Instantiating objects and object members

Question

For some reason the following doesn't crash like my program does, but I'm pretty sure it's similar in design. For one, the output's not correct. It outputs something similar to:

0x537ff4 5471612

While the main program outputs (nil) for the pointer address.

The key to the problem might be display_ in Drv.

Here's the code:

#include <iostream>
#include "debug.h"

class LCDText {
    public:
    int rows_;
    LCDText() { rows_ = 10; };
};

class Generic {
    LCDText *lcdText_;
    public:
    Generic(LCDText *lcdText) { lcdText_ = lcdText; };
    void Setup() {
        Error("%p %d", lcdText_, lcdText_->rows_);
    }
};

class Display : public LCDText {
    Generic *visitor_;
    public:
    Display(Generic *visitor) { visitor_ = visitor; };
};

class Drv : public Generic {
    Display *display_;
    public:
    Drv() : Generic((LCDText *)display_) {
        display_ = new Display((Generic *)this);
    };
    ~Drv() {  delete display_; };
};

int main()
{
    Drv drv;
    drv.Setup();
    return 0;
}

Answer 1

This code:

Drv() : Generic((LCDText *)display_) {
    display_ = new Display((Generic *)this);
};

first runs the parent class's ctor, with a yet-uninitialized value of display_, then independently sets display_, but, too late to change the parent class. So the pointer held by the parent class will never be set correctly. I guess you need to add a protected setter method (or make the parent-class-held pointer member itself protected).

Answer 2

Your Drv constructor passes the garbage, uninitialized value of Drv::display_ to Generic before initializing it in the constructor body. You can do a couple of things here, my preferred would be:

class Drv : public Generic {
    Display* display() { return (Display*)lcdText_; }
public:
    Drv() : Generic(new Display(this)) {}
}

Because it doesn't result in a duplicate field, but you can also have an abstract getLcdText() in Generic, which could be better if you are already using virtual methods.

Answer 3

In the constructor for Drv, when you first call the constructor for Generic display_ is still uninitialized. You don't new the pointer until later.