The following code example is taken from the book
C++ Templates - The Complete Guide
by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
© Copyright David Vandevoorde and Nicolai M. Josuttis 2002
template<typename T>
class IsFunctionT {
private:
typedef char One;
typedef struct { char a[2]; } Two;
template<typename U> static One test(...);
template<typename U> static Two test(U (*)[1]);
public:
enum { Yes = sizeof(IsFunctionT<T>::test<T>(0)) == 1 };
enum { No = !Yes };
};
template<typename T>
class IsFunctionT<T&> {
public:
enum { Yes = 0 };
enum { No = !Yes };
};
template<>
class IsFunctionT<void> {
public:
enum { Yes = 0 };
enum { No = !Yes };
};
template<>
class IsFunctionT<void const> {
public:
enum { Yes = 0 };
enum { No = !Yes };
};
// same for void volatile and void const volatile
//...
template<typename T>
class CompoundT { // primary template
public:
enum { IsPtrT = 0, IsRefT = 0, IsArrayT = 0,
IsFuncT = IsFunctionT<T>::Yes,
IsPtrMemT = 0 };
typedef T BaseT;
typedef T BottomT;
typedef CompoundT<void> ClassT;
};