Overload function for both arrays-of-const and pointers-to-const

Question

I can capture an array along with its (compile-time) size using a template function like so:

template<int N>
void foo(const int (&)[N]) {
    std::cout << "foo(const int (&)[N])\n";
}

However, I'd like to overload foo to also allow pointers-to-const, so that the first overload is used when the function is called on an array type, and the second one when it's called on a pointer directly.

void foo(const int *) {
    std::cout << "foo(const int *)\n";
}

int main() {
    int a[1] = { 0 };
    foo(a);
    const int b[1] = { 0 };
    foo(b);
}

Try it on ideone

Here, the first overload is called for a, the second one for b.

My guess is that for a, the compiler must perform a conversion-to-const which means that foo(const int *) is not a perfect match, but I'm lost to why this is not even an ambigous function call.

How can I change the code so that the first overload is called in both cases?

Answer 1

Why it doesn't work

In your example both overloads are considered by the compiler matches. Here comes overload resolution to the rescue to eliminate one and keep the best match. According to the draft standard N4527 13.3.3/1.6 Best viable function [over.match.best]:

F1 is not a function template specialization and F2 is a function template specialization

In our case F1 is void foo(const int *) and F2 is template<int N> void foo(const int (&)[N]). Thus, F1 will be prefered over F2 because F1 is not a template specialization while F2 is.

Solution

Pass the pointer by reference in the second overload:

void foo(const int *&) {
    std::cout << "foo(const int *)\n";
}

LIVE DEMO

Why the suggested solution works

Now as dyp already mentioned in the comments, if you pass the pointer by reference as shown earlier above, this match is broken because const int *& cannot match to int* neither to int[N].