I have the following class:
template <typename T, typename U = UDefault>
class A;
How to check whether types such as A<float> or A<float, int> are instances of the above templated class?
I tried modifying How to check if an object is an instance of a template class in C ? into:
template <typename T, typename U>
struct IsA : std::false_type
{};
template <typename T, typename U>
struct IsA<A<T, U>> : std::true_type
{};
but it's giving the following error
20:18: error: wrong number of template arguments (1, should be 2)
16:8: error: provided for 'template<class T, class U> struct IsA'
In function 'int main()':
40:34: error: wrong number of template arguments (1, should be 2)
16:8: error: provided for 'template<class T, class U> struct IsA'
How can this be solved?
CodePudding user response：
Your IsA class should be expected to take one template argument. The type you are testing.
template <typename Type>
struct IsA : std::false_type {};
template <typename T, typename U>
struct IsA< A<T,U> > : std::true_type {};
// ^^^^^^ The specialization of your one template argument.
Or put alternately, since the individual template parameters to A do not matter here:
template <typename ...AParams>
struct IsA< A<AParams...> > : std::true_type {};
// ^^^^^^^^^^^^^ The specialization of your one template argument.