系统出错了，急需要改正
Linking...StudentDoc.obj : error LNK2001: unresolved external symbol "public: __thiscall CAddDlg::CAddDlg(class CWnd *)" (??0CAddDlg@@QAE@PAVCWnd@@@Z)
Debug/Student.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.
Student.exe - 2 error(s), 0 warning(s)
这是一个什么错误啊，怎么改？
