C++ Global Object

How to use global variables in Objective-C?

Traditionally, global variables are declared in a header, and defined in a source file. Other source files only need to know how it is declared to use it (i.e. its type and its name). As long as the variable is defined somewhere in a source file, the linker will be able to find it and appropriately link all the references in other source files to the definition.

Somewhere in your header, you would declare a global variable like this:

extern int GlobalInt;

The extern part tells the compiler that this is just a declaration that an object of type int identified by GlobalInt exists. It may be defined later or it may not (it is not the compiler's responsibility to ensure it exists, that is the linker's job). It is similar to a function prototype in this regard.

In one of your source files, you define the GlobalInt integer:

int GlobalInt = 4;

Now, each file that includes the header will have access to GlobalInt, because the header says it exists, so the compiler is happy, and the linker will see it in one of your source files, so it too will be happy. Just don't define it twice!

However

You should consider whether or not this approach is useful. Global variables get messy for a number of reasons (trying to find out exactly where it is defined or declared, threading issues), there is usually not a need for global variables. You should perhaps consider using a singleton approach.

What is the need of global objects in c++?

The short answer is there is no need.

The long answer is that it can sometimes be convenient. Unfortunately convenience is subjective, and what one finds to convenient another might find to be too lenient.

Global objects have issues (among which muddying data flows in the code and access synchronization), but sometimes one might find it convenient nonetheless to use one because it makes something easier. It can indeed remain easier, or it can prove a maintenance burden, but that only the future can tell...

... If you can avoid global objects, in general you should be better off. In practice, though, you will rarely encounter issues before programming at scale; for toy examples and students' projects there is rarely an issue, which is why beginners fail to understand why more seasoned developers avoid them like the plague.

Declare a global object inside of a function (c++)

Try to declare the variable globally (outside of the function)
use the function to change the global variable.

Variables declared inside of a function are only accessible in that function; they are called local variables as they are local to the function in which they are declared.

The main() function cannot access the local variables of another function.

Globally declared variables are visible everywhere.

Creating A Global C++ Object

Don't define the object instance within the header file. Provide a declaration there and define the object in a single cpp file. For instance,

CoreEnvironment.h

class CoreEnvironment
{
    NetHandler* m_NET;
    NetPoint* Server;
    PhysicsHandler* m_Physics;
    irr::IrrlichtDevice* Device;
    irr::ITimer* Timer;

public:
    CoreEnvironment();
    ~CoreEnvironment();

    bool RunDevice();
    void Update();
};

extern CoreEnvironment CoreEnv;

CoreEnvironment.cpp

#include "CoreEnvironment.h"

CoreEnvironment CoreEnv;

And if there are only two other classes that require access to the CoreEnv object, maybe you should rethink the need for the global instance. Instead something like this might suffice.

int main()
{
    CoreEnvirnment CoreEnv;
    ActorHandler actor(CoreEnv); 
    WorldHandler world(CoreEnv);

    // do stuff with actor and world
}

where the constructors of ActorHandler and WorldHandler take (maybe const) references to CoreEnvironment.

c++ global object

We declare our globals as extern in a header file, in your case: global_obj.h, and the actual global variable in a source file: global_obj.cpp. In separate source files we #include "global_obj.h" to have access to them.

It should look like this:

global_obj.cpp

Class obj;

global_obj.h

extern Class obj;

main.cpp

#include "global_obj.h"

Global variable in Objective-C program

For a standard global variable (not persistent when the app is terminated and restarted) add this to a header file (*.h) of your choice:

extern NSInteger MYGlobalVariable;

Then put this in the implementation file; (*.m, *.c, *.cpp):

MYGlobalVariable = 0;  // Or any other default value.

That is how you do a bread and butter global variable.

MFC: How to use C++ Global Object in C

You cannot access classes from C++ in C without some sort of indirection and/or interface. If you really want to use it in C (Why?) then you will have to devise some kind of extern "C" interface to your object.

E.g. implement some cinterface.h:

#ifdef __cplusplus
extern "C" {
#endif

// It does not have to be void* but at this point it is the easiest thing to use.
typedef void * ObjCType;
ObjCType obj_get_obj (void);
int obj_get_value(ObjCType);

#ifdef __cplusplus
};
#endif

And then in cinterface.cpp implement the C language interface delegating to the Obj's member functions.:

#include <obj.hpp>
#include <cinterface.h>

// This is defined somewhere else.
extern Obj obj;

ObjCType obj_get_obj ()
{
  return &obj;
}

int obj_get_value(ObjCType o)
{
  return static_cast<Obj*>(o)->get_value ();
}

And finally, use the C interface in your source.c:

#include <cinterface.h>

int main ()
{
  ObjCType o = obj_get_obj ();
  int x = obj_get_value (o);
}