c++ - Unresolved external symbol in object files

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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?$allocator@D@2@@std@@@Z)
1>Form.obj
            : error LNK2019: unresolved external symbol "public: virtual void __thiscall
            Field::parse(class std::basic_stringstream            std::char_traits,class std::allocator > &)"
            (?parse@Field@@UAEXAAV?$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?$allocator@D@2@@std@@@Z)
1>Form.obj
            : error LNK2001: unresolved external symbol "public: virtual void __thiscall
            Field::prompt(void)" (?prompt@Field@@UAEXXZ)
1>Form.obj : error LNK2001:
            unresolved external symbol "public: virtual class std::basic_string            std::char_traits,class std::allocator > __thiscall
            Field::getName(void)"
            (?getName@Field@@UAE?AV?$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

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class="normal">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 */
}