Question

A beginner question:

How to put a condition on a template in a multi-templated class:

I tried this:

template <class T, class U>
typename std::enable_if<...>
class foo
{
};

And this:

template <class T,
          class U = std::enable_if<...>>
class foo
{
};

But they are not working. Any help would be appreciated 🙂

Answer 1

Declare an additional template parameter defaulted to void and specialize it with enable_if:

template <typename T, typename U, typename Enable = void>
class foo {};

template <typename T, typename U>
class foo<T, U, typename std::enable_if<...>::type>
{
};