8

I have a class which should be declared globally from main() and accessed from other declared classes in the program, how do I do that?

class A{ 
    int i; 
    int value(){ return i;}
};

class B{ 
   global A a; //or extern?? 
   int calc(){
       return a.value()+10;
   }
}

main(){
   global A a;
   B b;
   cout<<b.calc();
}
Georg Fritzsche
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Regof
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  • I guess singelton design pattern is a good point to start with http://stackoverflow.com/questions/1008019/c-singleton-design-pattern – Artem Barger May 09 '10 at 18:13
  • @Artem - to reemphasize what @gf says - don't do this! Global state makes code impossible to compose and tightly coupled. This is bad , amongst other reasons, because it makes it virtually impossible to test. Also, nearly every singleton implementation I have seen was not thread safe in some subtle and unpleasant way you don't notice until your code runs on a CPU with a weak memory model. – Stewart May 09 '10 at 18:35

2 Answers2

8

You probably really do not want to do this, but if you must - in the file that contains main:

#include "A.h"
A a;

int main() {
 ...
}

and then in the files that need to access the global:

#include "A.h" 
extern A a;

You will need to put the declaration of A in the A.h header file in order for this to work.

  • Thanks, will the externs be visible from inside the other classes? – Regof May 09 '10 at 18:19
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    @Regof If something is declared at file scope (to use an old term) and is not tagged as static, it can be accessed from _anywhere_. That's why creating such objects is A Bad Idea. I would never create such things in my own code, and I suggest you don't either. But the language, as I've indicated, does allow it. –  May 09 '10 at 18:25
  • let me try it out... thats just for an experiment, just playing around, dont worry :-) – Regof May 09 '10 at 18:49
2

In C++ declaring a global instance of a class is a no-no.

You should instead use the singleton pattern, which gives you a single instance of your object accessible from the entire application.

You can find a lot of literature on C++ singleton implementation, but wikipedia is a good place to start.

Thread safe singleton pattern implementation has already been discussed on stackoverflow.

Community
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Daniele Milan
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    Can't use global, so use singleton? What do you think a singleton is? It's global. Just use a global and ditch all the single-instance crap you don't need. – GManNickG May 09 '10 at 21:24
  • There actually is one very good reason to use Singleton: You don't have to remember to extern the class in every single file you want to use it from. Could lead to weird errors, probably not. But more importantly easier to use the class you've created in larger projects. – mczarnek Jan 09 '22 at 15:00