Class object defined in main(). How to get this instance in another class?

Question

I need to get functionA as a global function in functionB but to use only one instance of class A defined in main(). functionA will use in many classes and I'll want to get access as simple as it possible. I know defining of class in .h file is a bad practise but I think it's not critical in this case. In my case I get error, but how can I do this in the same manner? If I place A a(); before main() this works perfect, but A is initialized in the main() function.

/////A.h//////
class A
{
public:
    A(){//define constructor A//};
    functionA()
    {
        ...
    }
};

/////B.h//////
extern A a; //declare A//
class B
{
public:
    B(){//define constructor B//};
    functionB()
    {
        a.functionA();
    }
};

////main.cpp////

#include "A.h"
#include "B.h"
int main()
{
    B b(); //B b; 
    A a(); //A a; of course. Thanks for remark guys.
}

This is the error I am getting:

Error LNK2001: unresolved external symbol "class A a"

I think you might need to include #include "A.h" at the top of the B.h file. Let me know if this works. — Alex, Jul 23 '20 at 17:55
Not the problem but FWIW, `B b();` and `A a();` declare functions in `main`, not objects. You need `B b; A a;` or `B b{}; A a{};` — NathanOliver, Jul 23 '20 at 17:55
Is it an option to have a reference to an `A` inside `B` which gets initialised via constructor parameter? — Yunnosch, Jul 23 '20 at 17:58
`functionA will use in many classes` Then you'll have to make it `static`, make `a` global, or pass a reference to `a` to functions that need to use its member methods. — dxiv, Jul 23 '20 at 18:01
[When to use extern in C++](https://stackoverflow.com/questions/10422034/when-to-use-extern-in-c) covers what's gone wrong here. Also handy reading (even though it's discussing and referencing C): [How do I use extern to share variables between source files?](https://stackoverflow.com/questions/1433204/how-do-i-use-extern-to-share-variables-between-source-files) — user4581301, Jul 23 '20 at 18:01
That said, avoid globals and prefer to pass variables around instead. — user4581301, Jul 23 '20 at 18:01
@javierMarquez `extern A a;` This declares a global object `a`, but you don't define one anywhere, which is what LNK2001 complains about. — dxiv, Jul 23 '20 at 18:04
class B probably should take a reference to an `A` object instead of the global variable `a` — drescherjm, Jul 23 '20 at 18:08
@dxiv Sorry, my mistake, I define it in class A and initialize in main(). Look update. — javierMarquez, Jul 23 '20 at 18:24
***How to get this instance in another class?*** Get rid of the extern a; and the global variable and instead pass a reference to A in the constructor of B store that as a class member in B — drescherjm, Jul 23 '20 at 18:33
@javierMarquez `I define it in class A and initialize in main()` No, not at all. What you declare with `extern A a;` in the header file is a *global* object `a`. What you define in `main` is a *local* variable `a` which only happens to have the same name. The two `a`'s are not referring to the same object, and the global object `a` is not defined anywhere. — dxiv, Jul 23 '20 at 18:43
@dxiv I'm surprised the main () function also has a local variable. Thank you and all you guys. That's why initializing A before main() works fine. — javierMarquez, Jul 23 '20 at 20:01
@javierMarquez I assumed that's what you meant there, but `main` does not in fact have a local variable `a`. See Nathan Oliver's comment about why `A a();` is actually parsed as a function declaration. — dxiv, Jul 23 '20 at 20:38

Class object defined in main(). How to get this instance in another class?

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