How to conditionally add a function to a class template?

Question

I have a Matrix class template as follows:

template
class Matrix
{
    T data[nrows][ncols];
public:
         


        

Answer 1

The lazy and needlessly repetitive way

Just add a partial specialization:

template
class Matrix
{
    T data[N][N];
public:
    T& operator ()(std::size_t i, std::size_t j)
    {
        return data[i][j];
    }

    void setidentity(/*whatever params*/) { std::cout << "yay!"; }
};

Live Example

For general N * M matrices, the general template will be instantiated, whereas only for N * N matrics, this specialization is a better match.

Disadvantage: code repetition of all regular code. Could use a base class, but it's actually easier to do some SFINAE magic (below)

A slightly harder but more economical way

You can also use SFINAE by adding hidden template parameters N and M that default to nrows and ncols to setidentity, and to enable_if on the condition N == M.

template
class Matrix
{
    T data[nrows][ncols];
public:
    T& operator ()(std::size_t i, std::size_t j)
    {
        return data[i][j];
    }

    template * = nullptr>
    void setidentity(/*whatever params*/) { 
        static_assert(N == nrows && M == ncols, "invalid");
        std::cout << "yay!"; 
    }
};

Or, since the question was tagged C++11, use typename std::enable_if<(N == M)>::type instead.

Live Example