情况描述：
有两个ViewController，
FatherViewController和ChildViewController，
ChildViewController继承于FatherViewController
FatherViewController实现单例方法：
+ (*)shareInstance{
static FatherViewController *father = nil;
static dispatch_once_t once;
dispatch_once(&once, ^{
father = [[FatherViewController alloc] init];
});
return father;
}
当使用ChildViewController *child = [ChildViewController shareInstance];的时候会出现警告“incompatible pointer types initializing 'ChildViewController *' with an expression of type 'FatherViewController *'
原因就是返回类型是FatherViewController 但是是用ChildViewController来接收，起的冲突。
将FatherViewController单例的返回值改成id或者instancetype就可以消除警告
但是，在使用的时候发现，虽然警告不见了，但是child实质还是FatherViewController类，所以还是无法获取ChildViewController
修改方法：
将FatherViewController的单例改为
+ (id)shareInstance{
static NSMutableDictionary *dic = nil;
static dispatch_once_t once;
dispatch_once(&once, ^{
dic = [[NSMutableDictionary alloc] init];
[dic setObject:[[self alloc] init] forKey:(id<NSCopying>)[self class]];
});
return [dic objectForKey:[self class]];
}