为什么存储在NSMutableDictionary中的方法参数中传递的值变得空

Question

- (void) msg:(NSString *)msg from:(NSString *)from
{
     NSMutableDictionary *m = [[NSMutableDictionary alloc] init];
     [m setObject:msg forKey:@"msg"];
     [m setObject:from forKey:@"sender"];
     [m setObject:[NSString getCurrentTime] forKey:@"time"];


NSLog(@"MSG=>%@ : FROM=>%@ : TIME=>%@",[m objectForKey:msg], [m objectForKey:@"sender"], [m objectForKey:@"time"]);

NSLog(@"MSG=>%@",msg);

}

Reply of first log is

MSG=>(null) : FROM=>your friend! : TIME=>Apr 30, 2012 12:52:02 AM

And reply of second log is

MSG=>hello...how are you

I didn't understand why "msg" after going inside NSMutableDictionary getting null? after all parameter "from" is also displaying fine. And the interesting thing is logging msg parameter directly displaying perfectly.

I am using ARC enabled project and getting this null issue. I don't want to declare NSMutableDictionary out side of this method because it is getting called many times and every time new NSMutableDictionary created and stored in NSArray for further use.

Answer 1

[m objectForKey:msg]

is wrong, you're trying to get back the object using itself as the key. Correctly:

[m objectForKey:@"msg"]