</span> |
| </pre> |
| <p> |
| Next let us create some user-defined types we want to introspect. |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">AClass</span> <span class="special">{</span> <span class="special">};</span> |
| <span class="keyword">struct</span> <span class="identifier">Top</span> |
| <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">int</span> <span class="identifier">function1</span><span class="special">();</span> |
| <span class="identifier">AClass</span> <span class="identifier">function2</span><span class="special">(</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">short</span> <span class="special">*);</span> |
| <span class="special">};</span> |
| <span class="keyword">struct</span> <span class="identifier">Top2</span> |
| <span class="special">{</span> |
| <span class="keyword">long</span> <span class="identifier">function2</span><span class="special">(</span><span class="identifier">Top</span> <span class="special">&,</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">bool</span><span class="special">,</span><span class="keyword">short</span><span class="special">,</span><span class="keyword">float</span><span class="special">);</span> |
| <span class="keyword">static</span> <span class="identifier">Top</span> <span class="special">*</span> <span class="identifier">function3</span><span class="special">(</span><span class="keyword">long</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">AClass</span> <span class="special">&);</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| Finally we invoke our metafunction and return our value. This all happens at |
| compile time, and can be used by programmers doing compile time template metaprogramming. |
| </p> |
| <pre class="programlisting"><span class="identifier">has_function_function1</span><span class="special"><</span><span class="identifier">Top</span><span class="special">,</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span> |
| <span class="identifier">has_function_function1</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span> |
| |
| <span class="identifier">has_function_function2</span><span class="special"><</span><span class="identifier">Top</span><span class="special">,</span><span class="identifier">AClass</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">short</span> <span class="special">*></span> <span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span> |
| <span class="identifier">has_function_function2</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">AClass</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">short</span> <span class="special">*></span> <span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span> |
| |
| <span class="identifier">has_function_function3</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span> |
| <span class="identifier">has_function_function3</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">Top</span> <span class="special">*,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">long</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">AClass</span> <span class="special">&></span> <span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true;</span> |
| </pre> |
| <h4> |
| <a name="the_type_traits_introspection_library.tti_detail_has_function.h3"></a> |
| <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_function.metafunction_re_use"></a></span><a class="link" href="tti_detail_has_function.html#the_type_traits_introspection_library.tti_detail_has_function.metafunction_re_use">Metafunction |
| re-use</a> |
| </h4> |
| <p> |
| The macro encodes only the name of the function for which we are searching |
| and the fact that we are introspecting for a function within an enclosing type. |
| </p> |
| <p> |
| Because of this, once we create our metafunction for introspecting a function |
| by name, we can reuse the metafunction for introspecting any enclosing type, |
| having any function, for that name. |
| </p> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2011-2013 Tropic Software |
| East Inc<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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>) |
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