Question

#include <iostream>

template<typename T, int = 0>
struct test {
    typedef char type[3];
};

template<int N>
struct test<long *, N> {
    typedef char type[7];
};

int main()
{
    std::cout << sizeof( test<int*>::type ) << std::endl; // 3
    std::cout << sizeof( test<long*>::type ) << std::endl; // 7
    return 0;
}

I expected sizeof( test<long*>::type ) == 3. Why it is 7?

Answer 1

What you did is you specialized the template, saying that whenever the type argument is long*, you should use the second form of the template. For other arguments, it will use the initial form of the template (with size 3).