[c++] GCC C++ Linker errors: Undefined reference to 'vtable for XXX', Undefined reference to 'ClassName::ClassName()'

I'm setting up a C++ project, on Ubuntu x64, using Eclipse-CDT. I'm basically doing a hello world and linking to a commerical 3rd party library.

I've included the header files, linked to their libraries, but I still get linker errors. Are there some possible problems here other than the obvious (e.g. I am 99% sure I'm linking to the correct library).

1. Is there a way to confirm the static libraries I am linking to are 64bit?
2. Is there a way to confirm that the library has the class (and methods) I am expecting it to have?

Eclipse says:

Building target: LinkProblem
Invoking: GCC C++ Linker
g++ -L/home/notroot/workspace/somelib-3/somelib/target/bin -o"LinkProblem"  ./src/LinkProblem.o   -lsomelib1 -lpthread -lsomelib2 -lsomelib3
./src/LinkProblem.o: In function `main':
/home/notroot/workspace/LinkProblem/Debug/../src/LinkProblem.cpp:17: undefined reference to `SomeClass::close()'
./src/LinkProblem.o: In function `SomeOtherClass':
/home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:148: undefined reference to `SomeClass::SomeClass()'
/home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:148: undefined reference to `vtable for SomeOtherClass'
/home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:151: undefined reference to `SomeClass::~SomeClass()'
./src/LinkProblem.o: In function `~SomeOtherClass':
/home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:140: undefined reference to `vtable for SomeOtherClass'
/home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:140: undefined reference to `SomeClass::~SomeClass()'
/home/notroot/workspace/somelib-3/somelib/include/sql/somefile.h:140: undefined reference to `SomeClass::~SomeClass()'
collect2: ld returned 1 exit status
make: *** [LinkProblem] Error 1

I put this for future visitors:

if you are receiving the error on creating an Exception object, then the cause of it probably is a lack of definition for what() virtual function.

In regards to problems with Qt4, I couldn't use the qmake moc option mentioned above. But that wasn't the problem anyway. I had the following code in the class definition:

class ScreenWidget : public QGLWidget
{
   Q_OBJECT        // must include this if you use Qt signals/slots
...
};

I had to remove the line "Q_OBJECT" because I had no signals or slots defined.

In my case the problem occured when i forgot to add the =0 on one function in my pure virtual class. It was fixed when the =0 was added. The same as for Frank above.

class ISettings
{
public: 
    virtual ~ISettings() {};
    virtual void OKFunction() =0;
    virtual void ProblemFunction(); // missing =0   
};

class Settings : ISettings
{
    virtual ~Settings() {};
    void OKFunction();
    void ProblemFunction(); 
};

void Settings::OKFunction()
{
    //stuff
}

void Settings::ProblemFunction()
{
    //stuff
}

I had this error message when trying "hello world" like things with Qt. The problems went away by correctly running the qt moc (meta object compiler) and compiling+including these moc-generated files correctly.

The problem for me turned out to be pretty obscure. My class looked like this:

//-----------------------------------------
// libbase.h
class base {
public:
   base() { }
   virtual ~base() { }

   virtual int foo() { return 0; }
};
//-----------------------------------------

//-----------------------------------------
// libbase.cpp
#include "libbase.h"
//-----------------------------------------

//-----------------------------------------
// main.h
class derived : public base {
public:
    virtual int foo() ;
};
//-----------------------------------------

//-----------------------------------------
// main.cpp
int main () {
    derived d;
}
//-----------------------------------------

The problem is in the linker. My header file went in a library somewhere, but all the virtual functions were declared 'inline' in the class declaration. Since there was no code using the virtual functions (yet), the compiler or linker neglected to put actual function bodies in place. It also failed to create the vtable.

In my main code where I derived from this class, the linker tried to connect my class to the base class and his vtable. But the vtable had been discarded.

The solution was to declare at least one of the virtual functions' bodies outside the class declaration, like this:

//-----------------------------------------
// libbase.h
class base {
public:
   base() { }
   virtual ~base() ;   //-- No longer declared 'inline'

   virtual int foo() { return 0; }
};
//-----------------------------------------

//-----------------------------------------
// libbase.cpp
#include "libbase.h"
base::~base() 
{
}
//-----------------------------------------

I had this error message. The problem was that I declared a virtual destructor in the header file, but the virtual functions' body was actually not implemented.

Qt C++ will show this error when you change a class such that it now inherits from QObject (ie so that it can now use signals/slots). Running qmake -r will call moc and fix this problem.

If you are working with others via some sort of version control, you will want to make some change to your .pro file (ie add/remove a blank line). When everyone else gets your changes and runs make, make will see that the .pro file has changed and automatically run qmake. This will save your teammates from repeating your frustration.

This linker error usually (in my experience) means that you've overridden a virtual function in a child class with a declaration, but haven't given a definition for the method. For example:

class Base
{
    virtual void f() = 0;
}
class Derived : public Base
{
    void f();
}

But you haven't given the definition of f. When you use the class, you get the linker error. Much like a normal linker error, it's because the compiler knew what you were talking about, but the linker couldn't find the definition. It's just got a very difficult to understand message.

If you have a base class with pure virtual function, make sure your base class constructor and destructor has body otherwise linker fails.

I stumbled across the issue now, too. The application defined a pure virtual interface class and a user-defined class provided through a shared lib was supposed to implement the interface. When linking the application, the linker complained that the shared lib would not provide vtable and type_info for the base class, nor could they be found anywhere else. Turned out that I simply forgot to make one of the interface's methods pure virtual (i.e. omitted the " = 0" at the end of the declaration. Very rudimentary, still easy to overlook and puzzling if you can't connect the linker diagnostic to the root cause.

This error will also occur when we simply declare a virtual function without any definition in the base class.

For example:

class Base
{
    virtual void method1(); // throws undefined reference error.

}

Change the above declaration to the below one, it will work fine.

class Base
{
    virtual void method1()
    {
    }
}