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<div class="section">
<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">&lt;</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">&gt;</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">&lt;</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">)&gt;::</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">&lt;</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">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</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">&lt;</span><span class="identifier">U</span><span class="special">&gt;::</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">&lt;</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">&lt;</span><span class="identifier">OurType</span><span class="special">&gt;</span> <span class="special">&gt;::</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">&lt;</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">&gt;</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">&lt;</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">)&gt;::</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">&lt;</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">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</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">&lt;</span><span class="identifier">U</span><span class="special">&gt;::</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">&lt;</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">&lt;</span><span class="identifier">OurType</span><span class="special">&gt;</span> <span class="special">&gt;::</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>
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