Question

I have a class structure

Class A
{
   Object of classB
   Object of classC
}

Now for this purpose I have files A.h/A.m B.h/B.m , C.h/C.m
Now I have a separate file G.h which has a number of global variables which are used by all the class A,B,C
Now I have imported B.h , C.h , G.h inside A.h . I also have separately imported the file G.h inside B.h and C.h

Now when build the project , I receive a link error for duplicate symbol on the global varibles (which is probably due to multiple inclusion of the file G.h) .

How can I solve this ??? The following is my code structure

//A.h

#import "B.h"
#import "C.h"
#import "G.h"

@interface A : NSObject {
  B *b;
  C *c;
}
//B.h
#import "G.h"

//C.h

#import "G.h"

//G.h
A *a=nil;
@interface G : NSObject { //whole class is empty}

Answer 1

You should declare your globals in G.h but define them in G.m, so that they’re only linked once in the target. So if, for example, you have a string constant that’s globally defined, you would do:

G.h

extern NSString const *appName;

G.m

NSString const *appName = @"My Great App";

In your pasted code, it’s the A instance that’s being duplicated. It’s imported into each other translation unit, but with the same name every time which means that the compiled objects can’t be linked. Assuming you want a shared A instance called a, you’d do:

G.h

@class A;
extern A *a;

G.m

#import "A.h"
#import "G.h"

A *a = nil;