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...
operator+=
, operator-=
, ... ? 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;
}
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);
}
};