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- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="the_type_traits_introspection_library.tti_func_templates"></a><a class="link" href="tti_func_templates.html" title="Introspecting Function Templates">Introspecting
- Function Templates</a>
- </h2></div></div></div>
- <p>
- The one nested element which the TTI library does not introspect is function
- templates.
- </p>
- <p>
- Function templates, like functions, can be member function templates or static
- member function templates. In this respect they are related to functions. Function
- templates represent a family of possible functions. In this respect they are
- similar to class templates, which represent a family of possible class types.
- </p>
- <p>
- The technique for introspecting class templates in the TTI library is taken
- from the implementation of the technique in the Boost MPL library. In the case
- of <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_TEMPLATE</span></code> it
- directly uses the Boost MPL library functionality while in the case of <code class="computeroutput"><span class="identifier">BOOST_TTI_HAS_TEMPLATE_CHECK_PARAMS</span></code> it replicates
- much of the technique in the Boost MPL library. The technique depends directly
- on the fact that in C++ we can pass a template as a parameter to another template
- using what is called a "template template" parameter type.
- </p>
- <p>
- One obvious thing about a template template parameter type is that it is a
- class template. For whatever historical or technical reasons, no one has ever
- proposed that C++ have a way of passing a function template directly as a template
- parameter, perhaps to be called a "function template template" parameter
- type. I personally think this would be a good addition to C++ and would make
- the ability of passing a template as a parameter to another template more orthogonal,
- since both class templates and function templates would be supported. My efforts
- to discuss this on the major C++ newsgroups have met with arguments both against
- its practical usage and the justification that one can pass a function template
- to another template nested in a non-template class, which serves as a type.
- But of course we can do the same thing with class templates, which is in fact
- what Boost MPL does to pass templates as metadata, yet we still have template
- template parameters as class templates.
- </p>
- <p>
- Nonetheless the fact that we can pass class templates as a template parameter
- but not function templates as a template parameter is the major factor why
- there is no really good method for introspecting function templates at compile
- time.
- </p>
- <h4>
- <a name="the_type_traits_introspection_library.tti_func_templates.h0"></a>
- <span class="phrase"><a name="the_type_traits_introspection_library.tti_func_templates.instantiating_a_nested_function_template"></a></span><a class="link" href="tti_func_templates.html#the_type_traits_introspection_library.tti_func_templates.instantiating_a_nested_function_template">Instantiating
- a nested function template</a>
- </h4>
- <p>
- There is, however, an alternate but less certain way of introspecting a function
- template. I will endeavor to explain why this way is not currently included
- in the TTI library, but first I will explain what it is.
- </p>
- <p>
- It is possible to check whether some particular <span class="bold"><strong>instantiation</strong></span>
- of a nested function template exists at compile-time without generating a compiler
- error. Although checking if some particular instantiation of a nested function
- template exists at compile-time does not prove that the nested function template
- itself does or does not exist, since the instantiation itself may be incorrect
- and fail even when the nested function template exists, it provides a partial,
- if flawed, means of checking.
- </p>
- <p>
- The code to do this for member function templates looks like this ( similar
- code also exists for static member function templates ):
- </p>
- <pre class="programlisting"><span class="keyword">template</span>
- <span class="special"><</span>
- <span class="keyword">class</span> <span class="identifier">C</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">TestFunctionTemplate</span>
- <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="keyword">char</span> <span class="identifier">Bad</span><span class="special">;</span>
- <span class="keyword">struct</span> <span class="identifier">Good</span> <span class="special">{</span> <span class="keyword">char</span> <span class="identifier">x</span><span class="special">[</span><span class="number">2</span><span class="special">];</span> <span class="special">};</span>
- <span class="keyword">template</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">helper</span><span class="special">;</span>
- <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">static</span> <span class="identifier">Good</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">helper</span><span class="special"><&</span><span class="identifier">U</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">SomeFuncTemplateName</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">long</span><span class="special">,</span><span class="keyword">double</span><span class="special">></span> <span class="special">></span> <span class="special">*);</span>
- <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">static</span> <span class="identifier">Bad</span> <span class="identifier">check</span><span class="special">(...);</span>
- <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span><span class="special">=</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">check</span><span class="special"><</span><span class="identifier">C</span><span class="special">>(</span><span class="number">0</span><span class="special">))==</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">Good</span><span class="special">);</span>
- <span class="special">};</span>
- </pre>
- <p>
- where 'SomeFuncTemplateName' is the name of the nested function template, followed
- by some parameters to instantiate it. The 'class C' is the type of the enclosing
- class and the 'class T' is the type of the instantiated member function template
- as a member function.
- </p>
- <p>
- As an example if we had:
- </p>
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">AType</span>
- <span class="special">{</span>
- <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">X</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">Y</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">Z</span><span class="special">></span> <span class="keyword">double</span> <span class="identifier">SomeFuncTemplateName</span><span class="special">(</span><span class="identifier">X</span><span class="special">,</span><span class="identifier">Y</span> <span class="special">*,</span><span class="identifier">Z</span> <span class="special">&)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="number">0.0</span><span class="special">;</span> <span class="special">}</span>
- <span class="special">};</span>
- </pre>
- <p>
- then instantiating the above template with:
- </p>
- <pre class="programlisting"><span class="identifier">TestFunctionTemplate</span>
- <span class="special"><</span>
- <span class="identifier">AType</span><span class="special">,</span>
- <span class="keyword">double</span> <span class="special">(</span><span class="identifier">AType</span><span class="special">::*)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">long</span> <span class="special">*,</span><span class="keyword">double</span> <span class="special">&)</span>
- <span class="special">></span>
- </pre>
- <p>
- would provide a compile-time boolean value which would tell us whether the
- nested member function template exists for the particular instantiation provided
- above. Furthermore, through the use of a macro, the TTI library could provide
- the means for specifying the name of the nested member function template ('SomeFuncTemplateName'
- above) and its set of instantiated parameters ('int,long,double' above) for
- generating the template.
- </p>
- <p>
- So why does not the TTI library not provide at least this much functionality
- for introspecting member function templates, even if it represents a partially
- flawed way of doing so ?
- </p>
- <p>
- The reason is stunningly disappointing. Although the above code is perfectly
- correct C++ code ( 'clang' works correctly ), two of the major C++ compilers,
- in all of their different releases, can not handle the above code correctly.
- Both gcc ( g++ ) and Visual C++ incorrectly choose the wrong 'check' function
- even when the correct 'check' function applies ( Comeau C++ also fails but
- I am less concerned about that compiler since it is not used nearly as much
- as the other two ). All my attempts at alternatives to the above code have
- also failed. The problems with both compilers, in this regard, can be seen
- more easily with this snippet:
- </p>
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">AType</span>
- <span class="special">{</span>
- <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">AA</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">SomeFuncTemplate</span><span class="special">()</span> <span class="special">{</span> <span class="special">}</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">Test</span>
- <span class="special">{</span>
- <span class="keyword">template</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">helper</span><span class="special">;</span>
- <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">helper</span><span class="special"><&</span><span class="identifier">U</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">SomeFuncTemplate</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">></span> <span class="special">*)</span> <span class="special">{</span> <span class="special">}</span>
- <span class="special">};</span>
- <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
- <span class="special">{</span>
- <span class="identifier">Test</span><span class="special"><</span> <span class="keyword">void</span> <span class="special">(</span><span class="identifier">AType</span><span class="special">::*)()</span> <span class="special">>::</span><span class="identifier">check</span><span class="special"><</span><span class="identifier">AType</span><span class="special">>(</span><span class="number">0</span><span class="special">);</span>
- <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
- <span class="special">}</span>
- </pre>
- <p>
- Both compilers report compile errors with this perfectly correct code,
- </p>
- <p>
- gcc:
- </p>
- <pre class="programlisting"><span class="identifier">error</span><span class="special">:</span> <span class="identifier">no</span> <span class="identifier">matching</span> <span class="identifier">function</span> <span class="keyword">for</span> <span class="identifier">call</span> <span class="identifier">to</span> <span class="char">'Test<void (AType::*)()>::check(int)'</span>
- </pre>
- <p>
- and msvc:
- </p>
- <pre class="programlisting"><span class="identifier">error</span> <span class="identifier">C2770</span><span class="special">:</span> <span class="identifier">invalid</span> <span class="keyword">explicit</span> <span class="keyword">template</span> <span class="identifier">argument</span><span class="special">(</span><span class="identifier">s</span><span class="special">)</span> <span class="keyword">for</span> <span class="char">'void Test<T>::check(Test<T>::helper<&U::SomeFuncTemplate<int>> *)'</span>
- </pre>
- <p>
- There is a workaround for these compiler problems, which is to hardcode the
- name of the enclosing class, via a macro, in the generated template rather
- than pass it as a template type. In that case both compilers can handle both
- the member function code and the code snippet above correctly. In essence,
- when the line:
- </p>
- <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">helper</span><span class="special"><&</span><span class="identifier">U</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">SomeFuncTemplate</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">></span> <span class="special">*)</span> <span class="special">{</span> <span class="special">}</span>
- </pre>
- <p>
- gets replaced by:
- </p>
- <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">helper</span><span class="special"><&</span><span class="identifier">AType</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">SomeFuncTemplate</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">></span> <span class="special">*)</span> <span class="special">{</span> <span class="special">}</span>
- </pre>
- <p>
- both gcc and Visual C++ work correctly. The same goes for the 'check' line
- in the 'TestFunctionTemplate' above.
- </p>
- <p>
- But the workaround destroys one of the basic tenets of the TTI library, which
- is that the enclosing class be passed as a template parameter, especially as
- the enclosing class need not actually exist ( see <code class="computeroutput"><span class="identifier">BOOST_TTI_MEMBER_TYPE</span></code>
- and the previous discussion of 'Nested Types' ), without producing a compiler
- error. So I have decided not to implement even this methodology to introspect
- nested function templates in the TTI library.
- </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>)
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