C++ What will ...
What will be printed out as a result of the following code execution? 
template<class T, class U>
struct A{
	struct dummy{char _[2];};
	static char _(T);
	static dummy _(...);
	static const bool value=sizeof(_(U()))==sizeof(char);
};
struct B{};
struct C:B{};
...
cout << A<B,C>::value << A<B,B>::value << A<C,B>::value;
Explanation
If the class U is cast to type T (that is, it is the same class, it is as an inheritor, it provides a cast operator to type T or there is another trivial way to cast to T), the function _(T) will be called, otherwise _(...) will be called which accepts any number of parameters of any type.
After that, sizeof determines the size of the object returned by the called function.
template data-type-conversion type parameter struct class

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