Beginner with C++ - Need helps with overload member function error

I'm trying to do programming C++ for my project.
I got into trouble when I tried to do program for communicating with COM port(reading information in ComboBox).

The error " overloaded member function 'class CString(class CComboBox &a) not found in 'CXeedlg'. Could someone help me to overcome ?
overloaded member function not found in 'class'
The following is my code:
"
CString getCurStrInComboBox(const CComboBox &a)
{ CString str;
a.getLBText(a.GetCurSel(),str);
return str;
}
"

And the 2nd one is I dont know the reason why the complier informed that InsertString, AddString, GetCursel() arent members of ComboBox.

The deadline for my project is coming soon so I will really appreciate all your helps.

Thank you very much