How to make this operator= overload work?

Question

I was experimenting with making the operator= be virtual, like so: #include #include class Base { public: std::string field1; Base(std::string str) : field1(str) {} virtual Base& operator=(const Base& other) { field1 = other.field1; return *this; } }; class Derived : public Base { public: std::string field2; Derived(std::string str1, std::string str2) : Base(str1), field2(str2) {} virtual Derived& operator=(const Derived& other) override { Base::operator=(other); field2 = other.field2; return *this; } }; However, this gives a compiler error, because the Derived function is not actually overloading anything, the signatures are different. Is it possible to override the operator= to write code like this? Base* ptr = new Derived("old 1", "old 2"); Derived derived("new 1", "new 2"); *ptr = derived; // <- use the derived class operator= to assign both field1 and field2

Vlad from Moscow · Answer

This operator
virtual Derived& operator=(const Derived& other);

does not override the copy assignment operator declared in the base class.
You have to write
Derived& operator=(const Base& other) override;

That is the type of the parameter shall be const Base &.
Here is a demonstrative program.
#include <iostream>

struct A
{
    virtual A & operator =( const A & )
    {
        std::cout << "A::operator =n";
        return *this;
    }
};

struct B : A
{
    virtual B & operator =( const A &a ) override
    {
        A::operator =( a );
        std::cout << "B::operator =n";
        return *this;
    }
};

int main() 
{
    B b1;
    
    A &rb1 = b1;
    
    B b2;
    b2 = rb1;
    
    return 0;
}

Its output is
A::operator =
B::operator =

How to make this operator= overload work?

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