Nested template ambiguity

Question

Nested templates require the usage of template keyword, as explained in this answer. Here's a concise demonstration that results in a compiler error:

template <int T>
struct A {
  template <int U>
  static void b(int x) {
  }
};

template <int T>
void test() {
  A<T>::b<T>(T); // Should be:
  // A<T>::template b<T>(T);
}

int main() {
  test<0>();
}

As a challenge, I was trying to think of a program where two possibilities (with or without template) are both valid but have two different meanings, but I couldn't wrap my head around the operator overloading business. Is it possible to make A<T>::b<T>(T); a valid statement?

score -1 · Answer 1 · edited Sep 27 '19 at 02:40

If you change your struct A for:

template <int T>
struct A {
    static const int b = 5;
};

Then A<T>::b<T>(T); would compile.

In that case, A<T>::b would be 5.
T is 0.
Thus 5 < 0 is false (0)
0 > (0) is false too;

If you replace the code with something like:

auto x = A<T>::b<T>(T);
std::cout << typeid(x).name() << " " << x;

You will get:

bool 0

Thus it was not too hard to modify A so that A<T>::b<T>(T) is a valid statement.

Nested template ambiguity

1 Answers1