Template Specialization for T -> std::vector

Question

I have a template class method

template
T pop();

Now I want to do a template specialization as follows,

Answer 1

You need a temporary proxy to dispatch to different results:

Example:

#include 
#include 
#include 

class Stack
{
    private:
    std::vector m_data;

    class Pop {
        friend class Stack;

        public:
        Stack& stack;

        Pop(Stack& stack)
        :   stack(stack)
        {}

        private:
        Pop(const Pop&) = default;
        Pop& operator = (const Pop&) = default;

        public:
        operator int () {
            std::vector& data = stack.m_data;
            int result = -1;
            if( ! data.empty()) {
                result = data.front();
                data.erase(data.begin());
            }
            return result;
        }

        operator std::vector () {
            std::vector& data = stack.m_data;
            std::size_t size = std::min(data.size(), std::size_t(3));
            std::vector result(data.begin(), data.begin() + size);
            data.erase(data.begin(), data.begin() + size);
            return result;
        }
    };

    public:
    Stack()
    :   m_data( {0, 1, 2, 3, 4, 5, 6, 7, 8} )
    {}

    const std::vector& data() const { return m_data; }
    Pop pop() { return Pop(*this); }
};

int main()
{
    Stack stack;
    int i = stack.pop();
    std::vector v = stack.pop();
    std::cout << "i = " << i << '\n';
    std::cout << "v = {";
    for(auto i : v)
        std::cout << i;
    std::cout << "}\n";
}

Disclaimer: I consider the code totally useless (It might become useful if there is a pop() just returning a single value and a pop(std::size_t) converting to different containers, where the destructor of Pop is doing the erase).