I want to specialize a templated class with a templated type. Something like this..

#include <iostream>
#include <vector>
#include <string>

using namespace std;  // I know, I know.  For brevity here.

template < typename T > class MyClass
{
public:
    string print_sum();
    vector < T > m_Data;
};

template < typename T >
string MyClass< T >::print_sum()
{
    T sum = 0;
    for ( auto it = m_Data.begin(); it != m_Data.end(); ++it )
    {
        sum += *it;
    }
    return to_string( sum );
}

/*
// Does not work
template < typename S >
string MyClass< vector < S > >::print_sum()
{
    S sum = 0;

    for ( auto it = m_Data.begin(); it != m_Data.end(); ++it )
    {
        for ( auto it2 = (*it).begin(); it2 != (*it).end(); ++it2 )
        {
            sum += *it2;
        }

    }
    return to_string( sum );
}
*/

template <>
string MyClass< vector < int > >::print_sum()
{
    int sum = 0;

    for ( auto it = m_Data.begin(); it != m_Data.end(); ++it )
    {
        for ( auto it2 = (*it).begin(); it2 != (*it).end(); ++it2 )
        {
            sum += *it2;
        }

    }
    return to_string( sum );
}

int main()
{
    vector<int> d = {1, 2, 3, 4, 5};
    MyClass<int> foo;
    foo.m_Data = d;
    std::cout << foo.print_sum() << std::endl;
    
    vector < vector < int > > m = {d, d, d};
    MyClass < vector < int > > bar;
    bar.m_Data = m;
    std::cout << bar.print_sum() << std::endl;
    
    return 0;
}

I’d like something like the commented out version so I can have code that will work for various vector < vector < S > >.

Upon removing the comments gcc says:

main.cpp:26:43: error: invalid use of incomplete type ‘class MyClass >’
   26 | string MyClass< vector < S > >::print_sum()
      |                                           ^
main.cpp:7:31: note: declaration of ‘class MyClass >’
    7 | template < typename T > class MyClass
      |                               ^~~~~~~
main.cpp: In instantiation of ‘std::string MyClass<T>::print_sum() [with T = std::vector<int>; std::string = std::__cxx11::basic_string<char>]’:
main.cpp:51:31:   required from here

I can’t sort out how to make this work.

Bonus points if we can do some sort of typedef magic to make something like this work:

typedef
template < typename S >
class MyClass < vector < S > > MyVecClass < S >;


// To allow
MyVecClass < int > bar;
// instead of
MyClass < vector < int > > bar;