（一）boost::lambda源码分析

// is_instance_of_n<A, B>::value is true, if type A is
// an instantiation of a template B, or A derives from an instantiation
// of template B
//
// n is the number of template arguments for B
//
// Example:
// is_instance_of_2<std::istream, basic_stream>::value == true

#ifndef BOOST_LAMBDA_IS_INSTANCE_OF #define BOOST_LAMBDA_IS_INSTANCE_OF #include "boost/config.hpp" // for BOOST_STATIC_CONSTANT #include "boost/type_traits/conversion_traits.hpp" // for is_convertible #include "boost/preprocessor/enum_shifted_params.hpp" #include "boost/preprocessor/repeat_2nd.hpp" // is_instance_of -------------------------------- // // is_instance_of_n<A, B>::value is true, if type A is // an instantiation of a template B, or A derives from an instantiation // of template B // // n is the number of template arguments for B // // Example: // is_instance_of_2<std::istream, basic_stream>::value == true // The original implementation was somewhat different, with different versions // for different compilers. However, there was still a problem // with gcc.3.0.2 and 3.0.3 compilers, which didn't think regard // is_instance_of_N<...>::value was a constant. // John Maddock suggested the way around this problem by building // is_instance_of templates using boost::is_convertible. // Now we only have one version of is_instance_of templates, which delagate // all the nasty compiler tricks to is_convertible. //BOOST_PP_COMMA_IF(N) 如果 cond 展开为 0, 则该宏展开为空。否则展开为一个逗号。 #define BOOST_LAMBDA_CLASS(z, N,A) BOOST_PP_COMMA_IF(N) class #define BOOST_LAMBDA_CLASS_ARG(z, N,A) BOOST_PP_COMMA_IF(N) class A##N #define BOOST_LAMBDA_ARG(z, N,A) BOOST_PP_COMMA_IF(N) A##N #define BOOST_LAMBDA_CLASS_LIST(n, NAME) BOOST_PP_REPEAT(n, BOOST_LAMBDA_CLASS, NAME) #define BOOST_LAMBDA_CLASS_ARG_LIST(n, NAME) BOOST_PP_REPEAT(n, BOOST_LAMBDA_CLASS_ARG, NAME) #define BOOST_LAMBDA_ARG_LIST(n, NAME) BOOST_PP_REPEAT(n, BOOST_LAMBDA_ARG, NAME) //假设 INDEX = 3， 下面的宏可以展示为： // template< template<class,class, class> class F> // struct conversion_tester_3{ // template<A0,A1,A2> // conversion_tester_3(const F<A0,A1,A2>); // }; // // template<class From, template<class, class, class> class To> // struct is_instance_of_3 // { // private: // typedef ::boost::is_convertible< // From, // detail::conversion_tester_3<To> // >helper_type; // public: // BOOST_STATIC_CONSTANT(bool, value = helper_type::value); // }; // template <class From, class To> // struct is_convertible : public true_type-or-false_type {}; // // // 继承：如果一个类型为 From 的 imaginary lvalue（假想左值）可以转换为类型 To，则从 true_type 继承，否则从 false_type 继承。 // is_convertible<int, double> 从 true_type 继承。 // // is_convertible<const int, double>::type 是 true_type 类型。 // // is_convertible<int* const, int*>::value 是一个求值为 true 的 integral constant expression（整常表达式）。 // // is_convertible<int const*, int*>::value 是一个求值为 false 的 integral constant expression（整常表达式）：这个转换需要一个 const_cast。 // // is_convertible<int const&, long>::value 是一个求值为 true 的 integral constant expression（整常表达式）。 // // is_convertible<int>::value 是一个求值为 false 的 integral constant expression（整常表达式）。 // // is_convertible<T>::value_type 是 bool 类型。 namespace boost { namespace lambda { #define BOOST_LAMBDA_IS_INSTANCE_OF_TEMPLATE(INDEX) / / namespace detail { / / template <template<BOOST_LAMBDA_CLASS_LIST(INDEX,T)> class F> / struct BOOST_PP_CAT(conversion_tester_,INDEX) { / template<BOOST_LAMBDA_CLASS_ARG_LIST(INDEX,A)> / BOOST_PP_CAT(conversion_tester_,INDEX) / (const F<BOOST_LAMBDA_ARG_LIST(INDEX,A)>&); / }; / / } /* end detail */ / / template <class From, template <BOOST_LAMBDA_CLASS_LIST(INDEX,T)> class To> / struct BOOST_PP_CAT(is_instance_of_,INDEX) / { / private: / typedef ::boost::is_convertible< / From, / BOOST_PP_CAT(detail::conversion_tester_,INDEX)<To> / > helper_type; / / public: / BOOST_STATIC_CONSTANT(bool, value = helper_type::value); / }; #define BOOST_LAMBDA_HELPER(z, N, A) BOOST_LAMBDA_IS_INSTANCE_OF_TEMPLATE( BOOST_PP_INC(N) ) // Generate the traits for 1-4 argument templates BOOST_PP_REPEAT_2ND(4,BOOST_LAMBDA_HELPER,FOO) //BOOST_LAMBDA_HELPER(z,0,FOO), //BOOST_LAMBDA_HELPER(z,1,FOO), //BOOST_LAMBDA_HELPER(z,2,FOO), //BOOST_LAMBDA_HELPER(z,3,FOO) //这样展开为： //BOOST_LAMBDA_IS_INSTANCE_OF_TEMPLATE( 1 ), //BOOST_LAMBDA_IS_INSTANCE_OF_TEMPLATE( 2 ), //BOOST_LAMBDA_IS_INSTANCE_OF_TEMPLATE( 3 ), //BOOST_LAMBDA_IS_INSTANCE_OF_TEMPLATE( 4 ) #undef BOOST_LAMBDA_HELPER #undef BOOST_LAMBDA_IS_INSTANCE_OF_TEMPLATE #undef BOOST_LAMBDA_CLASS #undef BOOST_LAMBDA_ARG #undef BOOST_LAMBDA_CLASS_ARG #undef BOOST_LAMBDA_CLASS_LIST #undef BOOST_LAMBDA_ARG_LIST #undef BOOST_LAMBDA_CLASS_ARG_LIST } // lambda } // boost #endif