How to use a copy constructor with a base class?

Question

I'm confused about base classes and copy constructors.

Say I have a class

class A {
    public:
    A(int m) : m(m) { return; }
    virtual ~A() { return; }
    int m;
}

And a class that inherits this

class B : public A {
    public:
    B(int n, int m) : A(m), n(n) { return; }
    vitual ~B() { return; }
    int n;
}

When I copy class B, how do I ensure that the m value in class A is copied as well?

To be sure you can replace m and n with classes that have copy constructors. — Minimus Heximus, Aug 02 '13 at 14:31
@LuchianGrigore Unrelated, but how do I know when to define my own copy constructor? — Tyler Jandreau, Aug 02 '13 at 14:35
When the class manages a manual resource, usually when you have raw pointers and new. Members have their copy constructors called automatically by the default copy constructor. That is fine for a basic type such as int, or a class with a proper copy constructor such as std::string or std::vector. But regular pointers just copy their address - the resource they point to is not copied. Try to use smart containers and objects to avoid needing custom copy constructors as much as possible. — Neil Kirk, Aug 02 '13 at 14:39
@TylerJandreau, see http://stackoverflow.com/questions/3278625/when-do-we-have-to-use-copy-constructors — Mark Ransom, Aug 02 '13 at 14:40

score 4 · Accepted Answer · answered Aug 02 '13 at 14:37

4

The default copy constructor will copy all of the member variables, no matter whether they reside in the base class or a derived class.

If you create your own copy constructor for class B you will need to copy the class A members yourself, or better yet use the copy constructor for class A in the initializer list.

class B : public A {
    public:
    // ...
    B(const B & b) : A(b), n(b.n) {}
    // ...
};

answered Aug 02 '13 at 14:37

Mark Ransom

299,747
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What happens if you do not include A(b) in the initialization list? – Neil Kirk Aug 02 '13 at 14:46
Probably will not compile – UnknownError1337 Aug 02 '13 at 14:49
2

@NeilKirk: It will try to default-construct the base object. In this case, it will fail to compile since there's no default constructor available. If there were, you'd have no errors but surprising behaviour. – Mike Seymour Aug 02 '13 at 14:54

score 1 · Answer 2 · answered Aug 02 '13 at 14:29

The base copy constructor will be called before the derived copy constructor automatically. It's the same as for a regular constructor. (More accurately, the base constructor is called by derived initialization list before derived constructor continues). In your case, the default copy constructors are sufficient.

Be careful, however, that you do not copy a B into an A (search object splicing).

Minimus Heximus · Answer 3 · 2013-08-02T15:05:49.223

This code is your code just copy constructors are added with a class:

#include <iostream>

using namespace std;

struct Int
{

    Int ()
    {
    }

    Int(const Int&)
    {
        cout<< "Int copied" "\n";
    }
};


class A
{
    Int m;
public:
    A(Int m) : m(m)
    {

    }
    virtual ~A()
    {

    }

};

class B : public A
{
    Int n;
public:
    B(Int n, Int m) : A(m), n(n)
    {

    }


    virtual ~B()
    {

    }

};

void f(B)
{

}

int main()
{
   Int m,n;
   B b(m,n);

   cout<< "\n" "start:" "\n";
   f(b);
   cout<< "end";

}

Ubuntu Qt Creator Output says both are copied.

One can think there's a hidden member with type B in A and this hidden member can be initialized or copied as a usual member as in the above program with:

B(Int n, Int m) : A(m), n(n)

score -1 · Answer 4 · answered Aug 02 '13 at 14:44

-1

You should not return from a constructor/destructor, not even void.

answered Aug 02 '13 at 14:44

paper.plane

1,201
10
17

I'm not familiar with this rule. Why's that? – Tyler Jandreau Aug 02 '13 at 14:45
1

You can't return a value from a constructor, but you can call `return;` just fine. Furthermore, this should have been a comment, not an answer. – SirGuy Aug 02 '13 at 14:49
The `return` here is unnecessary, but not wrong; and this doesn't answer the question. – Mike Seymour Aug 02 '13 at 14:55

How to use a copy constructor with a base class?

4 Answers4