Generic Stack Implementation C++, What to return in case stack is empty?

Question

I wrote a generic stack for practicing. The question is about what type I should return in case stack empty. The first thing comes to my mind is returning -1 :

   U pop_back()
    {
        if (m_pos <= 0)
        {
            std::cout << "Stack <pop_back> No element in stack will return default " << std::endl;
            return -1;
        }
        return m_data[m_pos--];
    }

But this will limit my generic container to integer types. And also -1 can be a data for integer types instead of error. Other options in my mind handle error with exceptions or default ctors .

Please advice me the best way of returning error in those kind of situations.

Whole code can be seen below.

template <typename U, std::size_t n>
class Stack
{
public:
    Stack()
    {
        m_pos = -1;
    }


    U pop_back()
    {
        if (m_pos <= 0)
        {
            std::cout << "Stack <pop_back> No element in stack will return default " << std::endl;
            return -1;
        }
        return m_data[m_pos--];
    }

    void push_back(U val)
    {
        if (m_pos >= m_data.size())
        {
            std::cout << " Stack <push_back> Capasity is full won't add more" << std::endl;
            return;
        }
        m_data[++m_pos] = val;
    }

private:

    int m_pos;
    std::array<U, n> m_data;
};