Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_LoginReturn", referenced from:
objc-class-ref in LoginViewController.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
在LoginViewController中使用的类LoginReturn没有加入到链接库或没有声明
原文：http://www.cnblogs.com/zhengyumin/p/5252559.html