Unresolved external symbol in object files

Question

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

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 */ }

Answer 2

Make sure you decorate your header files with

#ifndef YOUR_HEADER_H
#define YOUR_HEADER_H

// your header code here

#endif

Bad things -including this- can happen if you don't

Answer 3

I'm doing some C++ for the first time in a long time, and I'm getting this error when I forget to add the ClassName:: prefix for the function definition, since this is a little unique to C++. So remember to check for that too!

Answer 4

sometimes if a new header file is added, and this error starts coming due to that, you need to add library as well to get rid of unresolved external symbol.

for example:

#include WtsApi32.h

will need:

#pragma comment(lib, "Wtsapi32.lib") 

Answer 5

See Linker Tools Error LNK2019 at MSDN, it has a detailed list of common problems that cause LNK2019.

Answer 6

In addition to the excellent answer by Chris Morris above, I found a very interesting way you can receive this same fault if you are calling to a virtual method that hasn't been set to pure but doesn't its own implementation. It is the exact same reason (the compiler can't find an implementation of the method and therefore crooks), but my IDE did not catch this fault in the least bit.

for example, the following code would get a compilation error with the same error message:

//code testing an interface
class test
{
   void myFunc(); 
}

//define an interface
class IamInterface
{
    virtual void myFunc();
}

//implementation of the interface
class IamConcreteImpl
{
    void myFunc()
    {
       1+1=2;
    }
}

However, changing IamInterface myFunc() to be a pure virtual method (a method that "must" be implemented, that than a virtual method which is a method the "can" be overridden) will eliminate the compilation error.

//define an interface
class IamInterface
{
    virtual void myFunc() = 0;
}

Hopes this helps the next StackOverFlow person stepping through code!