C++: Unresolved external symbol

I got this error in visual studio when i try to compile my project

>main.obj : error LNK2019: unresolved external symbol "public: class std::vector<class enveloppexml::Element const ,class std::allocator<class enveloppexml::Element const > > __thiscall enveloppexml::Element::obtenirEnfants(void)const " (?obtenirEnfants@Element@enveloppexml@@QBE?AV?$vector*****CBVElement@enveloppexml@@V?$allocator*****CBVElement@enveloppexml@@@std@@@std@@XZ) referenced in function "void __cdecl traiteElement(class enveloppexml::Element const &,int)" (?traiteElement@@YAXABVElement@enveloppexml@@H@Z)

i have tried differents solutions i google but still not work. does any one know what it means.
And i want to know, given a solution in visual studio is there a way to know if it is a win32 console app or just a win32 project?
thnks