摘要:
int ival = 1024;
int &refVal = ival; // ok: refVal refers to ival
int &refVal2; // error: a reference must be initialized
int &refVal3 = 10; // error: initializer must be an object 阅读全文
摘要:
The NSNull class defines a singleton object used to represent null values in collection objects (which don’t allow nil values).
+ (NSNull *)null:
Returns the singleton instance of NSNull.
如果要向一个字典,或数组 加入空对象可以加 [NSNull null] 阅读全文