CHECK MEMBER TYPE

检查类里是否存在某种类型的几种方法，以检查xxx类型为例：
方法1：

template<class T>
class has_member_type_Type
{
    struct big { char a[]; };
    template<class C> static big  probe(typename C::xxx*); // match here if type T::Type exists
    template<class C> static char probe(...);
public:
    static const bool value = sizeof(probe<T>(nullptr)) > ;
};

方法2：

template<typename T, typename = typename T::xxx>
static std::true_type has_xxx_impl(int);

template<typename T>
static std::false_type has_xxx_impl(...);

template<typename T>
struct has_xxx : decltype(has_xxx_impl<T>())
{
};

方法3：

template<typename... Ts> struct make_void { typedef void type; };
template<typename... Ts> using void_t = typename make_void<Ts...>::type;

template<typename T, typename = void>
struct has_yyy : std::false_type
{
};

template<typename T>
struct has_yyy < T, void_t<typename T::xxx>> : std::true_type
{
};

//用宏简化
#define HAS_TYPE_MEMBER(MEMBER)\
    template<typename T, typename = void>\
    struct has_type_##MEMBER : std::false_type\
    {\
    };\
    template<typename T>\
    struct has_type_##MEMBER < T, void_t<typename T::MEMBER>>:\
    std::true_type\
    {\
    };

    HAS_TYPE_MEMBER(xxx)

测试代码：

struct MyStruct
{
    typedef char xxx;
};

int main()
{
    static_assert(has_xxx<MyStruct>::value, "no xxx");
    static_assert(has_type_xxx<MyStruct>::value, "no xxx");
}