Passing pointer to object in constructor

Question

I want to create two objects A and B and each object contains each other.

class B;

class A
{
public:
    A(B * b) : _b(b) {}
    void printB(void)
    {
        if (0 != _b)
        {
            std::cout << "B is not null" << std::endl;
        }
        else
        {
            std::cout << "B is null" << std::endl;
        }
    }
private:
    B * _b;
};

class B
{
public:
    B(A * a) : _a(a) {}
    void printA(void)
    {
        if (0 != _a)
        {
            std::cout << "A is not null" << std::endl;
        }
        else
        {
            std::cout << "A is null" << std::endl;
        }
    }
private:
    A * _a;
};

int main(int argc, char ** argv)
{
    A * a = 0;
    B * b = 0;

    a = new A(b);
    b = new B(a);

    a->printB();
    b->printA();

    delete a;
    delete b;

    return 0;
}

As you can see object 'a' contains null pointer 'b'. What is the best way to re-write this code so that 'a' contains a reference to 'b'? (note that object 'a' and 'b' needs to use 'new')

Many thanks!

Two objects cannot contain one another, since every object is the subobject of a unique complete object. — Kerrek SB
– Kerrek SB, Commented Jun 2, 2014 at 8:37
you should use setter after object creation. Otherwise one has to be null because of circular reference. — Rakib
– Rakib, Commented Jun 2, 2014 at 8:37
References cannot be null. So you want instance of A to hold a reference to instance of B, which holds a reference to instance of A, which holds a reference to instance of B, which holds... — Spook
– Spook, Commented Jun 2, 2014 at 8:48
@chesschi, I am assuming that you mean pointer when you say reference because you have not yet learned about C++ References yet. If that is what you mean, it would be best if you edited your question appropriately. — merlin2011
– merlin2011, Commented Jun 2, 2014 at 8:54

merlin2011 · Accepted Answer · 2014-06-02 08:40:04Z

6

Just add a setB() method and call it after you are done constructing both.

#include <iostream>

class B;

class A
{
public:
    A(B * b) : _b(b) {}
    void setB(B* b) {
        this->_b = b;
    }
    void printB(void)
    {
        if (0 != _b)
        {
            std::cout << "B is not null" << std::endl;
        }
        else
        {
            std::cout << "B is null" << std::endl;
        }
    }
private:
    B * _b;
};

class B
{
public:
    B(A * a) : _a(a) {}
    void printA(void)
    {
        if (0 != _a)
        {
            std::cout << "A is not null" << std::endl;
        }
        else
        {
            std::cout << "A is null" << std::endl;
        }
    }
private:
    A * _a;
};

int main(int argc, char ** argv)
{
    A * a = 0;
    B * b = 0;

    a = new A(b);
    b = new B(a);
    a->setB(b);

    a->printB();
    b->printA();

    delete a;
    delete b;

    return 0;
}

answered Jun 2, 2014 at 8:40

merlin2011

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4 Comments

Spook Over a year ago

How does it solve the problem What is the best way to re-write this code so that 'a' contains a reference to 'b' ? A still contains a pointer to B instead of a reference. Or maybe I got the question wrong.

merlin2011 Over a year ago

@Spook, Given the phrasing of the OP's question and the structure of his current code, I deduce that he means pointer when he says reference, rather than a literal & reference.

Spook Over a year ago

Yeah, reference in C++ means something very specific. If it is the case, this seems to be a proper solution.

chesschi Over a year ago

Sorry about that if it is not clear. Basically what I meant is when B changes, A will get B's latest change and vice versa. And the setter is the solution. Thanks!

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