c++ template 判断是否为类类型

/* The following code example is taken from the book
* "C++ Templates - The Complete Guide"
* by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
*
* (C) Copyright David Vandevoorde and Nicolai M. Josuttis 2002.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include <iostream>
 
template<typename T>
class IsClassT {
private:
    typedef char One;
    typedef struct { char a[2]; } Two;
    template<typename C> static One test(int C::*);
    template<typename C> static Two test(...);
public:
    enum { Yes = sizeof(test<T>(0)) == 1 };
    enum { No = !Yes };
};
 
 
class MyClass {
};
 
struct MyStruct {
};
 
union MyUnion {
};
 
void myfunc()
{
}
 
enum E { e1 } e;
 
// check by passing type as template argument
template <typename T>
void check()
{
    if (IsClassT<T>::Yes) {
        std::cout << " IsClassT " << std::endl;
    }
    else {
        std::cout << " !IsClassT " << std::endl;
    }
}
 
// check by passing type as function call argument
template <typename T>
void checkT(T)
{
    check<T>();
}
 
int main()
{
    std::cout << "int:     ";
    check<int>();
 
    std::cout << "MyClass: ";
    check<MyClass>();
 
    std::cout << "MyStruct:";
    MyStruct s;
    checkT(s);
 
    std::cout << "MyUnion: ";
    check<MyUnion>();
 
    std::cout << "enum:    ";
    checkT(e);
 
    std::cout << "myfunc():";
    checkT(myfunc);
}