Function overloading using template

Question

Here are two overloading declarations of a function:

void fun(char& arg);
void fun(int&  arg);
void fun(long& arg);

The definitions are doing the same job:

void fun(char& arg) { ++arg; }
void fun(int&  arg) { ++arg; }
void fun(long& arg) { ++arg; }

How to declare and define the function once by using template, which accepts only int, char and long types for the argument? Error should appear as soon as possible (before runtime) if the function is misused (e.g. a variable of double type is passed).

Answer 1

You can just static_assert in the body of the function with a bunch of std::is_same

template<typename T>
void fun(T& arg) 
{ 
    static_assert(
        std::is_same_v<T, char> || 
        std::is_same_v<T, int> || 
        std::is_same_v<T, long>, 
        "bad call"); 
    ++arg; 
}

Answer 2

You can apply SFINAE with std::enable_if and std::is_same.

template <typename T>
std::enable_if_t<std::is_same_v<T, int> || 
                 std::is_same_v<T, char> || 
                 std::is_same_v<T, long>> 
fun(T& arg) { ++arg; }

For other types you'll get a no matching function for call error.