type_traits

#include <iostream>namespace namespace269 {        /** 2两个空白类,没有任何成员,不会带来额外负担,却能通过类型代表真假,可以用于函数特化参数 */    struct __true_type { };    struct __false_type { };        template <class _Tp>    struct __type_traits {        typedef __true_type this_dummy_member_must_be_first;              typedef __false_type has_trivial_default_constructor;        typedef __false_type has_trivial_copy_constructor;        typedef __false_type has_trivial_assignment_operator;        typedef __false_type has_trivial_destructor;        typedef __false_type is_POD_type;    };}template <class T>void test_tpye_traits(T& t, namespace269::__true_type) {    std::cout << "true" << std::endl;}template <class T>void test_tpye_traits(T& t, namespace269::__false_type) {    std::cout << "false" << std::endl;}template <typename T>void test_tpye_traits(T& t) {    typedef typename namespace269::__type_traits<T>::is_POD_type is_POD_type;        test_tpye_traits(t, typename namespace269::__type_traits<T>::is_POD_type());}class A {public:};class B {};namespace namespace269 {    template <>    struct __type_traits<A> {        typedef __true_type is_POD_type;    };    template <>    struct __type_traits<B> {        typedef __false_type is_POD_type;    };}int main(void) {    A a;    B b;    test_tpye_traits(a);   // true    test_tpye_traits(b);   // false    return 0;}