类型转换

NSInteger <--> NSString

NSString *inStr = [NSString stringWithFormat:@"%d", (int)month];

NSInteger = [ NSString  intValue];

NSData--> NSMutableData

NSData *_data = [[NSData alloc] init];
NSMutableData *_mdata = [NSMutableData dataWithData:_data];
[_data release];

NSstring --> NSdata

NSString *aString = @"ab";
NSData *aData = [aString dataUsingEncoding: NSUTF8StringEncoding];