Question

Maybe you will be able to clear something for me, because I don’t know exactly where my thinking is flawed. First some code:

Talker.h:

class talker       
{
    public:
    talker();
    void sayHello() {cout << "Hello";} ;
};

anotherClass.h:

class anotherClass
{
    public:
    anotherClass();
    void doSomethingYourself() { cout << "will do"; };
    void askHimToSayHello() { pointerToTalker->sayHello; };  
                          //Access violation, bad pointer(?)
};

common.h:

 static talker *pointerToTalker;       
      // if I add here "= new talker", code works fine

main.cpp:

#include "common.h"
int main()
{
    pointerToTalker = new talker;            // Here is the bug, but why?
    pointerToTalker -> sayHello;             // says Hello alright

    anotherClass *pointerToAnotherClass = new anotherClass;
    pointerToAnotherClass -> doSomething ();  //Does something OK

    pointerToAnotherClass -> askHimToSayHello(); // Causes access violation 
}

Of course functions are a bit more complex, and each is implemeted in coresponding .cpp including “common.h”. My question is – why pointerToTalker, if initialized inside main() does not work inside anotherClass::AskHimToSayHello()? It should pointing to valid memory by the time it is used there.
It is my “Hello world, OOP!” btw, so please be gentle if there is no hope for me 🙂

Sorry for the childish style btw. It helps me cut down the code I have to something more compact without me loosing the big picture :).

Answer 1

Because

static talker *pointerToTalker;   

is not a global. In this context, static gives the variable internal linkage for each translation unit (cpp file + included files) in which common.h is included.

You need to declare it as extern:

extern talker *pointerToTalker;   

and initialize it in a single implementation file.

Declaring it static will create a copy of pointerToTalker for each translation unit. So you’re initializing the one from main.cpp. Others are left uninitialized, and thus you run into undefined behavior. The proper way is:

//common.h:
extern talker *pointerToTalker;    

//common.cpp
#include "common.h"
talker* pointerToTalker = new talker;