Initializing a static pointer in C++

Question

I have a class with a static member that's a pointer like so :

animation.h

class Animation
{
public:
    Animation();
    static QString *m;

};

animation.cpp

#include "animation.h"

QString* Animation::m = 0;

Animation::Animation()
{
}

When I try to initialize that 'm' pointer from another class like so :

Animation::m = new QString("testing");

It works.

But when I do it this way :

QString x("Testing");
Animation::m = &x;

The program crashes.

What is wrong with this second method ?

Also I would like to have that static pointer as private so I can make static getter and setter functions to it. The setter should use the second method as the 'x' will come in a parameter so I'm stuck.

Thanks for any help!

Answer 1

I bet it's not crashing on that line, but afterwards.

The problem is that you're taking the address of a variable located in automatic memory, and probably try to access it afterwards. The variable x will be destroyed when it's scope ends, but Animation::m will still point to that memory (memory you no longer own after x went out of scope). This results in undefined behavior.

Just like the following would be illegal:

int* x = NULL;
{
   int k = 3;
   x = &k;
}
*x = 4;

Workaround assign to the value, not the pointer (provided it was previously assigned to a valid QString*):

QString x("Testing");
*(Animation::m) = x;