NSString 一般来说是个常量:__NSCFConstantString
根据创建时候的不同,它还有其它类型
NSString *string1 = @"string 1";__NSCFConstantString
NSString *string2 = [NSString stringWithString:@"string 2"];__NSCFConstantString
NSString *string3 = [NSString stringWithFormat:@"string 3"];NSTaggedPointerString
NSString *string4 = [[NSString alloc] initWithString:@"string 4"];__NSCFConstantString
NSString *string5 = [[NSMutableString alloc] initWithString:@"string 5"];__NSCFString
NSString *string6 = string1;__NSCFConstantString