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I am declaring class variable of type NSString in my "class.h":

@interface class : NSObject<GADInterstitialDelegate, 

GADBannerViewDelegate, GADRewardBasedVideoAdDelegate, 

GADNativeAppInstallAdLoaderDelegate, GADNativeContentAdLoaderDelegate>

{

   ...

   NSString* appId;

}

I the "class.mm" file In function "a" i am assigning a value to the variable:

appId = [[dic objectForKey:@"appid"] stringValue]

The string value at this point is correct.

The problem happen when other function call - function "b".

[GoogleMobileAdsMediationTestSuite presentWithAppID:appId onViewController:rootViewController delegate:nil];

when i try to use this appId - it doesn't contain the assigned, instead I assume it contain a memory address.

How can I keep the value of appId in all future references?

You know how, see when you assign value it just keep reference form original object but soon you initialize new object with new alloc or call retain, it's a copy of original. You can keep retain or use

NSString *entityName = [[NSString alloc] initWithString:[[dic objectForKey:@"appid"] stringValue]];` 

Or may be you can define appId as:

@property (strong, nonatomic) NSString *appId; 

in interface and, refer it with self.appId.

So i got it to work by doing this: appId = [[dic objectForKey:@"appid"] retain];

If that compiles, it means that you've turned off ARC (automatic reference counting) for that file or for your whole project. There's little reason to do that these days, especially if you're not very familiar with the manual reference counting rules. The best solution is almost certainly to turn ARC back on and remove the retain call.

So that means that the appId my assumption was correct and appId was referencing the "dic" object all this time? The question is how can i detach the appId from the "dic" object so it will stay alive even when the object is cleard?

Basically, you were assigning a string to appId without retaining the string. When the dictionary you got it from was released, it also released all the objects that it contained, including the string that appId pointed to. With manual reference counting, which you are apparently using, you have to retain any objects that you keep a reference to, and release those objects when you no longer need that reference. If you create an object with alloc/init or new or copy (or some variant of those), you don't need to retain that object, but you do need to release it. You can read more about it in Memory Management Rules.

Try to alloc appID variable like: [NSString string] in "class.m" and then you can change value of this variable.
You try to assign value to null