| |
| // (C) Copyright Tobias Schwinger |
| // |
| // Use modification and distribution are subject to the boost Software License, |
| // Version 1.0. (See http://www.boost.org/LICENSE_1_0.txt). |
| |
| //------------------------------------------------------------------------------ |
| // |
| // This example implements a utility to accept a type expression, that may |
| // contain commas to a macro. |
| // |
| // |
| // Detailed description |
| // ==================== |
| // |
| // Accepting a type as macro argument can cause problems if the type expression |
| // contains commas: |
| // |
| // #define MY_MACRO(a_type) |
| // ... |
| // MY_MACRO(std::map<int,int>) // ERROR (wrong number of macro arguments) |
| // |
| // This problem can be solved by pasing using a parenthesized type |
| // |
| // MY_MACRO((std::map<int,int>) // OK |
| // |
| // but then there is no way to remove the parentheses in the macro argument |
| // with the preprocessor. |
| // We can, however, form a pointer to a function with a single argument (the |
| // parentheses become part of the type) and extract the argument with template |
| // metaprogramming: |
| // |
| // // Inside the macro definition |
| // |
| // typename mpl::front< parameter_types<void(*)a_type> >::type |
| // |
| // This code snippet does not read too expressive so we use another macro |
| // to encapsulate the solution: |
| // |
| // // Inside the macro definition |
| // |
| // BOOST_EXAMPLE_MACRO_TYPE_ARGUMENT(a_type) |
| // |
| // As a generalization of this technique we can accept a comma-separated list of |
| // types. Omitting the mpl::front invocation gives us an MPL-sequence. |
| // |
| // |
| // Limitations |
| // =========== |
| // |
| // - only works for types that are valid function arguments |
| // |
| // Acknowledgments |
| // =============== |
| // |
| // Thanks go to Dave Abrahams for letting me know this technique. |
| |
| #ifndef BOOST_EXAMPLE_MACRO_TYPE_ARGUMENT_HPP_INCLUDED |
| #define BOOST_EXAMPLE_MACRO_TYPE_ARGUMENT_HPP_INCLUDED |
| |
| #include <boost/function_types/parameter_types.hpp> |
| #include <boost/mpl/front.hpp> |
| |
| #define BOOST_EXAMPLE_MACRO_TYPE_ARGUMENT(parenthesized_type) \ |
| boost::mpl::front< \ |
| BOOST_EXAMPLE_MACRO_TYPE_LIST_ARGUMENT(parenthesized_type) >::type |
| |
| #define BOOST_EXAMPLE_MACRO_TYPE_LIST_ARGUMENT(parenthesized_types) \ |
| ::boost::function_types::parameter_types< void(*) parenthesized_types > |
| |
| |
| #endif |
| |
