Question

Here’s a sample code:

#include <stack>
#include <cstddef>

template <std::size_t N,
         template <class> class Stack = std::stack
         >   
class Tower : protected Stack<int>
{   
    public:
        Tower() : Stack<int>(N)
        {   
        }   
};  


int main(int argc, char **argv)
{   
    Tower<5L> tower1();
} 

And I see the compiler (gcc) is not happy:

file.cpp: In function 'int main(int, char**)':
file.cpp:18:11: error: type/value mismatch at argument 2 in template parameter 
list for 'template<long unsigned int N, template<class> class Stack> class Tower'
file.cpp:18:11: error: expected a template of type 'template<class> class Stack',
got 'template<class _Tp, class _Sequence> class std::stack'
file.cpp:18:21: error: invalid type in declaration before ';' token

The standard stack container has this form:

template <class Type, class Container = deque<Type> > class stack;

Meaning I should be fine to pass only one template argument here!

Any thoughts on how to resolve this?
Thanks

Answer 1

'template<class> class Stack',
got 'template<class _Tp, class _Sequence> class std::stack' shows the problem.

Here is what std::stack looks like

template<
    class T,
    class Container = std::deque<T>
> class stack;

As you can see there is a second parameter.

Adding:

#include <deque>
template <std::size_t N,
         template <class T, class = std::deque<T>> class Stack = std::stack
         >   

should make it compile.