123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248 |
- <html>
- <head>
- <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
- <title>Introspecting an inner type</title>
- <link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
- <meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
- <link rel="home" href="../index.html" title="Chapter 1. The Type Traits Introspection Library">
- <link rel="up" href="../index.html" title="Chapter 1. The Type Traits Introspection Library">
- <link rel="prev" href="tti_detail.html" title="Macro Metafunctions">
- <link rel="next" href="tti_detail_has_template.html" title="Introspecting an inner class template">
- </head>
- <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
- <table cellpadding="2" width="100%"><tr>
- <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
- <td align="center"><a href="../../../../../index.html">Home</a></td>
- <td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
- <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
- <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
- <td align="center"><a href="../../../../../more/index.htm">More</a></td>
- </tr></table>
- <hr>
- <div class="spirit-nav">
- <a accesskey="p" href="tti_detail.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="tti_detail_has_template.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="the_type_traits_introspection_library.tti_detail_has_type"></a><a class="link" href="tti_detail_has_type.html" title="Introspecting an inner type">Introspecting
- an inner type</a>
- </h2></div></div></div>
- <p>
- The TTI macro <code class="computeroutput"><a class="link" href="../BOOST_TTI_HAS_TYPE.html" title="Macro BOOST_TTI_HAS_TYPE">BOOST_TTI_HAS_TYPE</a></code>
- introspects a nested type of a class.
- </p>
- <p>
- The BOOST_TTI_HAS_TYPE macro takes a single parameter which is the name of
- an inner type whose existence the programmer wants to check. The macro generates
- a metafunction called "has_type_'name_of_inner_type'".
- </p>
- <p>
- The main purpose of the generated metafunction is to check for the existence
- by name of the inner type. The metafunction can also be used to invoke an MPL
- lambda expression which is passed the inner type. One of the most common usages
- of the added functionality is to check whether or not the inner type is a typedef
- for another type.
- </p>
- <p>
- The metafunction is invoked by passing it the enclosing type to introspect.
- A second type may be passed to the metafunction, an MPL lambda expression taking
- the inner type and returning a boolean constant.
- </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 value is true or false depending
- on whether the inner type exists or not.
- </p>
- <p>
- If a second optional type is passed, this type must be an MPL lambda expression
- and the expression will be invoked only if the inner type exists. In that case
- the metafunction returns true or false depending on whether the lambda expression
- returns true or false. If the inner type does not exist, the lambda expression,
- even if specified, is never invoked and the metafunction returns false.
- </p>
- <h4>
- <a name="the_type_traits_introspection_library.tti_detail_has_type.h0"></a>
- <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.generating_the_metafunction"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.generating_the_metafunction">Generating
- the metafunction</a>
- </h4>
- <p>
- You generate the metafunction by invoking the macro with the name of an inner
- type:
- </p>
- <pre class="programlisting"><span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">AType</span><span class="special">)</span>
- </pre>
- <p>
- generates a metafunction called 'has_type_AType' in the current scope.
- </p>
- <h4>
- <a name="the_type_traits_introspection_library.tti_detail_has_type.h1"></a>
- <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.invoking_the_metafunction"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.invoking_the_metafunction">Invoking
- the metafunction</a>
- </h4>
- <p>
- You invoke the metafunction by instantiating the template with an enclosing
- type to introspect and, optionally, an MPL lambda expression. A return value
- called 'value' is a compile time bool constant.
- </p>
- <pre class="programlisting"><span class="identifier">has_type_AType</span><span class="special"><</span><span class="identifier">Enclosing_Type</span><span class="special">>::</span><span class="identifier">value</span>
- <span class="identifier">has_type_AType</span><span class="special"><</span><span class="identifier">Enclosing_Type</span><span class="special">,</span><span class="identifier">ALambdaExpression</span><span class="special">>::</span><span class="identifier">value</span>
- </pre>
- <h4>
- <a name="the_type_traits_introspection_library.tti_detail_has_type.h2"></a>
- <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.examples"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.examples">Examples</a>
- </h4>
- <p>
- First we generate metafunctions for various inner type 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_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
- <span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">MyTypeDef</span><span class="special">)</span>
- <span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">AType</span><span class="special">)</span>
- <span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">ATypeDef</span><span class="special">)</span>
- <span class="identifier">BOOST_TTI_HAS_TYPE</span><span class="special">(</span><span class="identifier">MyType</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">Top</span>
- <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="keyword">int</span> <span class="identifier">MyTypeDef</span><span class="special">;</span>
- <span class="keyword">struct</span> <span class="identifier">AType</span> <span class="special">{</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">typedef</span> <span class="keyword">long</span> <span class="identifier">ATypeDef</span><span class="special">;</span>
- <span class="keyword">struct</span> <span class="identifier">MyType</span> <span class="special">{</span> <span class="special">};</span>
- <span class="special">};</span>
- </pre>
- <p>
- Finally we invoke our metafunction and return our value.
- </p>
- <pre class="programlisting"><span class="identifier">has_type_MyTypeDef</span><span class="special"><</span><span class="identifier">Top</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
- <span class="identifier">has_type_MyTypeDef</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
- <span class="identifier">has_type_AType</span><span class="special"><</span><span class="identifier">Top</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
- <span class="identifier">has_type_AType</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
- <span class="identifier">has_type_ATypeDef</span><span class="special"><</span><span class="identifier">Top</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
- <span class="identifier">has_type_ATypeDef</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
- <span class="identifier">has_type_MyType</span><span class="special"><</span><span class="identifier">Top</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
- <span class="identifier">has_type_MyType</span><span class="special"><</span><span class="identifier">Top2</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_type.h3"></a>
- <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.examples___using_lambda_expressions"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.examples___using_lambda_expressions">Examples
- - using lambda expressions</a>
- </h4>
- <p>
- We can further invoke our metafunction with a second type, which is an MPL
- lambda expression.
- </p>
- <p>
- An MPL lambda expression, an extremely useful technique in template metaprogramming,
- allows us to pass a metafunction to other metafunctions. The metafunction we
- pass can be in the form of a placeholder expression or a metafunction class.
- In our case the metafunction passed to our has_type_'name_of_inner_type' metafunction
- as a lambda expression must return a boolean constant expression.
- </p>
- <h4>
- <a name="the_type_traits_introspection_library.tti_detail_has_type.h4"></a>
- <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.example___using_a_lambda_expression_with_a_placeholder_expression"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.example___using_a_lambda_expression_with_a_placeholder_expression">Example
- - using a lambda expression with a placeholder expression</a>
- </h4>
- <p>
- We will first illustrate the use of a lambda expression in the form of a placeholder
- expression being passed as the second template parameter to our has_type_'name_of_inner_type'
- metafunction. A popular and simple placeholder expression we can use is 'boost::is_same<_1,SomeType>'
- to check if the inner type found is a particular type. This is particularly
- useful when the inner type is a typedef for some other type.
- </p>
- <p>
- First we include some more header files and a using declaration for convenience.
- </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">mpl</span><span class="special">/</span><span class="identifier">placeholders</span><span class="special">.</span><span class="identifier">hpp</span>
- <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">is_same</span><span class="special">.</span><span class="identifier">hpp</span>
- <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">;</span>
- </pre>
- <p>
- Next we invoke our metafunction:
- </p>
- <pre class="programlisting"><span class="identifier">has_type_MyTypeDef</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">is_same</span><span class="special"><</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">int</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_type_MyTypeDef</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">is_same</span><span class="special"><</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">long</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_type_ATypeDef</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">int</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_type_ATypeDef</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">_1</span><span class="special">,</span><span class="keyword">long</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_type.h5"></a>
- <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.example___using_a_lambda_expression_with_a_metafunction_class"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.example___using_a_lambda_expression_with_a_metafunction_class">Example
- - using a lambda expression with a metafunction class</a>
- </h4>
- <p>
- We will next illustrate the use of a lambda expression in the form of a metafunction
- class being passed as the second template parameter to our has_type_'name_of_inner_type'
- metafunction.
- </p>
- <p>
- A metafunction class is a type which has a nested class template called 'apply'.
- For our metafunction class example we will check if the inner type is a built-in
- integer type. First let us write out metafunction class:
- </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">type_traits</span><span class="special">/</span><span class="identifier">is_integral</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
- <span class="keyword">class</span> <span class="identifier">OurMetafunctionClass</span>
- <span class="special">{</span>
- <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">struct</span> <span class="identifier">apply</span> <span class="special">:</span>
- <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_integral</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span>
- <span class="special">{</span>
- <span class="special">};</span>
- <span class="special">};</span>
- </pre>
- <p>
- Now we can invoke our metafunction:
- </p>
- <pre class="programlisting"><span class="identifier">has_type_MyTypeDef</span><span class="special"><</span><span class="identifier">Top</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
- <span class="identifier">has_type_AType</span><span class="special"><</span><span class="identifier">Top</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
- <span class="identifier">has_type_ATypeDef</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
- <span class="identifier">has_type_MyType</span><span class="special"><</span><span class="identifier">Top2</span><span class="special">,</span><span class="identifier">OurMetafunctionClass</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_type.h6"></a>
- <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_type.metafunction_re_use"></a></span><a class="link" href="tti_detail_has_type.html#the_type_traits_introspection_library.tti_detail_has_type.metafunction_re_use">Metafunction
- re-use</a>
- </h4>
- <p>
- The macro encodes only the name of the inner type for which we are searching
- and the fact that we are introspecting for an inner type within an enclosing
- type.
- </p>
- <p>
- Because of this, once we create our metafunction for introspecting an inner
- type by name, we can reuse the metafunction for introspecting any enclosing
- type, having any inner type, for that name.
- </p>
- <p>
- Furthermore since we have only encoded the name of the inner type for which
- we are introspecting, we can not only introspect for that inner type by name
- but add different lambda expressions to inspect that inner type for whatever
- we want to find out about it using the same metafunction.
- </p>
- </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 © 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>)
- </p>
- </div></td>
- </tr></table>
- <hr>
- <div class="spirit-nav">
- <a accesskey="p" href="tti_detail.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="tti_detail_has_template.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
- </div>
- </body>
- </html>
|