samplecache.m

static id cachedObject;
static NSLock *lock;

id getCachedObject() {
    [lock lock];
    id result = [cachedObject retain];
    [lock unlock];
    return result;
}

void setCachedObject(id newObject) {
    [lock lock];
    [cachedObject release];
    cachedObject = [newObject retain];
    [lock unlock];
}

//then in client code…

id object = getCachedObject();
[object doSomething];
[object release]; //not locked