解决编译时出现的警告：format string is not a string literal (potentially insecure)

   转载自：http://www.cocoachina.com/bbs/read.php?tid=87645

       在Xcode 4.2（iOS 5）之前，我猜大家都和我一样很喜欢下面的调试输出写法：

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NSString *str = @ "Attention" ;            // first warning      NSLog (str);            // second warning      NSLog ([str stringByAppendingFormat:@ ", %@" , @ "Xcode 4.2 do not support this format!" ]);            // third warning      NSLog ([ NSString stringWithFormat:@ "%@, Xcode 4.2 do not support this format!" , @ "Attention" ]);            // fourth warning      NSException *exception = [ NSException exceptionWithName:@ "Attention"                                                       reason:@ "Xcode 4.2 do not support this format!"                                                     userInfo: nil ];            NSLog (exception);

但是在Xcode 4.2(iOS 5)之后，貌似苹果更新的编译器，出了支持ARC的Apple LLVM compiler 3.0。然后我发现每次编译，以前的这些输出都会产生一个warning(警告，黄色三角形)。
在StackOverflow和iPhone Dev SDK查找相关答案之后，发现在最新版的编译器里面NSLog为了安全，只接受格式化的字符串，因为NSLog底层也是用printf来格式化输出的。
所以上面的写法都会给出警告，可以把上面的写法修改为以下合法模式：

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NSString *str = @ "Attention" ;            // first warning      NSLog (str);                           // warning      NSLog (@ "%@" , str);         // solution      NSLog (str, nil );                    // solution            // second warning      NSLog ([str stringByAppendingFormat:@ ", %@" , @ "Xcode 4.2 do not support this format!" ]);                         // warning      NSLog (@ "%@" , [str stringByAppendingFormat:@ ", %@" , @ "Xcode 4.2 do not support this format!" ]);         // solution      NSLog ([str stringByAppendingFormat:@ ", %@" , @ "Xcode 4.2 do not support this format!" ], nil );                    //solution            // third warning      NSLog ([ NSString stringWithFormat:@ "%@, Xcode 4.2 do not support this format!" , @ "Attention" ]);             // warning      NSLog (@ "%@, Xcode 4.2 do not support this format!" , @ "Attention" );                                                                 // solution      NSLog ([ NSString stringWithFormat:@ "%@, Xcode 4.2 do not support this format!" , @ "Attention" ], nil );       // solution            // fourth warning      NSException *exception = [ NSException exceptionWithName:@ "Attention"                                                       reason:@ "Xcode 4.2 do not support this format!"                                                     userInfo: nil ];            NSLog (exception);                         // warning      NSLog (@ "%@" , exception);       // solution