Question

Here is the code first, it comes from ‘Ruminations on C++’ chapter 10

// TestCode.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <iostream>
#include <string>
#include <conio.h>


using namespace std;

class P_Node
{
    friend class Picture;
protected:
    P_Node() : use(1)
    {

    }
    virtual ~P_Node()
    {

    }
private:
    int use;
};

class Picture
{
    friend Picture frame(const Picture&);
public:
    Picture() : p(new P_Node)
    {
        cout << "Constructor\t" << "Picture::Picture()" << "\tcalled" << endl;
        cout << "Picture p count\t" << p->use << endl;
    }
    Picture(const Picture& orig) : p(orig.p)
    {
        cout << "Copy Constructor\t" << "Picture::Picture(const Picture&)" << "\tcalled" << endl;
        cout << "Picture p count\t" << p->use << endl;
        orig.p->use++;
    }
    ~Picture()
    {
        cout << "Destructor\t" << "Picture::~Picture()" << "\tcalled" << endl;
        cout << "Picture p count before decrease\t" << p->use << endl;
        if(--p->use == 0)
        {
            cout << "Picture p count after decrease\t" << p->use << endl;
            cout << "Deleted" << endl;
            delete p;
        }
    }
    Picture& operator=(const Picture& orig)
    {
        cout << "operator=\t" << "Picture& Picture::operator=(const Picture& orig)" << "\tcalled" << endl;
        cout << "Picture p count before decrease\t" << p->use << endl;
        orig.p->use++;
        if(--p->use == 0)
        {
            cout << "Picture p count after decrease\t" << p->use << endl;
            delete p;
        }
        p = orig.p;
        return *this;
    }
private:
    Picture(P_Node* p_node) : p(p_node)
    {
        // Why not p_node->use++?
        cout << "Picture::Picture(P_Node* p_node)\tcalled" << endl;
    }
    P_Node *p;
};

class Frame_Pic : public P_Node
{
    friend Picture frame(const Picture&);
private:
    Frame_Pic(const Picture& pic) : p(pic)
    {
        cout << "Frame_Pic::Frame_Pic(const Picture& orig)" << "\tcalled" << endl;
    }
    Picture p;
};

Picture frame(const Picture& pic)
{
    return new Frame_Pic(pic);
}

int main(int argc, char* argv[])
{
    Picture my_pic;
    frame(my_pic);
    return 0;
}

The result is:

Constructor Picture::Picture()  called
Picture p count 1
Copy Constructor    Picture::Picture(const Picture&)    called
Picture p count 1
Frame_Pic::Frame_Pic(const Picture& orig)   called
Picture::Picture(P_Node* p_node)    called
Destructor  Picture::~Picture() called
Picture p count before decrease 1
Picture p count after decrease  0
Deleted
Destructor  Picture::~Picture() called
Picture p count before decrease 2
Destructor  Picture::~Picture() called
Picture p count before decrease 1
Picture p count after decrease  0
Deleted

I have two questions about this code:

1. Why is the copy constructor called before Frame_Pic‘s Constructor? In my mind, the copy constructor is called because frame(my_pic) is returning a Picture by value. But that should be called after Frame_Pic‘s Constructor.
2. In Picture::Picture(P_Node* p_node), why not increase the use count? isn’t this creating a new Picture?

Thanks for any help.

I’m using VC6 under Windows XP.

Answer 1

1, Why is the Copy Constructor called before Frame_Pic’s Constructor?

Because the p member is being copy-constructed in the initialization list of Frame_pic’s constructor. The initialization list runs before the constructor’s body is entered.

In my mind, the Copy Constructor is called because frame(my_pic) is returning a Picture by value. But that should be called after Frame_Pic’s Constructor.

frame() is declared to return a Picture instance by value, but it is coded to return a Frame_pic* instead. Frame_pic derives from P_node, and Picture has a constructor that accepts a P_node*, and that constructor is accessible to frame() so the compiler allows it.

2, In Picture::Picture(P_Node* p_node), why not increase the use count? isn’t this creating a new Picture?

The use count is on P_node, not Picture. The Picture that frame() returns owns the Frame_pic that frame() creates, whose use count member is already 1 by the Frame_pic constructor. That is why that Picture constructor does not increment the use count – it is already at the correct value.

The Frame_pic contains its own Picture that is copy-constructed from another Picture, so that Picture constructor needs to increment the use count of the original Picture.