Question

Until a test I’ve just made, I believed that only Constructors were not inherited in C++. But apparently, the assignment operator= is not too…

1. What is the reason of that ?
2. Is there any workaround to inherit the assignment operator ?
3. Is it also the case for operator+=, operator-=, … ?
4. Are all other functions (apart from constructors/operator=) inherited ?

In fact, I encountered this problem as I was doing some CRTP :

template<class Crtp> class Base
{
    inline Crtp& operator=(const Base<Crtp>& rhs) {/*SOMETHING*/; return static_cast<Crtp&>(*this);}
};

class Derived1 : public Base<Derived1>
{
};

class Derived2 : public Base<Derived2>
{
};

Is there any solution to get that working ?

EDIT : OK, I have isolated the problem. Why the following isn’t working ? How to solve the problem ?

#include <iostream>
#include <type_traits>

// Base class
template<template<typename, unsigned int> class CRTP, typename T, unsigned int N> class Base
{
    // Cast to base
    public:
        inline Base<CRTP, T, N>& operator()()
        {
            return *this;
        }

    // Operator =
    public:
        template<typename T0, class = typename std::enable_if<std::is_convertible<T0, T>::value>::type>
        inline CRTP<T, N>& operator=(const T0& rhs)
        {
            for (unsigned int i = 0; i < N; ++i) {
                _data[i] = rhs;
            }
            return static_cast<CRTP<T, N>&>(*this);
        }

    // Data members
    protected:
        T _data[N];
};

// Derived class
template<typename T, unsigned int N> class Derived : public Base<Derived, T, N>
{
};

// Main
int main()
{
    Derived<double, 3> x;
    x() = 3; // <- This is OK
    x = 3;   // <- error: no match for 'operator=' in ' x=3 '
    return 0;
}

Answer 1

The assignment operator is technically inherited; however, it is always hidden by an explicitly or implicitly defined assignment operator for the derived class (see comments below).

(13.5.3 Assignment) An assignment operator shall be implemented by a
non-static member function with exactly one parameter. Because a copy
assignment operator operator= is implicitly declared for a a class if
not declared by the user, a base class assignment operator is always
hidden by the copy assignment operator of the derived class.

You can implement a dummy assignment operator which simply forwards the call to the base class operator=, like this:

// Derived class
template<typename T, unsigned int N> class Derived : public Base<Derived, T, N>
{
public:
    template<typename T0, class = typename std::enable_if<std::is_convertible<T0, T>::value>::type>
    inline Derived& operator=(const T0& rhs)
    {
        return Base<Derived, T, N>::operator=(rhs);
    }
};