Defining a Function Template Specialization

When we specialize a function template, we must supply arguments for every template parameter in the original template. To indicate that we are specializing a template, we use the keyword template followed by an empty pair of angle brackets (< >). The empty brackets indicate that arguments will be supplied for all the template parameters of the original template:

// special version of compare to handle pointers to character arrays
template <>
int compare(const char* const &p1, const char* const &p2)
{
    return strcmp(p1, p2);
}

The hard part in understanding this specialization is the function parameter types. When we define a specialization, the function parameter type(s) must match the corresponding types in a previously declared template. Here we are specializing:

template <typename T> int compare(const T&, const T&);

in which the function parameters are references to a const type. As with type aliases, the interaction between template parameter types, pointers, and const can be surprising (§ 2.5.1, p. 68).

We want to define a specialization of this function with T as const char*. Our function requires a reference to the const version of this type. The const version of a pointer type is a constant pointer as distinct from a pointer to const (§ 2.4.2, p. 63). The type we need to use in our specialization is const char* const &, which is a reference to a const pointer to const char.