Question

I have a code base, in which for Matrix class, these two definitions are there for () operator:

template <class T> T& Matrix<T>::operator() (unsigned row, unsigned col)
{
    ......
}


template <class T> T Matrix<T>::operator() (unsigned row, unsigned col) const
{
    ......
}

One thing I understand is that the second one does not return the reference but what does const mean in the second declaration? Also which function is called when I do say mat(i,j)?

Answer 1

Which function is called depends on whether the instance is const or not. The first version allows you to modify the instance:

 Matrix<int> matrix;
 matrix(0, 0) = 10;

The const overload allows read-only access if you have a const instance (reference) of Matrix:

 void foo(const Matrix<int>& m)
 {
     int i = m(0, 0);
     //...
     //m(1, 2) = 4; //won't compile
 }

The second one doesn’t return a reference since the intention is to disallow modifying the object (you get a copy of the value and therefore can’t modify the matrix instance).

Here T is supposed to be a simple numeric type which is cheap(er) to return by value. If T might also be a more complex user-defined type, it would also be common for const overloads to return a const reference:

 template <class T>
 class MyContainer
 {
      //..,
      T& operator[](size_t);
      const T& operator[](size_t) const;
 }