NSString *str = @"I LOVE COCOA";
这种方式最简单地创建了一个NSString对象，很多人也这样用。但是这样有很大缺陷。
The simplest way to create a string object in source code is to use the Objective-C @”…” construct:
NSString *temp = @”/tmp/scratch”;
Note that, when creating a string constant in this fashion, you should avoid using anything but 7-bit ASCII characters. Such an object is created at compile time and exists throughout your program’s execution. The compiler makes such object constants unique on a per-module basis, and they’re never deallocated, though you can retain and release them as you do any other object. You can also send messages directly to a string constant as you do any other string:
BOOL same = [@"comparison" isEqualToString:myString];
上文的大概含义是，以@方式生成的字符串，会作为字符串常量，在程序过程中，会一直存在，占用着内存。
这样的字符串常量不能被释放，虽然可以进行retain或release，也可以直接对字符串常量发送方法信息，如同对其它的字符串
所以，建议使用stringwithxxx方法，可以建立一个宏：
#define OC(str) [NSString stringWithCString:(str) encoding:NSUTF8StringEncoding]
或
[[NSString alloc] initWithUTF8String:"hello UTF8String"];
于是就可以方便使用了：NSLog(@”%@”, OC(“Hello World!”) );