Inheritence and copy constructor - how to init private fields from base class?

Question

I have 2 classes, A and B. In A I have 3 private fields. In class B I would like to write a copy constructor, and initialize private fields from class A. However, this does not work:

#include <iostream>
#include <string>
using namespace std;

class A
{
    private:

        string *field1;
        string *field2;
        string *field3;
        double num1;

    public:

        A(string *o, string *n, string *m, double a=0)
        {
            field1 = new string(*o);
            field2 = new string(*n);
            field3 = new string(*m);
            num1 = a;
        }

        A(const A& other) {
            field1 = new string(*other.field1);
            field2 = new string(*other.field2);
            field3 = new string(*other.field3);
            num1 = other.num1;
        }

        void show()
        {
            cout << *field1 << " " << *field2 << " " << *field3 << "\n";
        }

        ~A()
        {
            delete field1;
            delete field2;
            delete field3;
        }
};

/*--------------------------------------------------------------------------------------------*/

class B : public A
{
    private :

        double num2;
        double num3;

    public:

        B(double num2, double num3, string *o, string *n, string *num, double a=0) : A(o,n,num,a)
        {
            this->num2 = num2;
            this->num3 = num3;
        }

        B(const B& other) : A(other.field1, other.field2, other.field3, other.num1)
        {
            num2 = other.num2;
            num3 = other.num3;
        }

        void show()
        {
            cout << num2 << " " << num3 << "\n";
            A::show();
        }
};

int main()
{
    string o = "TEXT 111";
    string *optr = &o;

    string n = "TEXT 222";
    string *nptr = &n;

    string *numptr = new string("9845947598375923843");

    A ba1(optr, nptr, numptr, 1000);
    ba1.show();

    A ba2(ba1);
    ba2.show();

    A ba3 = ba2;
    ba3.show();

    B vip1(20, 1000, optr, nptr, numptr, 3000);
    vip1.show();

    B vip2(vip1);
    vip2.show();

    delete numptr;
    return 0;
}

I DO understand that when I change from private to protected it should work (and works, of course) - but how to deal with the situation that I have in my code? The question is: how to initialize, in copy constructor, private fields from base class? I get the following errors with the current code:

/home/yak/test.cpp|9|error: ‘std::string* A::field1’ is private|
/home/yak/test.cpp|61|error: within this context|
/home/yak/test.cpp|10|error: ‘std::string* A::field2’ is private|
/home/yak/test.cpp|61|error: within this context|
/home/yak/test.cpp|11|error: ‘std::string* A::field3’ is private|
/home/yak/test.cpp|61|error: within this context|
/home/yak/test.cpp|12|error: ‘double A::num1’ is private|
/home/yak/test.cpp|61|error: within this context|
||=== Build failed: 8 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

You make them `protected` as you've suggested, then access them from B's ctor body. What's the problem with making them `protected`? — RJFalconer, Apr 07 '16 at 17:10
If you cannot change `A`, then what you're asking is impossible. And your code is leaking and not exception-safe. Why use pointers to strings?! — Christian Hackl, Apr 07 '16 at 17:11
@RJFalconer: I'm curious, what I need to do to initialize `private` fields from the child class. As we see, with initialization list it does not work — yak, Apr 07 '16 at 17:12
@ChristianHackl, of course it is possible! See my answer. (the pointer-to-string part is valid, of course) — SergeyA, Apr 07 '16 at 17:13
@SergeyA: Yes, of course you can use `A`'s copy constructor. But you cannot initialise them directly. — Christian Hackl, Apr 07 '16 at 17:14
@SergeyA: (ignoring the fact that `A` does not initialise any of its members anyway, because the constructors use assignments) — Christian Hackl, Apr 07 '16 at 17:16
@ChristianHackl, unneccessary pedantic and moot, and seems to be spoken only to support your initial incorrect claim. — SergeyA, Apr 07 '16 at 17:23
@SergeyA: I disagree. The OP apparently does not fully understand that `private` really means private, even for derived classes and constructors calling base constructors. — Christian Hackl, Apr 07 '16 at 17:24
@ChristianHackl, OP explicitly accepted the answer below, and explained that they understood the issue fully. Op didn't need to access the fields, only to make sure they are initialized when child copy ctor is called, and didn't know how to call base copy ctor directly. — SergeyA, Apr 07 '16 at 17:27

NathanOliver · Accepted Answer · 2016-04-07T17:18:29.327

6

All you need to do is call the copy constructor of A when you copy construct B like

B(const B& other) : A(other)
{
    num2 = other.num2;
    num3 = other.num3;
}

Since B inherits from A this is legal and A will copy the A part of other.

Also note that all of these pointer are unnecessary and make the code more complicated. We could rewrite it like:

class A
{
private:
    string field1;
    string field2;
    string field3;
    double num1;

public:
    A(const string& o, const string& n, const string& m, double a = 0) : field1(o), field2(n), feild3(m), num1(a) {}

    A(const A& other) field1(other.field1), field2(other.field2), feild3(other.feild3), num1(other.num1) {}

    void show()
    {
        cout << field1 << " " << field2 << " " << field3 << "\n";
    }
};

/*--------------------------------------------------------------------------------------------*/

class B : public A
{
private:

    double num2;
    double num3;

public:

    B(double num2, double num3, const string& o, const string& n, const string& m, double a = 0) : A(o, n, num, a), num2(num2), num3(num3) {}

    B(const B& other) : A(other), num2(other.num2), num3(other.num3) {}

    void show()
    {
        cout << num2 << " " << num3 << "\n";
        A::show();
    }
};

edited Apr 07 '16 at 17:18

answered Apr 07 '16 at 17:12

NathanOliver

171,901
28
288
402

How do `A` know what 'part' of `other` it should copy? It's because I have a copy constructor in `A`? – yak Apr 07 '16 at 17:14
@yak That and [object slicing](http://stackoverflow.com/questions/274626/what-is-object-slicing) (not an issue here). – LogicStuff Apr 07 '16 at 17:15
@LogicStuff: Thank you – yak Apr 07 '16 at 17:15
@yak: `A` doesn't know. But the compiler does. – IInspectable Apr 07 '16 at 17:15
But if I wouldn't have copy constructor in `A`? It should work as well? – yak Apr 07 '16 at 17:16
3

If `A` doesn't implement a copy-constructor (explicitly or implicitly), then the compiler will issue an error when trying to compile `A(other)`. – IInspectable Apr 07 '16 at 17:18
I think that I understand it now. Many thanks to all of you guys. – yak Apr 07 '16 at 17:20
@yak No problem. Not sure if you saw my the updated code where you do not have to use `new` and `delete` at all. – NathanOliver Apr 07 '16 at 17:21

score 1 · Answer 2 · answered Apr 07 '16 at 17:19

When using the copy constructor for class B, you should call the copy constructor for class A.

So, substitute your code by this one:

    B(const B& other) : A(other)
    {
        num2 = other.num2;
        num3 = other.num3;
    }

Also, declare the destructor from class A as virtual. And I suppose you also want to do that for show() method.

score 0 · Answer 3 · answered Apr 07 '16 at 17:11

0

You need to invoke parent copy constructor from your copy constructor, giving it the same argument.

answered Apr 07 '16 at 17:11

SergeyA

61,605
5
78
137

Inheritence and copy constructor - how to init private fields from base class?

3 Answers3