Question

I have a template class. The class has a private member variable which I like to have preset to a certain value which differs for each template type.

I was thinking about different constructors for different types, but since the constructor has no parameters I have no idea how to do it.

Is it possible anyway ?

Thanks,

Answer 1

You can do it with a specialization on the type, the easiest form is:

#include <iostream>

template <typename T>
struct make {
  static int value() {
    return -1; // default
  }
};

template <>
struct make<int> {
  static int value() {
    return 1; 
  }
};

template <>
struct make<double> {
  static int value() {
    return 2; 
  }
};

template <typename T>
struct foo {
  const int val;
  foo() : val(make<T>::value()) {}
};

int main() {
  std::cout << foo<int>().val << ", " << foo<double>().val << "\n";
}

But you could also arrange it as an overload:

#include <iostream>

int value(void *) {
  return -1; // default
}

int value(double *) {
  return 2;
}

int value (int *) {
  return 1;
}

template <typename T>
struct foo {
  const int val;
  foo() : val(value(static_cast<T*>(nullptr))) {}
};

int main() {
  std::cout << foo<int>().val << ", " << foo<double>().val << "\n";
}