C++: Declare a global class and access it from other classes?

Question

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();
}

Solution

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.

Comments

[GManNickG]

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.

[mczarnek]

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.