The template class defines its member type as a synonym for the return type of a function call described by its template argument Ty. The template argument must be of the form Fty(T1, T2, ..., TN), where Fty is a callable type. The template determines the return type according to the first of the following rules that applies:

• if Fty is a pointer to function type R(*)(U1, U2, ..., UN) the return type is R;

• 如果Fty是个R(*)(U1, U2, ..., UN) 函数指针，那么返回类型是R

• if Fty is a reference to function type R(&)(U1, U2, ..., UN) the return type is R;

• 如果Fty是个R(&)(U1, U2, ..., UN) 函数指针，那么返回类型是R

• if Fty is a pointer to member function type R(U1::*)(U2, ..., UN) the return type is R;

• 如果Fty是个R(U1::*)(U2, ..., UN)成员函数指针，那么返回类型是R

• if Fty is a pointer to data member type R U1::* the return type is R;

• 如果Fty是个R U1::* 成员变量指针，那么返回类型是R

• if Fty is a class with a member typedef result_type the return type is Fty::result_type;

• 如果Fty是一个类并且其中定义了result_type ，那么返回类型是Fty::result_type

• if N is 0 (that is, Ty is of the form Fty()) the return type is void;

• 如果Fty函数不带有任何的参数，那么返回类型是void

• if Fty is a class with a member template named result the return type is Fty::result<T1, T2, ..., TN>::type;

• 如果Fty是个类并且有个成员模板是result,那么返回类型是Fty::result<T1, T2, ..., TN>::type

• in all other cases it is an error.

• 其它情况就会出错