Assigning base data from derived class with move produces an error

Question

I am new to the new feature of C++ such as moving semantic (moving constructor and moving assignment operator). The code below produces an error in IDE as follows.

How to fix it?

#include <iostream>
using namespace std;

class HeavyData {};

class Base
{
protected:
    HeavyData data;

public:
    Base(HeavyData data) :data{ data } {}
    Base(Base const&& src) :data{ move(src.data) } {}

    Base& operator = (Base const&& rhs)
    {
        this->data = move(rhs.data);
        return *this;
    }
};

class  Derived : public Base
{
private:
    HeavyData extra;
public:
    Derived(HeavyData data, HeavyData extra) :
        extra{ extra }, Base(data) {}
    Derived(Derived const&& src) :
        extra{ move(src.extra) }, data{ move(src.data) }  {}

    Derived& operator = (Derived const&& rhs)
    {
        this->data = move(rhs.data);
        this->extra = move(rhs.extra);
        return *this;
    }
};
void main()
{
    HeavyData data, extra;
    Derived d1(data, extra);
    Derived d2 = move(d1);
}

jkb · Accepted Answer · 2020-09-07T01:53:24.273

3

Don't initialize base-class data in the derived class's constructor. Invoke the base class constructor first, then your derived class's data.

Derived(Derived const &&src) :
  Base(std::move(src)), extra{std::move(src.extra)} ()

edited Sep 07 '20 at 01:53

answered Sep 07 '20 at 01:32

jkb

2,376
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Sorry, please show me the code. – Second Person Shooter Sep 07 '20 at 01:35
@WissenMachtFrei I've edited the answer to provide sample code. – jkb Sep 07 '20 at 01:40
@AdrianMole Quite right, thanks. I've edited the answer. – jkb Sep 07 '20 at 01:54

olkivan · Answer 2 · 2020-09-07T11:25:44.673

2

tl;dr;You need to call base class constructor.

Since base class should be initialized before derived class, base class members should be initialized during base class constuction. You cannot affect base class members creation from the derived class, but you can call base class constructor like this:

 Derived(Derived const&& src) :
        Base(move(src.data)),
        extra{ move(src.extra) }{}

Edit: delegating -> base class

edited Sep 07 '20 at 11:25

answered Sep 07 '20 at 01:41

olkivan

89
1
3

@JaMiT you're completely right. I've edited the post. Thanks! – olkivan Sep 07 '20 at 11:26

artm · Answer 3 · 2020-09-07T01:37:51.313

0

You need:

Derived(Derived const&& src) : 
    Base(std::move(src.data)), 
    extra{ move(src.extra) }
{}

edited Sep 07 '20 at 01:37

answered Sep 07 '20 at 01:26

artm

17,291
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In my understanding, protected data member in base class must be visible in the derived class. – Second Person Shooter Sep 07 '20 at 01:27
It's nothing to do with the fact that `data` is `protected`! You can't initialize a base-class member in a derived class using an initializer list ... you have to do it in the *body* of the constructor. – Adrian Mole Sep 07 '20 at 01:29
Changing `protected` to `public` does not solve the problem. I just tried it. – Second Person Shooter Sep 07 '20 at 01:29
@WissenMachtFrei yeah fixed it – artm Sep 07 '20 at 01:39
Why does this work? Code-only answers rarely explain anything. – JaMiT Sep 07 '20 at 03:49

Assigning base data from derived class with move produces an error

3 Answers3