Specialize one function?

Question

There's two things I'd like to specialize. An int, and any type of pointer. But let's stick with int for now.

In the below code, if T is an int, I'd like a different implementation of Test::a() which returns 0. I tried:

template<>
int a() { return 0; }

but that's obviously wrong, as I got a compile error suggesting what I wrote made no sense.

Is there a way I can do this? Do I need to specialize all of Test if I want to do something like this?

My real class is a few hundred lines, so I would rather only reimplement the one or two functions, if possible.

template<class T>
class Test{
    T*t;
public:
    Test(T*t) : t(t) {}

    T a() { return 1; }
    T b() { return 2; }
    //if T == int
    //int a() { return 0; }
};

int main(int argc, char *argv[])
{
    Test<int> t(0);
    return t.a();
}

Answer 1

Specialization is arguably overkill for this sort of problem.

You can use constexpr if (which is spelled if constexpr) to achieve this just as efficiently as specialization would, in a single function.

#include <type_traits>

template<class T>
class Test{
    T*t;
public:
    Test(T*t) : t(t) {}

    T a() {
      if constexpr ( std::is_same_v<T,int> ) {
        return 0;
      } else {
        return 1;
      }
    }
    T b() { return 2; }
};

int main(int argc, char *argv[])
{
    Test<int> t(0);
    return t.a();
}