template overloading plus pointer to pointer

Question

I have the following code:

template<class A, class B>
void test(A& a, const B* b)
{ std::cout << "hi" << std::endl; }

template<class A, class B>
void test(A& a, const B** b)
{ std::cout << "hello" << std::endl; }

class TestClass
{};


int main()
{
    int a = 5;
    TestClass b;
    TestClass* c = &b;
    test(a, &c);
    return 0;
}

Somehow the output is "hi" although it seems that better match would be the second template function. When I remove consts as the qualifiers to B* and B** then I get "hello" which corresponds to the second template function. How does the compiler choose the function to call in this case? Thanks!

Answer 1

Given that there is no conversion from T** to T const** the second isn't a match at all (there is no such conversion because it would allow nonconst access to a const object). There is a conversion from T** to T* const*, however. Thus, the corresponding overload is the only viable and used one.

Answer 2

&c is a TestClass** which may be promoted to TestClass* const * but not to const TestClass**.

You may force the error by explicitly using test<int, TestClass>(a, &c); which will show you the impossible conversion.