[/ (C) Copyright Edward Diener 2011,2012 Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt). ] [section:tti_detail_has_template Introspecting an inner class template] [section:tti_detail_has_template_macro Using the BOOST_TTI_HAS_TEMPLATE macro] The TTI macro [macroref BOOST_TTI_HAS_TEMPLATE] introspects an inner class template of a class. The macro must specify, at the least, the name of the class template to introspect. [heading Two forms of introspection] There are two general forms of template introspection which can be used. The first is to find a class template with any number of only template type parameters ( template parameters starting with `class` or `typename` ). In this form only the name of the class template needs to be specified when invoking the macro. We will call this form of the macro the `template type parameters` form. An example of a class template of this form which could be successfully introspected would be: template class AClassTemplate { /* etc. */ }; The second is to find a class template with specific template parameters. In this form both the name of the class template and the template parameters are passed to the macro. We will call this form of the macro the `specific parameters` form. An example of a class template of this form which could be successfully introspected would be: template class Y, int Z> BClassTemplate { /* etc. */ }; When using the specific form of the macro, there are two things which need to be understood when passing the template parameters to the macro. First, the actual names of the template parameters passed are irrelevant. They can be left out completely or be different from the names in the nested class template itself. Second, the use of 'typename' or 'class', when referring to a template type parameter, is completely interchangeable, as it is in the actual class template itself. [heading Variadic and non-variadic macro usage] When using the BOOST_TTI_HAS_TEMPLATE macro we distinguish between compilers supporting variadic macros or not supporting variadic macros. The programmer can always tell whether or not the compiler supports variadic macros by checking the value of the macro BOOST_PP_VARIADIC after including the necessary header file `boost/tti/has_template.hpp` in order to use the BOOST_TTI_TEMPLATE macro. A value of 1 indicates the compiler supports variadic macros while a value of 0 indicates the compiler does not support variadic macros. Modern C++ compilers, in supporting the latest C++11 standard, normally support variadic macros. Even before the latest C++11 standard a number of C++ compilers already supported variadic macros. If you feel your compiler supports variadic macros and BOOST_PP_VARIADIC is 0 even after including `boost/tti/has_template.hpp`, you can predefine BOOST_PP_VARIADIC to 1 before including `boost/tti/has_template.hpp`. [heading Non-variadic macro usage] We start with syntax for compilers not supporting variadic macros since this syntax can also be used by compilers which do support variadic macros. The form for non-variadic macros always takes two macro parameters. The first macro parameter is always the name of the class template you are trying to introspect. The second macro parameter, when using the `specific parameters` form of the macro, is the template parameters in the form of a Boost preprocessor library array data type. When using the `template type parameters` form of the macro the second macro parameter is BOOST_PP_NIL. If the second parameter is neither a Boost preprocessor library array data type or BOOS_PP_NIL you will get a compiler error if your compiler only supports non-variadic macros. The non-variadic macro form for introspecting the class templates above using the `template type parameters` form would be: BOOST_TTI_TEMPLATE(AClassTemplate,BOOST_PP_NIL) BOOST_TTI_TEMPLATE(BClassTemplate,BOOST_PP_NIL) Invoking the metafunction in the second case would always fail since the BClassTemplate does not have all template type parameters. The non-variadic macro form for introspecting the class templates above using the `specific parameters` form would be: BOOST_TTI_TEMPLATE(AClassTemplate,(4,(class,typename,class,typename))) BOOST_TTI_TEMPLATE(BClassTemplate,(3,(class, template class, int))) You need to be careful using the non-variadic `specific parameters` form to specify the correct number of array parameters. This can sometimes be tricky if you have a template template parameter, or a non-type template parameter which has parentheses surrounding part of the type specification. In the latter case, when parentheses surround a comma ( ',' ), do not count that as creating another Boost PP array token. Two examples: template class CClassTemplate { /* etc. */ }; template class T> class DClassTemplate { /* etc. */ }; BOOST_TTI_TEMPLATE(CClassTemplate,(1,(void (*)(int,long)))) BOOST_TTI_TEMPLATE(DClassTemplate,(2,(template class))) In the case of using the macro to introspect CClassTemplate the number of Boost PP array parameters is 1, even though there is a comma separating the tokens in `void (*FunctionPointer)(int,long)`. This is because the comma is within parentheses. In the case of using the macro to introspect DClassTemplate the number of Boost PP array parameters is 2, because there is a comma separating the tokens in `template class T`. [heading Variadic macro usage] Having the ability to use variadic macros makes the syntax for using BOOST_TTI_TEMPLATE easier to specify in both the `template type parameters` form and the `specific parameters` form of using the macro. This is because variadic macros can take a variable number of parameters. When using the variadic macro form the first macro parameter is always the name of the class template you are trying to introspect. You only specify further parameters when using the `specific parameters` form of the macro, in which case the further parameters to the macro are the specific template parameters. Introspecting the first class template above using the `template type parameters` form the variadic macro would be: BOOST_TTI_TEMPLATE(AClassTemplate) Introspecting the other class templates above using the `specific parameters` form the variadic macros would be: BOOST_TTI_TEMPLATE(BClassTemplate,class,template class, int) BOOST_TTI_TEMPLATE(CClassTemplate,void (*)(int,long)) BOOST_TTI_TEMPLATE(DClassTemplate,template class) Here we have no problem with counting the number of tuple tokens for the Boost PP array, nor do we have to specify BOOST_PP_NIL if we are using the `template type parameters` form. Also for the specific parameters form we simply use the template parameters as the remaining tokens of the variadic macro. [heading The resulting metafunction] Using either form of the macro, whether using variadic or non-variadic syntax, the macro generates a metafunction called "has_template_'name_of_inner_class_template'". The metafunction can be invoked by passing it the enclosing type to introspect. 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 is true or false depending on whether the inner class template exists or not. [endsect] [section:tti_detail_has_template_metafunction Using the has_template_(xxx) metafunction] [heading Generating the metafunction] You generate the metafunction by invoking the macro with the name of an inner class template: // `template type parameters` form BOOST_TTI_HAS_TEMPLATE(AClassTemplate,BOOST_PP_NIL) // non-variadic macro BOOST_TTI_HAS_TEMPLATE(AClassTemplate) // variadic macro // `specific parameters` form BOOST_TTI_HAS_TEMPLATE(AClassTemplate,(2,(class,int))) // non-variadic macro BOOST_TTI_HAS_TEMPLATE(AClassTemplate,class,int) // variadic macro generates a metafunction called 'has_template_AClassTemplate' in the current scope. If you want to introspect the same class template name using both the `template type parameters` form and the `specific parameters` form you will have the problem that you will be generating a metafunction of the same name and violating the C++ ODR rule. In this particular case you can use the alternate BOOST_TTI_TRAIT_HAS_TEMPLATE macro to name the particular metafunction which will be generated. [heading Invoking the metafunction] You invoke the metafunction by instantiating the template with an enclosing type to introspect. A return value called 'value' is a compile time bool constant. has_template_AType::value [heading Examples] First we generate metafunctions for various inner class template names: #include // Using variadic macro, `template type parameters` BOOST_TTI_HAS_TEMPLATE(Template1) BOOST_TTI_HAS_TEMPLATE(Template2) BOOST_TTI_HAS_TEMPLATE(Template3) BOOST_TTI_HAS_TEMPLATE(Template4) BOOST_TTI_HAS_TEMPLATE(Template5) // or using non-variadic macro, `template type parameters` BOOST_TTI_HAS_TEMPLATE(Template1,BOOST_PP_NIL) BOOST_TTI_HAS_TEMPLATE(Template2,BOOST_PP_NIL) BOOST_TTI_HAS_TEMPLATE(Template3,BOOST_PP_NIL) BOOST_TTI_HAS_TEMPLATE(Template4,BOOST_PP_NIL) BOOST_TTI_HAS_TEMPLATE(Template5,BOOST_PP_NIL) // Using variadic macro, `specific parameters` BOOST_TTI_HAS_TEMPLATE(Template6,class,int) BOOST_TTI_HAS_TEMPLATE(Template7,typename,template struct,long) BOOST_TTI_HAS_TEMPLATE(Template8,double,typename) BOOST_TTI_HAS_TEMPLATE(Template9,typename,class,typename,class,typename,short) // or using non-variadic macro, `specific parameters` BOOST_TTI_HAS_TEMPLATE(Template6,(2,(class,int))) BOOST_TTI_HAS_TEMPLATE(Template7,(4,(typename,template struct,long))) BOOST_TTI_HAS_TEMPLATE(Template8,(2,(double,typename))) BOOST_TTI_HAS_TEMPLATE(Template9,(6,(typename,class,typename,class,typename,short))) Next let us create some user-defined types we want to introspect. struct Top { template struct Template1 { }; template class Template2 { }; template class Template3 { }; }; struct Top2 { template class Template3 { }; template struct Template4 { }; template class Template5 { }; }; struct Top3 { template struct Template6 { }; template struct B,long C> class Template7 { }; }; struct Top4 { template struct Template8 { }; template class Template9 { }; }; Finally we invoke our metafunction and return our value. This all happens at compile time, and can be used by programmers doing compile time template metaprogramming. has_template_Template1::value; // true has_template_Template1::value; // false has_template_Template2::value; // true has_template_Template2::value; // false has_template_Template3::value; // false, not all typename/class template parameters has_template_Template3::value; // true has_template_Template4::value; // false has_template_Template4::value; // true has_template_Template5::value; // false has_template_Template5::value; // true has_template_Template6::value; // true has_template_Template6::value; // false has_template_Template7::value; // true has_template_Template7::value; // false has_template_Template8::value; // false has_template_Template8::value; // true has_template_Template9::value; // false has_template_Template9::value; // true [heading Metafunction re-use] The macro encodes the name of the inner class template for which we are searching, the fact that we are introspecting for a class template within an enclosing type, and optionally the template parameters for that class template. Once we create our metafunction for introspecting an inner class template by name, we can reuse the metafunction for introspecting any enclosing type, having any inner class template, for that name. However we need to understand that we are restricted in our reuse of the metafunction by whether we originally use the template type parameters form or the specific form. In either case we are always introspecting an inner class template which matches that form. In the case of the template type parameters form, any inner class template for which we are introspecting must have all template type parameters, as well as the correct name. In the case of the specific parameters form, any inner class template for which we are introspecting must have template parameters which match the specific template parameters passed to the macro, as well as the correct name. [endsect] [endsect]