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Derived classes problem: 'undefined reference to vtable' linker error with derived classes

Torin (38)
Hi,

I'm beginning to work on a plugin framework for an application, which will work by dynamically loading shared/dynamic libraries that contain classes (unknown to the application) that derive from base classes (known to the application).

I'm using MinGW with GCC and GNU-Make. The compiler output is below, including the rather strange linker error.

g++ -DBUILDING_DLL -c -o plugin.o classes.cpp
g++ -shared -o plugin.dll plugin.o
plugin.o:classes.cpp:(.text$_ZN9baseClassC2Ev[baseClass::baseClass()]+0x8): undefined reference to `vtable for baseClass'
collect2: ld returned 1 exit status
mingw32-make: *** [plugin.dll] Error 1


The problem appears to be in baseClass' constructor, which I haven't defined. I get the same error, but twice, if I define baseClass' constructor as 'baseClass() {}', as I did with derivedClass (see code).

The code isn't too long, I'll post it below. Help is appreciated. I think I'm doing something wrong with constructors, but I'm not sure what.


classes.h
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// classes.h

#ifndef CLASSES_H
#define CLASSES_H

#ifdef BUILDING_DLL
#define DLL_INTERFACE __declspec(dllexport)
#else
#define DLL_INTERFACE __declspec(dllimport)
#endif


class DLL_INTERFACE baseClass
{
public:
	virtual void testFunc();
};


class DLL_INTERFACE derivedClass : public baseClass {

public:
	derivedClass() {};
	~derivedClass() {};
	void testFunc();
	
private:
	void privateFunc();
};


#endif



classes.cpp:
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// classes.cpp

#include <iostream>
#include "classes.h"

void derivedClass::testFunc()
{
	this->privateFunc();
	return;
}

void derivedClass::privateFunc()
{
	std::cout << "derivedClass::privateFunc()\n";
	return;
}



Makefile:
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CPP = g++

all: plugin.dll

plugin.dll: plugin.o
	$(CPP) -shared -o $@ $?
	
plugin.o: classes.cpp classes.h
	$(CPP) -DBUILDING_DLL -c -o $@ classes.cpp
vlad from moscow (6539)
I think the problem is that you did not define testFunc in the base class. Also you shall define destructor as virtual.
codekiddy (783)
Try this,

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// classes.h

#ifndef CLASSES_H
#define CLASSES_H

#ifdef BUILDING_DLL
#define DLL_INTERFACE __declspec(dllexport)
#else
#define DLL_INTERFACE __declspec(dllimport)
#endif


class DLL_INTERFACE baseClass
{
public:
        virtual ~baseClass() { };
	virtual void testFunc() = 0;
};


class DLL_INTERFACE derivedClass : public baseClass {

public:
	derivedClass() {};
	~derivedClass() {};
	void testFunc() override;
	
private:
	void privateFunc();
};
#endif
Torin (38)
@vlad
Thanks. Defining testFunc in the base class did it:

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class DLL_INTERFACE baseClass
{
public:
	virtual void testFunc() {};
	virtual ~baseClass() {};
};


As you can see, I also took your advice and made the destructor virtual. Omitting it didn't make a difference to compilation, but if you think it's a good idea, then thanks for the advice!

Also, this makes a lot more sense to me after reading this, which I just found after some searching: http://gcc.gnu.org/faq.html#vtables

Thanks again.
clanmjc (666)
If you do not declare the base class destructor as virtual, then derived classes that need to free resources in their destructors will never get called from a base class pointer.
Torin (38)
@clanmjc

Okay, that makes sense. Thanks.
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