Specialize one function?

July 22, 2023

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 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 and 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 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();
}

>Solution :

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();
}