Partial template specialization of free functions - best practices
- by Poita_
As most C++ programmers should know, partial template specialization of free functions is disallowed. For example, the following is illegal C++:
template <class T, int N>
T mul(const T& x) { return x * N; }
template <class T>
T mul<T, 0>(const T& x) { return T(0); }
// error: function template partial specialization ‘mul<T, 0>’ is not allowed
However, partial template specialization of classes/structs is allowed, and can be exploited to mimic the functionality of partial template specialization of free functions. For example, the target objective in the last example can be achieved by using:
template <class T, int N>
struct mul_impl
{
static T fun(const T& x) { return x * N; }
};
template <class T>
struct mul_impl<T, 0>
{
static T fun(const T& x) { return T(0); }
};
template <class T, int N>
T mul(const T& x)
{
return mul_impl<T, N>::fun(x);
}
It's more bulky and less concise, but it gets the job done -- and as far as users of mul are concerned, they get the desired partial specialization.
My questions is: when writing templated free functions (that are intended to be used by others), should you automatically delegate the implementation to a static method function of a class, so that users of your library may implement partial specializations at will, or do you just write the templated function the normal way, and live with the fact that people won't be able to specialize them?
