Why does it work? SFINAE [closed]

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and c开发者_运维百科annot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center. Closed 11 years ago.

template<class T, class U>
struct is_convertible
{
    typedef char yes;
    typedef struct
    {char _[2];}no;

    static yes test(U);
    static no test(...);
    enum {value = (sizeof(test(0)) == sizeof(yes)) ? 1 : 0};
//THE PART I'M INTERESTED IN IS (test(0)). Why 0 (zero) works here?

};

Please see comment in the code.

---

Code "works" when it meets its specification.

This code does not meet the specification implied by the function name, and no clearer specification has been given.

Currently, the code yields is_convertible<T, U>::value true when U is copyable and an implicit conversion exists in the context of struct is_convertible from int or any pointer to U, may not compile if U is not copy-constructible, and false otherwise.

The assumed specification, based on a combination of the name and existing code, is that is_convertible<T, U>::value should be true if U is copy-constructible (in the context of struct is_convertible) and a value of type T is implicitly convertible (in the context of struct is_convertible) to U.

A slight modification is required to make the code meet the implied specification:

enum {value = (sizeof(test(*(T*)0)) == sizeof(yes)) ? 1 : 0};