Ios5 iOS 5链接器错误

编辑一些代码后，我发现4个构建错误：

Undefined symbols for architecture i386:
"_OBJC_CLASS_$_Playlist", referenced from:
  objc-class-ref in PlayerViewController.o
"_OBJC_CLASS_$_Song", referenced from:
  objc-class-ref in PlayerViewController.o
"_currentIndex", referenced from:
  -[PlayerViewController viewDidLoad] in PlayerViewController.o
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)

---

关于如何解决这个问题有什么提示吗？

有时候Xcode会忘记在编译列表中添加类

---

转到项目设置-目标-构建阶段-编译源代码，检查是否所有需要的文件都在那里。

有时候Xcode忘记在编译列表中添加类

转到项目设置-目标-构建阶段-编译源代码并检查是否存在所有需要的文件