# C++ Linker Errors Debugging
C++ compilers generally do not produce very helpful error messages (it is part of the training of a C++ code to learn to interpret the error messages -- one needs to understand that the error message is a symptom that can lead to the error and rarely the error itself, except in the most trivial cases). C++ linkers are worse.
I had the really funny error today that the linker could not find a function I knew very well was in the library that I was providing. This is what I was getting from the C++ compiler:
error LNK2019: unresolved external symbol "public: bool __thiscall imageRead::seqStart(char *,class ATL::CStringT<wchar_t,class StrTraitMFC_DLL<wchar_t,class ATL::ChTraitsCRT<wchar_t> > >)"
The function outlined was definitely in the library, which was properly linked. I checked 10 times.
Visual studio comes with the very handy dumpbin.exe utility. It allows one to see the functions inside a .obj or .lib file. It is usually contained in the following directory:
C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\bin
Here is the microsoft help page for the tool. Here is the usage that helped me find my problem:
dumpbin.exe /SYMBOLS foo.lib dumpbin.exe /SYMBOLS foo.obj
By using dumpbin and grepping for ‘seqStart’, I got the following:
public: bool __thiscall imageRead::seqStart(char *,class ATL::CStringT<char,class StrTraitMFC_DLL<char,class ATL::ChTraitsCRT<char> > >))
The linker error indicates that it is looking for the following function:
public: bool __thiscall imageRead::seqStart(char *,class ATL::CStringT<wchar_t,class StrTraitMFC_DLL<wchar_t,class ATL::ChTraitsCRT<wchar_t> > >)
… which means that the only difference is that the library code somehow generated wchar CString, and the linker is looking for char CString. My guess is that there is a #define somewhere that defines the behavior of CString, and that it was somehow different in the two libraries. I just replaced a few to those CString with std::string and then the linker error disappeared.