No match to call for template member function for operator<< overload

Question

I am trying to write an output function for a matrix class by overloading the operator<< overload. The matrix class also has an indexing function, which is created by overloading the `op.

The code is as follows:

template<typename T, unsigned int N>
T& LU_Matrix<T, N>::operator() (unsigned int i, unsigned int j)
{
    return data_[N * i + j];
}

template<typename T, unsigned int N>
std::ostream& operator<< (std::ostream& os, const LU_Matrix<T, N> mat)
{
    for (unsigned int i = 0; i < N; ++i)
    {
        for (unsigned int j = 0; j < N; ++j)
        {
            os << mat(i, j) << " ";
        }
        os << std::endl;
    }
    return os;
}

int main()
{
    LU_Matrix<double, 3> lu_mat();
    // instantiate lu_mat here
    std::cout << lu_mat << std::endl;
    return 0;
}

When I run this, it throws the error

no match to call for '(const LU_Matrix<double, 3>) (unsigned int, unsigned int)'

It seems like the compiler should have created the stated function, given the templated function overloading operator<<. What's going wrong here?

Answer 1

What's going wrong here?

As others have pointed out in comment section, Your LU_Matrix::operator() isn't const-qualified, therefore it cannot be used with const qualified LU_Matrix objects.

In your operator<< overload, you have the above-mentioned case:

std::ostream& operator<< (std::ostream& os, const LU_Matrix<T, N> mat)
//                                          ^^^^^^ --------------------> this

Here the compiler does not find a const operator() operator overload to call with the const object, hence the error!

You need to change the declaration and definition to:

const T& operator() (unsigned int i, unsigned int j) const;
^^^^^^^^^                                            ^^^^^^

Not that, the return parameter also must be const qualified, as you return the element from a member of the class from a const member function.

---

In addition, as an improvement operator<<, neither modify the object nor require a copy, therefore you can actually pass const-ref to LU_Matrix<T, N> to the operator<< overload:

template<typename T, unsigned int N>
std::ostream& operator<< (std::ostream& os, const LU_Matrix<T, N>& mat) /* noexcept */
{
    // ...
    return os;
}

(See a demo)