Does it worth to have a ivar instead of doing Gestalt(gestaltSystemVersion, (SInt32*)&systemVersion)?

I want to support OSX 10.6 and 10.7 so I am doing some things conditionally. some of them are done several times in very short periods of time so I wonder if there is a gain in having a ivar that will tell me the systemVersion instead of doing

SInt32 systemVersion
Gestalt(gestaltSystemVersion, (SInt32*)&systemVersion);

That will be used :

if (systemVersion >= 0x1070){ //OSX 10.7
}else{//OSX 10.6 
}

I've never used Gestalt in the past. Is Gestalt doing some kind of hard stuff or is cheap to call it every time?

Gestalt is incredibly cheap, especially when compared to something like running a separate sw_vers -productVersion to figure it out. That said, it certainly wouldn't hurt to cache it as a static variable in your implementation file. You could do something like this:

MDObject.m:

enum {
    MDUndeterminedVersion    = 0,
    MDTiger                  = 0x1040,
    MDLeopard                = 0x1050,
    MDSnowLeopard            = 0x1060,
    MDLion                   = 0x1070,
    MDMountainLion           = 0x1080,
    MDMavericks              = 0x1090,
    MDUnknownVersion         = 0x1100 // ??
};

static SInt32 MDSystemVersion = MDUndeterminedVersion;

@implementation

+ (void)initialize {
    if (MDSystemVersion == MDUndeterminedVersion) {
        SInt32 MDFullSystemVersion = 0;
        Gestalt(gestaltSystemVersion, &MDFullSystemVersion);
        MDSystemVersion = MDFullSystemVersion & 0xfffffff0;
    }
}

- (void)someMethod {
   if (MDSystemVersion >= MDLion) {

   } else {

   }
}

@end

+initialize is called once and (usually) only once, before an instance of that class is ever created. So, it provides a convenient place to make sure the static variable is properly determined before any of the objects are actually used.