Template specialization to use default type if class member typedef does not exist
- by Frank
Hi Everyone,
I'm trying to write code that uses a member typedef of a template argument, but want to supply a default type if the template argument does not have that typedef. A simplified example I've tried is this:
struct DefaultType { DefaultType() { printf("Default "); } };
struct NonDefaultType { NonDefaultType() { printf("NonDefault "); } };
struct A {};
struct B { typedef NonDefaultType Type; };
template<typename T, typename Enable = void> struct Get_Type { typedef DefaultType Type; };
template<typename T> struct Get_Type< T, typename T::Type > { typedef typename T::Type Type; };
int main()
{
Get_Type::Type test1;
Get_Type::Type test2;
}
I would expect this to print "Default NonDefault", but instead it prints "Default Default". My expectation is that the second line in main() should match the specialized version of Get_Type, because B::Type exists. However, this does not happen.
Can anyone explain what's going on here and how to fix it, or another way to accomplish the same goal?
Thank you.
