Pure virtual assignment operator in c++

Question

I have a base class

class Base{
public:
virtual ~Base();

};

I derive two classes from Base:

class D1:public Base{
//...some fields
//assignment operator, it does the deep copy of the members
D1& operator=(const D1&);
};

class D2:public Base{
//...some fields
//assignment operator, it does the deep copy of the members
D2& operator=(const D2&);
};

Next, in main I have two objects of let's say D1. The problem is that the overriden assignment operator is never called, however default one for base is called. I tried to make assignment operator virtual in Base, but it didn't help.

D1 *d1 = new D1();
D1 *d1_another = new D1();
//this doesn't work:
d1 = d1_another

D2 *d2 = new D2();
D2 *d2_another = new D2();
//this doesn't work:
d2 = d2_another

UPD Also I would like to know how to deal with

Base *d1 = new D1();
Base *d1_another = new D1();
//?
d1 = d1_another

you assign the pointers... [this way](http://coliru.stacked-crooked.com/a/728183b3c839551e) — user1810087, Feb 01 '19 at 10:39
@user1810087, but what to do if I need to assign `Base* d1 = new D1()` to `Base* d1_another = new D1()`? if I write `*d1 = *d1_another` assignment operator prints nothing — Pavlo Kovalov, Feb 01 '19 at 10:57
Base and D1 are distict types in this case and the compiler does not know how to assign them, since D1 can have additional "fields". You need to define your own assginments for **all** types you want to cover. [like this](http://coliru.stacked-crooked.com/a/9946c26f371f6fc5). see [here](https://stackoverflow.com/a/669894/1810087) a good discussion. — user1810087, Feb 01 '19 at 11:17
Assignment and derived classes don't play well togetger. Combining them is a design error more often than not. You hovever are never assugning any base or derived objects. You are only assigning pointers. — n. 1.8e9-where's-my-share m., Feb 01 '19 at 11:36

score 1 · Accepted Answer · answered Feb 01 '19 at 11:20

1

Try it like this

#include <iostream>
#include <string>

using namespace std;

class Base {
    public:
    virtual ~Base() {}

};


class D1 : public Base {
public:
    virtual ~D1() {}
    //...some fields
    //assignment operator, it does the deep copy of the members
    D1& operator=(const D1&) {
        cout << "D1:operator=(const D1&)\n";
        return *this;
    }
};

class D2 : public Base {
public:
    virtual ~D2() {}
    //...some fields
    //assignment operator, it does the deep copy of the members
    D2& operator=(const D2&) {
        cout << "D2:operator=(const D2&)\n";
        return *this;
    }
};

main

    D1 *d1 = new D1();
    D1 *d1_another = new D1();
    //this doesn't work:
    *d1 = *d1_another;

    D2 *d2 = new D2();
    D2 *d2_another = new D2();
    //this doesn't work:
    *d2 = *d2_another;

answered Feb 01 '19 at 11:20

Hamza.S

1,243
8
18

Why did you add virtual destructors to `D1` and `D2`? Not virtual won't work? – Pavlo Kovalov Feb 01 '19 at 11:22
@PavloKovalov because if you want to work with some raw pointers , then it is necessary – Hamza.S Feb 01 '19 at 11:44
2

if the base class destructor is virtual then the derived class destructor is also virtual you don't need to specify it virtual explicitly – Hamza.S Feb 01 '19 at 11:49
so in derived class `~destructor()` is the same as `virtual ~destructor()`, in case when base has virtual destructor? – Pavlo Kovalov Feb 01 '19 at 12:06
@PavloKovalov yes exactly – Hamza.S Feb 01 '19 at 12:07
If it's not too much to ask, could you please suggest smth regarding upd? I've updated the question. I suppose I have to do some casting, but not sure how to do that – Pavlo Kovalov Feb 01 '19 at 12:12
Okay, but I don’t have enough reputation( – Pavlo Kovalov Feb 01 '19 at 12:16
try it the same way . `*d1 = *d1_another;` – Hamza.S Feb 01 '19 at 12:20
Ok, just a minute – Pavlo Kovalov Feb 01 '19 at 12:35
https://stackoverflow.com/questions/54479931/use-of-assignment-operator-of-derived-class-in-base here it is – Pavlo Kovalov Feb 01 '19 at 12:52
@PavloKovalov you probably should **not** ask a new question. there are already a lot of them if you search for inheritance, assignment... well to late... – user1810087 Feb 01 '19 at 12:53

Pure virtual assignment operator in c++

1 Answers1

Try it like this

main