“_OBJC_CLASS_$_”，引用自：xcode 4.3.2 中的错误

所以今天我遇到了一个大问题。我在 xcode 4.3.2 上遇到涉及视图控制器的问题，如以下错误消息所示：

Undefined symbols for architecture i386: "_OBJC_CLASS_$_SecondViewController",
referenced from: objc-class-ref in Birdflix_ProViewController.o "_OBJC_CLASS_$_ThirdViewController", 
referenced from: objc-class-ref in Birdflix_ProViewController.o "_OBJC_CLASS_$_FourthViewController",
referenced from: objc-class-ref in Birdflix_ProViewController.o "_OBJC_CLASS_$_Author",
referenced from: objc-class-ref in Birdflix_ProViewController.o
ld: symbol(s) not found for architecture i386 clang: error: linker command failed with exit code 1 (use -v to see invocation)

我注意到这些错误仅在我编写将视图控制器链接到其他类的 IBActions 时才会出现。如果我删除 IBActions，错误就会消失。任何帮助表示赞赏。

终于找到了答案。在目标设置中，找到构建阶段并导航至编译源。一旦出现，只需将出现错误的 .m 文件添加到列表中即可。