Question

I am trying to overload the operator= in a template class.

I have this template class:

template <class T>
class Matrice
{
    T m,n;
public:
    template <class V>
    friend Matrice<V>& operator=(const Matrice<V> &);
};

template <class T>
Matrice<T>& Matrice<T>::operator=(const Matrice<T> &M)
{
    /*...*/
    return *this;
}

and I also tried:

template <class T>
class Matrice
{
    T m,n;
public:
    template <class V>
    Matrice<V>& operator=(Matrice<V> &);
};

template <class T>
Matrice<T>& operator=(Matrice<T> &M)
{
    /*...*/
    return *this;
}

but I still get this error:

error C2801: 'operator =' must be a non-static member

Answer 1

error C2801: ‘operator =’ must be a non-static member

The bolded word is key here. friend is not a member; it’s a friend. Remove that friend keyword and treat operator= as member:

The syntactically proper version is:

template <class T>
class Matrice
{
    T m,n;
public:
    template <class V>
    Matrice<V>& operator=(const Matrice<V> &);
};

template <class T>
template <class V>
Matrice<V>& Matrice<T>::operator=(const Matrice<V> &M)
{
    /*...*/
    return *this;
}

Although I think that it’s wrong to use that template <class V> there; the sematically proper version would be

template <class T>
class Matrice
{
    T m,n;
public:
    Matrice<T>& operator=(const Matrice<T> &);
};

template <class T>
Matrice<T>& Matrice<T>::operator=(const Matrice<T> &M)
{
    /*...*/
    return *this;
}

Explanation: you don’t generally want to assign Type<V> to Type<T> in this way; if you have to, then it is probably sign of bad design.