[SOLVED] C++ class member pointer changes after function returns

Snark1994 · 03-30-2011, 04:19 PM

I'm venturing into the world of OpenGL, but in the process of writing my graphics library I ran into a problem... My code is:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>

class myGraphics {
	private:
		Display *display;
		char* myChar;
	public:
		int createWindow(int,int);
		void printMahPointer(void);
};

int myGraphics::createWindow(int width, int height){
	Display *display = XOpenDisplay(0);
	myChar = (char*) malloc(1);
	printf("%p %p\n",(void*) display,myChar);
	return 0;
}

void myGraphics::printMahPointer(void){
	printf("%p %p\n",(void*) display,myChar);
}
 
int main(void){
	myGraphics graphics;
	graphics.createWindow(100,100);
	graphics.printMahPointer();
	return 0;
}

I then compiled this with

Code:

g++ graphics.cpp -o graphics -lX11 -Wall

and got output:

Code:

0x601010 0x602680
0x7fff7f3f3fa0 0x602680

The pointer to 'display' seems to change its value when the function returns, and the value it changes to each time is different which suggests to me it's referencing invalid memory. Certainly, if I try to use it in any functions that draw to it then it throws a segfault. These problems don't occur if I use the exact same code inside the createWindow() function.

Can anyone point me to a resource on pointers which explains what's going on here? (no pun intended...) Thank you

dwhitney67 · 03-30-2011, 04:26 PM

Oh boy, that code looks more C-like than C++. Anyhow, the issue is that you re-declared "display" in your createWindow() function. Obviously you meant to use the class member data variable.

Snark1994 · 03-30-2011, 04:34 PM

Cheers

I knew it would be something stupid like that... Should have read it more carefully.

And yeah, it kinda is. I'm new to C++ (and, well, pretty new to C) but I started using it for the classes. Should I try to make changes to my coding style for C++, or is this more of a "whatever works" type thing?

dwhitney67 · 03-30-2011, 04:39 PM

Do you want to excel in C++, or just do the minimum to get by?

99% of software development has to do with style, and unfortunately it's not yours, but that of your peers. If you adhere to the design and style expected by your peers (or company or teacher), then you will be fine.

You are at the beginning of your project (obviously not much there), so it is hard to comment on your design (you have one, right??).

As for C++, one should allocate memory using "new", not "malloc"... and never allocate one measly byte! If you require a string, use the STL string class object. But in the end, you do what you want. After all, it is your project.

Snark1994 · 03-30-2011, 04:44 PM

Ah, myChar was just to show that the 'char' pointer didn't change when I popped out of the function

In terms of code, I'm just having fun really. So I don't really have an "expected" design or style. However, thanks for the heads-up on "new". And... unless I'm misunderstanding you, I don't really have a design

I'm just putting together some functions for other programmes to call, so I can re-use my graphics and window creation code.

tuxdev · 03-30-2011, 06:46 PM

Quote:

As for C++, one should allocate memory using "new", not "malloc"... and never allocate one measly byte! If you require a string, use the STL string class object. But in the end, you do what you want. After all, it is your project.

Well, really you shouldn't use "new" either.. except in a constructor, and then make sure to do the "delete" in the destructor. RAII is a very important resource management C++ idiom.

Personally, I almost always let SDL handle opening the window context. There's some limitations to that and I don't know if you need more capability, but like I said, I almost never need more.