Adding functionality to a default copy constructor

Question

I have a question. Say I have a class A, for which I'm perfectly happy with the default copy constructor.

Can I add functionality to this default copy constructor without re-writing all its work again?

Trivial example:

class A 
{
public:
A(int n) : data(n) { };
private:
int data;
};

Suppose I want to print the message "Copy constructor!" each time a copy constructor is called. For this simple case I would just write my own copy constructor, which takes charge of the shallow copy explicitly, and also prints out the message. Is there a way to add the message printing (or whatever other functionality I want) on top of the default copy constructor, without writing explictly the shallow coyp?

You may do something like [this](https://ideone.com/hUJ9x3) – Jarod42 Mar 08 '17 at 19:19 — Jarod42, Mar 08 '17 at 19:19

πάντα ῥεῖ · Accepted Answer · 2017-03-09T11:11:06.890

1

Suppose I want to print the message "Copy constructor!" each time a copy constructor is called. For this simple case I would just write my own copy constructor, which takes charge of the shallow copy explicitly, and also prints out the message.

Yes, you need to provide your copy constructor explicitely and add that printing out the message.

class A 
{
    public:
    A(int n) : data(n) { };
    // You need to add tis:
    A(const& A rhs) : data(rhs.data)  {
        std::cout << "Copy constructor!" << '\n';
    }
    private:
    int data;
};

Is there a way to add the message printing (or whatever other functionality I want) on top of the default copy constructor, without writing explictly the shallow coyp?

No, there isn't.

edited Mar 09 '17 at 11:11

answered Mar 08 '17 at 18:56

πάντα ῥεῖ

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You actually forget to copy `data`, you just do the extension. – Jarod42 Mar 08 '17 at 19:20
1

And something like [this](https://ideone.com/hUJ9x3) allow to add functionality on top of default copy constructor. – Jarod42 Mar 08 '17 at 19:22
This is bad practice; you need to remember to add the copying every time you add a member. Single responsibility rule should be applied here. – sp2danny May 29 '17 at 13:56
@sp2danny _"Single responsibility rule should be applied here."_ Can you elaborate please? A private `init()` function or what? – πάντα ῥεῖ May 29 '17 at 17:01
I would suggest a member, whose only purpose is to be loud about being copied, that way the containing class can have all special members implicitly defaulted. – sp2danny May 29 '17 at 18:01
@sp2danny I wouldn't like such overengineering. – πάντα ῥεῖ May 29 '17 at 18:03

Adding functionality to a default copy constructor

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