c++ - Unresolved external symbol in object files

During coding in Visual Studio I got an unresolved external symbol error
and I've got no idea what to do. I don't know what's wrong.
Could you please decipher me? Where should I be looking for what kind of errors?

1>Form.obj : error LNK2019: unresolved external symbol "public: class Field * __thiscall Field::addField(class Field *)" (?addField@Field@@QAEPAV1@PAV1@@Z) referenced in function "public: void __thiscall Form::parse(class std::basic_stringstream,class std::allocator > &)" (?parse@Form@@QAEXAAV?$basic_stringstream@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z)
1>Form.obj : error LNK2019: unresolved external symbol "public: virtual void __thiscall Field::parse(class std::basic_stringstream,class std::allocator > &)" (?parse@Field@@UAEXAAV?$basic_stringstream@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) referenced in function "public: __thiscall InputField::InputField(class std::basic_stringstream,class std::allocator > &)" (??0InputField@@QAE@AAV?$basic_stringstream@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z)
1>Form.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Field::prompt(void)" (?prompt@Field@@UAEXXZ)
1>Form.obj : error LNK2001: unresolved external symbol "public: virtual class std::basic_string,class std::allocator > __thiscall Field::getName(void)" (?getName@Field@@UAE?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ)

1>Form.obj : error LNK2001: unresolved external symbol "public: virtual class std::basic_string,class std::allocator > __thiscall Field::getType(void)" (?getType@Field@@UAE?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ)
1>Form.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Field::describe(void)" (?describe@Field@@UAEXXZ)
1>C:\Users\tomy\Documents\Visual Studio 2010\Projects\zapoctovkac++\Debug\zapoctovkac++.exe : fatal error LNK1120: 6 unresolved externals

Answer

This error often means that some function has a declaration, but not a definition.

Example:

// A.hpp

class A
{
public:
  void myFunc(); // Function declaration
};

// A.cpp

// Function definition
void A::myFunc()

{
  // do stuff
}

In your case, the definition cannot be found. The issue could be that you are including a header file, which brings in some function declarations, but you either:

1. do not define the functions in your cpp file (if you wrote this code yourself)


2. do not include the lib/dll file that contains the definitions

A common mistake is that you define a function as a standalone and forget the class selector, e.g. A::, in your .cpp file:

Wrong: void myFunc() { /* do stuff */ }

Right: void A::myFunc() { /* do stuff */ }