C++如何正确使用静态变量？

Question

I'm making a c++ OpenGL project and I'm having trouble with static variables.

I have a header "Scene.h" like so:

#pragma once

#include "A.h"

class Scene
{
    //class body
};

static Scene* active = new Scene();

And my Ah file looks like this:

#pragma once

#include "Scene.h"

class A
{
    active->SomeMethod(); //Here I get error C2065: undeclared identifier
};

In my source file I have included only my Scene.h, since it includes Ah already, and I have no problem there.

I have also tried to use a static Scene object like so:

class Scene
{
    static Scene* active;
};

And then to access it like so:

Scene::active->DoSomething();

But then I get error C2653: Scene is not a class or namespace name. I read somewhere that to do this I need precompiled headers, and that is no option for me.

What is the correct way to have a static pointer in this case?

Answer 1

If you declare a global variable static in C++, you make it internal linkage. This means, it is not visible to other compilation units, and since you want to use it in class A - which probably resides within a different source file - this is not what you want.

What you probably want is a static class member (your second example), but be aware that in addition to the declaration you need to define it:

class Scene
{
    public:
       static Scene* active;
};

Scene* Scene::active = new Scene();

Also note that members of C++ classes by default are private , so your second example is lacking the public access specifier, so A would not be able to access a private member of Scene .

As Scheff has already pointed, you're having issues with circular dependencies. If class Scene does not use A in any way, remove #include "Ah" in the "Scene.h". Otherwise move active->SomeMethod(); to the implementation of class A and use forward declaration instead of including "Scene.h" in Ah

[edit] As a note, since you only show excerpts of your code: This answer was written under the assumption that you have a header file and a source file for Scene , and a header file and a source file for A . If you keep both Scene and A within their headers only, things will not work out because of circular dependencies. Unless you have a strong reason for going header-only, I would suggest in any case that you have a clean separation between clas declaration (header file) and implementation (source file), avoid too many mutual includes if possible and rather use forward declarations.

Answer 2

See working example (by the use of the hint from scheff ) in onlineGdb .

First of all I do not recommend use of global variables! Even worse is there use at global namespace. Better design is some namespace or if applicable (acessed only from one file) definition in an anonymos namespace. Think about using the singleton-Pattern - but that does not adress your Question.

Second I do not recomend to use a raw pointer: Objects you created with new you have to destroy, but at which time you'll want to do it. If ever You stick to a pointer use some smartpointer ... - My enhancment uses an Objecte, and prove by cout the call of the destructor.

the Prob in your code is: You can't call a method inside a class definition.