Question

I’m sure this has something to do with virtual functions, but I’m struggling to work out how.

Here is my (simplified) situation:

Roughly what the program does is have one pair of files (computer.h) draw a computer with a blank screen, and another pair (program.h) which has a function that needs to draw on that computer screen

The computer class is going to be re-used in many different situations, so the screen draw function needs to be passed in a generic fashion

in computer.h:

include "screen.h"

class computer {

    void drawComputer(); //this function draws a picture of a computer
    void drawMonitor();

};

in computer.cpp:

void computer::drawComputer(){

    //draws all the components then the monitor

    drawMonitor(); //this is where the external function (from class screen) needs to execute
}

void computer::drawMonitor(){
    //draws the background and border of screen
}

in program.h:

class program {

    //many other program functions

    void drawScreen();

};

in program.cpp:

//many other program functions

void program::drawScreen(){
    //this function draws the contents of the screen
}

My question is, from program.cpp, how do I ‘send’ the drawScreen() function to execute within the drawMonitor() function in computer.cpp?

Edit

@Chris’ solution seems to be nearly exactly what I am after, however when I attempt to implement it I get the following errors:

testApp.h:40: error: 'testApp::prog' cannot appear in a constant-expression
testApp.h:40: error: `&' cannot appear in a constant-expression
testApp.h:40: error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression
testApp.h:40: error: ISO C++ forbids initialization of member 'isprog'
testApp.h:40: error: making 'isprog' static
testApp.h:40: error: invalid in-class initialization of static data member of non-integral type 'IScreen*'
testApp.h:41: error: 'isprog' has not been declared
testApp.h:42: error: ISO C++ forbids declaration of 'comp1' with no type
testApp.h:42: error: expected ';' before '.' token

The lines are

39    Program prog;
40    IScreen *isprog = dynamic_cast<IScreen*>(&prog);
41    OP1 comp1(isprog);
42    comp1.drawScreen();

Anyone know where I’m going wrong with the implementation?

Answer 1

Well you’re half-way there. For something like this, yes I’d use virtual functions (in order to define an abstract interface). Here’s the basic outline of how I’d do it:

// First create a class to define an interface for drawing a screen
class IScreen
{
public:
    // Defines an interface named drawScreen
    virtual void drawScreen() = 0;
};

// Next actually implement the interface
class Program : public IScreen
{
public:
    // Here we actually implement it
    virtual void drawScreen()
    {
        // Draw some stuff here
    }
};

// You can implement this more than once if you want
class BlueScreenOfDeathScreen : public IScreen
{
public:
    virtual void drawScreen()
    {
        // Draw a BSOD on the screen
    }
};

// Finally, use the interface
class Computer
{
private:
    IScreen* myScreen;

public:
    Computer(IScreen* screen)
        : myScreen(screen)
    {
    }

    void drawComputer()
    {
        // ...
    }

    void drawMonitor()
    {
        // Draw the monitor
        // ...

        // Draw the screen
        myScreen->drawScreen();
    }
};

Doing it this way, you can easily define multiple IScreen implementations and quickly swap them out with minimal changes to your code.

// Render using the "Program" class
Program prog;
IScreen *iprog = dynamic_cast<IScreen*>(&prog);
Computer comp1(iprog);
comp1.drawScreen();

// Render using the "BlueScreenOfDeathScreen" class
BlueScreenOfDeathScreen bsod;
IScreen *ibsod = dynamic_cast<IScreen*>(&bsod);
Computer comp2(ibsod);
comp2.drawScreen();

Easy, no?