Getting confused with C++ template

Question

I am looking at some c++ code and do not understand the purpose of the template declaration in this situation:

template<> void operator>>(const ClassA& s, const ClassB d) {...}

What is the semantic of template<>?

Answer 1

This is, indeed, template specialization, as others before mentioned. There must be some previously declared function template, such as:

template<typename T, typename U>
void operator>>(const T& s, const U d) {...}

However, it is quite misguided. It is much better to remove the template<> altogether, so operator>> would just be overloaded. The problem with function template specialization is that it may lead to unexpected behaviour in the presence of overloaded functions (and operator>> has lots of overloads), since the specialization does not overload. This means that the compiler first selects the most appropriate overload for the function and then, if the selected overload is a function template, it looks for template specializations to see if there is an appropriate one.

A classical example (unfortunately, I don't remember where I read it). Consider this overloaded function template:

template <typename T>
void Function(T param);

template <typename T>
void Function(T* param);

template <>
void Function(int* param);

main()
{
  int i = 5;
  Function(&i);
}

As expected, the template specialization for int* is called. But just change the order of the function definitions:

template <typename T>
void Function(T param);

template <>
void Function(int* param);

template <typename T>
void Function(T* param);

main()
{
  int i = 5;
  Function(&i);
}

Now the general template for T* is called, since we are specializing the template for T, not for T*, and this second template is better suited for our call. This would be avoided if we overloaded the function instead of specializing the template:

void Function(int* param);

Now the order of declaration does not matter, we will always call the overload for int*.

UPDATE: Now I know who to credit. I read about this in an article by Herb Sutter. The example was provided by Peter Dimov and Dave Abrahams.