enable_if
is very useful in lots of scenarios, but in cases like these I'm usually more inclined to use tag dispatching. To my eye, the code looks cleaner, behaves more predictably, and if you try to use it wrongly, the error messages make slightly more sense.
struct C
{
template <class T>
void operator()(const T& container) const
{
operator()(container, Tag<typename T::value_type>());
}
private:
template <class V> struct Tag {};
template <class T>
void operator()(const T& container, Tag<A>) const
{
std::cout << "A\n";
}
template <class T>
void operator()(const T& container, Tag<B>) const
{
std::cout << "B\n";
}
};
int main()
{
std::vector<A> a;
std::list<B> b;
C c;
c(a);
c(b);
}