Template default argument

Question

How do I specify a template class as a default value for a template typename? e.g. the following doesn't work.

template <typename A, typename B> class X {};

template <typename T=template <typename, typename> class X> class Y {};

int main()
{
    Y<> y;
    return 0;
} 

tmp.cc:4:22: error: expected type-specifier before ‘template’
 template <typename T=template <typename, typename> class X> class Y {};
                      ^~~~~~~~
tmp.cc:4:22: error: expected ‘>’ before ‘template’
tmp.cc: In function ‘int main()’:
tmp.cc:8:7: error: template argument 1 is invalid
     Y<> y;
       ^

Answer 1

The problem of your code is that for Y you ask a type template parameter and you want to use a template template parameter.

template <typename A, typename B> class X {};

//           type                template template
//........VVVVVVVVVV VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
template <typename T=template <typename, typename> class X> class Y {};

int main()
{
    Y<> y;
    return 0;
} 

If you want to use X as default, T must be be a template template, so

template <typename, typename>
class X
 {};

template <template <typename, typename> class T = X>
class Y
 {};

int main ()
 {
   Y<> y;
 } 

Or also using variadics to improve flexibility

template <template <typename...> class T = X>
class Y
 {};