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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="the_type_traits_introspection_library.tti_func_sig"></a><a class="link" href="tti_func_sig.html" title="Nested Types and Function Signatures">Nested |
| Types and Function Signatures</a> |
| </h2></div></div></div> |
| <p> |
| The strength of <code class="computeroutput"><span class="identifier">BOOST_TTI_MEMBER_TYPE</span></code> |
| to represent a type which may or may not exist, and which then can be subsequently |
| used in other macro metafunctions whenever a type is needed as a template parameter |
| without producing a compiler error, should not be underestimated. It is one |
| of the reasons why we have two different ways of using our generated metafunction |
| when introspecting for member data, a member function, or a static member function |
| of an enclosing type. |
| </p> |
| <p> |
| In the cases where we specify a composite syntax when using <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_MEMBER_DATA</span></code>, |
| <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span></code>, |
| or <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION</span></code>, |
| the signature for the member data, member function, or static member function |
| is a single type. For <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_MEMBER_DATA</span></code> |
| the signature is a pointer to member data, for <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span></code> |
| the signature is a pointer to a member function, and for <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION</span></code> |
| the signature is divided between an enclosing type and a function in composite |
| format. This makes for a syntactical notation which is natural to specify, |
| but because of the notation we can not use the nested type functionality in |
| <code class="computeroutput"><span class="identifier">BOOST_TTI_MEMBER_TYPE</span></code> for potential |
| parts of these composite types. If any part of this signature, which specifies |
| a composite of various types, is invalid, a compiler time error will occur. |
| </p> |
| <p> |
| But in the more specific cases, when we use <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_MEMBER_DATA</span></code>, |
| <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span></code>, |
| and <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION</span></code>, |
| our composite type in our signatures is broken down into their individual types |
| so that using <code class="computeroutput"><span class="identifier">BOOST_TTI_MEMBER_TYPE</span></code> |
| for any one of the individual types will not lead to a compile time error if |
| the type specified does not actually exist. |
| </p> |
| <p> |
| A few examples will suffice. |
| </p> |
| <p> |
| Given known types T and U, and the supposed type Ntype as a nested type of |
| U, we want to find out if type T has a member function whose signature is |
| <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">aMemberFunction</span><span class="special">(</span><span class="identifier">U</span><span class="special">::</span><span class="identifier">Ntype</span><span class="special">)</span></code>. |
| </p> |
| <p> |
| First using <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span></code> |
| using our composite form we would code: |
| </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_member_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| |
| <span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span><span class="special">(</span><span class="identifier">aMemberFunction</span><span class="special">)</span> |
| |
| <span class="identifier">has_member_function_aMemberFunction</span><span class="special"><</span><span class="keyword">void</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">Ntype</span><span class="special">)>::</span><span class="identifier">value</span><span class="special">;</span> |
| </pre> |
| <p> |
| If the nested type U::Ntype does not exist, this leads to a compiler error. |
| We really want to avoid this situation, so let's try our alternative. |
| </p> |
| <p> |
| Second using <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span></code> |
| using our specific form we would code: |
| </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">member_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <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_member_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| |
| <span class="identifier">BOOST_TTI_HAS_MEMBER_TYPE</span><span class="special">(</span><span class="identifier">Ntype</span><span class="special">)</span> |
| <span class="identifier">BOOST_TTI_HAS_MEMBER_FUNCTION</span><span class="special">(</span><span class="identifier">aMemberFunction</span><span class="special">)</span> |
| |
| <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">has_member_type_Ntype</span><span class="special"><</span><span class="identifier">U</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">OurType</span><span class="special">;</span> |
| <span class="identifier">has_member_function_aMemberFunction</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">void</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">OurType</span><span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> |
| </pre> |
| <p> |
| If the nested type U::Ntype does exist and T does have a member function whose |
| signature is <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">aMemberFunction</span><span class="special">(</span><span class="identifier">U</span><span class="special">::</span><span class="identifier">Ntype</span><span class="special">)</span></code> our |
| 'value' is true, otherwise it is false. We will never get a compiler error |
| in this case. |
| </p> |
| <p> |
| As a second example we will once again use the suppositions of our first example; |
| given known types T and U, and the supposed type Ntype as a nested type of |
| U. But this time let us look for a static member function whose signature is |
| <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">aStaticMemberFunction</span><span class="special">(</span><span class="identifier">U</span><span class="special">::</span><span class="identifier">Ntype</span><span class="special">)</span></code>. |
| </p> |
| <p> |
| First using <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION</span></code> |
| using our composite form we would code: |
| </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_static_member_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| |
| <span class="identifier">BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION</span><span class="special">(</span><span class="identifier">aStaticMemberFunction</span><span class="special">)</span> |
| |
| <span class="identifier">has_static_member_function_aStaticMemberFunction</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">void</span> <span class="special">(</span><span class="identifier">U</span><span class="special">::</span><span class="identifier">Ntype</span><span class="special">)>::</span><span class="identifier">value</span><span class="special">;</span> |
| </pre> |
| <p> |
| Once again if the nested type U::Ntype does not exist, this leads to a compiler |
| error, so let's try our alternative. |
| </p> |
| <p> |
| Second using <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION</span></code> |
| using our specific form we would code: |
| </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">member_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <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_static_member_function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| |
| <span class="identifier">BOOST_TTI_HAS_MEMBER_TYPE</span><span class="special">(</span><span class="identifier">Ntype</span><span class="special">)</span> |
| <span class="identifier">BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION</span><span class="special">(</span><span class="identifier">aStaticMemberFunction</span><span class="special">)</span> |
| |
| <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">has_member_type_Ntype</span><span class="special"><</span><span class="identifier">U</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">OurType</span><span class="special">;</span> |
| <span class="identifier">has_static_member_function_aStaticMemberFunction</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">void</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">OurType</span><span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> |
| </pre> |
| <p> |
| If the nested type U::Ntype does exist and T does have a member function whose |
| signature is <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">aMemberFunction</span><span class="special">(</span><span class="identifier">U</span><span class="special">::</span><span class="identifier">Ntype</span><span class="special">)</span></code> our |
| 'value' is true, otherwise it is false. We will never get a compiler error |
| in this case. |
| </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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