C template default argument based on next argument-CodePudding

I need to do something like this:

template<class A=B, class B>
    A fn(B x) { return A(x); };

int main()
{
    int i = fn(5); // Error, no matching overload found
    double d = fn<double>(5);
};

Hence, a function template which deduces the types automatically from the function arguments, but the caller can change the first one if needed. Any way to do this?

CodePudding user response：

If you want the default value of the fn()'s first template parameter to be the second parameter, you can add another template specialization.

template <class Ret, class Arg>
Ret fn(Arg x) {
  return Ret(x);
};

template <class Arg>
Arg fn(Arg x) {
  return fn<Arg, Arg>(x);
};

int main() {
  int i    = fn(5);         // ==>  fn<int, int>()
  double d = fn<double>(5); // ==>  fn<double, int>()
};

CodePudding user response：

You can simply use constexpr if in such cases like:

struct NO_TYPE;

template<class RET_TYPE = NO_TYPE, class IN_TYPE>
    auto fn(IN_TYPE x)
{
    if constexpr ( std::is_same_v< RET_TYPE, NO_TYPE> )
    {    
        std::cout << "1" << std::endl;
        return IN_TYPE(x);
    }    
    else 
    {    
        std::cout << "2" << std::endl;
        return RET_TYPE(x);
    }    
}

int main()
{
    int i = fn(5); // Error, no matching overload found
    double d = fn<double>(5);
};

In hope I did not misunderstand your question.