In C++ How do I template the return value to be unique from the parameter value?

Question

In C++ How do I template the return value to be unique from the parameter value? My code looks as follows - now if data type of number is int going in I want it as a double going out.

template <class T>
T divide(T number)
 {
    return number/10;
 }

in main I have this

 divide(5);
 divide(2.5);

In the case of 2.5 the value going in and out will be type double so there isn't a problem but in the case of 5 it goes in as an int but I need it to not be truncated and returned as type double.

template <class T, Class T1>
T1 divide(T number)
 {
    return number/10;
 }

That doesn't work because its looking for two parameters in the functions.

T1 template <class T>
T1 divide(T number)
 {
    return number/10;
 }

this gives me no declaration of storage class or type specifier - when I add the word class to that line before T1 I get the error message: function returns incomplete type.

Answer 1

The most generic method is to have a helper template

template <typename T>
struct calc_return {
  using type = T;
};

template <class T>
typename calc_return<T>::type divide(T number)
{
   return number/10;
}

Now if there are exceptions to the rule, you specialize calc_return using whatever logic you'd like:

// Prior to the declaration of divide()
template <>
struct calc_return<int> {
  using type = double;
};