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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> |
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