Question

I have this function template

template < class TParam >
bool greaterThan (TParam A, TParam B) {
  if (typeid(TParam) == typeid(string)) {   
    return A.compare(B) > 0;   <--------------- Error
  } else {
    return A > B;
  }
}

But the compiler won’t allow me to use this with an int.

I get a compiler error in the place shown above that says

Member reference base type 'int' is not a structure or union.

The if statement does not run if I’m calling the function on an int.

I commented it out and checked. I don’t know whats wrong.

Answer 1

When TParam is int, with this:

A.compare(B)

You’re trying to call the compare method of an int. Such a method does not exist. What you need is template specialisation so that you can make the template do something special when the type is a certain type:

template<class TParam>
bool greaterThan(TParam A, TParam B) {
    return A > B; // this will be called for any type except string because of
                  // our following specialisation
}

// make a specialisation for when the template parameter is string
template<>
bool greaterThan<string>(string A, string B) { 
   return A.compare(B) > 0;
}

The syntax is a tiny bit foreign but if you read up on it, template specialisation is a very powerful addition to templates.

Note that string does have operator< so you don’t even need to specialise it if you don’t want to, but this is a good opportunity to learn about template specialisation.