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

You can do it with std::enable_if in the following mode

template 
typename std::enable_if::type setIdentity ()
 { /* do something */ }

A full example

#include 


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

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

    template 
    typename std::enable_if::type setIdentity ()
     { /* do something */ }
};

int main()
 {
   Matrix  mi3;
   Matrix  mnoi;

   mi3.setIdentity();
   // mnoi.setIdentity(); error

  return 0;
}

--- EDIT ---

As pointed in a comment by Niall (regarding the TemplateRex's answer, but my solution suffer from the same defect) this solution can be circonvented expliciting the number of rows and columns in this way

mi3.setIdentity<4, 4>();

(but this isn't a real problem (IMHO) because mi3 is a square matrix and setIdentity() could work with real dimensions (nrows and ncols)) or even with

mnoi.setIdentity<4, 4>()

(and this is a big problem (IMHO) because mnoi isn't a square matrix).

Obviously there is the solution proposed by Niall (add a static_assert; something like

    template 
    typename std::enable_if::type setIdentity ()
     {
       static_assert(r == nrows && c == ncols, "no square matrix");

       /* do something else */ 
     }

or something similar) but I propose to add the same check in std::enable_if.

I mean

template 
typename std::enable_if<    (r == c)
                         && (r == nrows)
                         && (c == ncols)>::type setIdentity ()
 { /* do something */ }