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

A basic, but simple solution not mentioned by any other answer: you can use std::conditional and inheritance.
It follows a minimal, working example:

#include
#include

struct HasSetIdentity {
    void setIdentity() { }
};

struct HasNotSetIdentity {};

template
class Matrix: public std::conditional<(nrows==ncols), HasSetIdentity, HasNotSetIdentity>::type
{
    T data[nrows][ncols];
public:
    T& operator ()(std::size_t i, std::size_t j)
    {
        return data[i][j];
    }
};

int main() {
    Matrix m1;
    m1.setIdentity();
    Matrix m2;
    // Method not available
    // m2.setIdentity();
}

You can still move data down the hierarchy if you need them to be shared by all the subobjects.
It mostly depends on the real problem.