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

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