How can I declare my return type from generic function in generic class-CodePudding

I have generic class in c

template<typename A>
struct  series { 

A member;
}

template<typename A,typename B,typename C>
series <C> & operator   (A first, const B& second) {
return first second;
}

I need to return type that is implicit conversion of type A and type B. Fox example : double a int b → implicit conversion is double how can I do that, because C cannot be determined like this I was trying something with decltype and typedef but it didn't work Thanks in advance

CodePudding user response：

The standard library provides a number of templates for situations like this. For the moment, let's deal with the really simple case:

template <class T, class U>
returnType operator (T t, U u) {
    return t   u;
}

So what should we use for returnType? In this case, we can use std::common_type, which tells us the common type to which both its operand types will be converted when used in an expression together.

template <class T, class U>
typename std::common_type<T, U>::type add(T t, U u) {
    return t   u;
}

So, if we passed this an int and a double, the return type would be double.

In most cases, the result type is pretty obvious, but in a few cases, the result can be...interesting, to put it mildly. For example, on an implementation that used 16 bits for both short and int (like many MS-DOS compilers did), it's entirely possible that unsigned short int could have a common type of long.

CodePudding user response：

You can use C 11 trailing return types with decltype:

template<typename A,typename B>
auto operator (A first, const B& second) -> decltype(first second) {
  return first second;
}