Undefined reference to vtable

Question

When building my C++ program, I\'m getting the error message

undefined reference to \'vtable...

What is the cause of this probl

Answer 1

There is lot of speculation going on in various answers here. I'll below give a fairly minimal code that reproduces this error and explain why it is occuring.

Fairly Minimal Code to Reproduce This Error

IBase.hpp

#pragma once

class IBase {
    public:
        virtual void action() = 0;
};

Derived.hpp

#pragma once

#include "IBase.hpp"

class Derived : public IBase {
    public:
        Derived(int a);
        void action() override;
};

Derived.cpp

#include "Derived.hpp"
Derived::Derived(int a) { }
void Derived::action() {}

myclass.cpp

#include 
#include "Derived.hpp"

class MyClass {

    public:
        MyClass(std::shared_ptr newInstance) : instance(newInstance) {

        }

        void doSomething() {
            instance->action();
        }

    private:
        std::shared_ptr instance;
};

int main(int argc, char** argv) {
    Derived myInstance(5);
    MyClass c(std::make_shared(myInstance));
    c.doSomething();
    return 0;
}

You can compile this using GCC like this:

g++ -std=c++11 -o a.out myclass.cpp Derived.cpp

You can now reproduce the error by removing = 0 in IBase.hpp. I get this error:

~/.../catkin_ws$ g++ -std=c++11 -o /tmp/m.out /tmp/myclass.cpp /tmp/Derived.cpp
/tmp/cclLscB9.o: In function `IBase::IBase(IBase const&)':
myclass.cpp:(.text._ZN5IBaseC2ERKS_[_ZN5IBaseC5ERKS_]+0x13): undefined reference to `vtable for IBase'
/tmp/cc8Smvhm.o: In function `IBase::IBase()':
Derived.cpp:(.text._ZN5IBaseC2Ev[_ZN5IBaseC5Ev]+0xf): undefined reference to `vtable for IBase'
/tmp/cc8Smvhm.o:(.rodata._ZTI7Derived[_ZTI7Derived]+0x10): undefined reference to `typeinfo for IBase'
collect2: error: ld returned 1 exit status

Explanation

Notice that above code does not require any virtual destructors, constructors or any other extra files for compile to be successful (although you should have them).

The way to understand this error is as follows: Linker is looking for constructor of IBase. This it will need it for the constructor of Derived. However as Derived overrides methods from IBase, it has vtable attached to it that will reference IBase. When linker says "undefined reference to vtable for IBase" it basically means that Derived has vtable reference to IBase but it can't find any compiled object code of IBase to look up to. So the bottom line is that class IBase has declarations without implementations. This means a method in IBase is declared as virtual but we forgot to mark it as pure virtual OR provide its definition.

Parting Tip

If all else fails then one way to debug this error is to build minimal program that does compile and then keep changing it so it gets to the state you want. In between, keep compiling to see when it starts to fail.

Note on ROS and Catkin build system

If you were compiling above set of classes in ROS using catkin build system then you will need following lines in CMakeLists.txt:

add_executable(myclass src/myclass.cpp src/Derived.cpp)
add_dependencies(myclass theseus_myclass_cpp)
target_link_libraries(myclass ${catkin_LIBRARIES})

The first line basically says that we want to make an executable named myclass and the code to build this can be found files that follows. One of these files should have main(). Notice that you don't have to specify .hpp files anywhere in CMakeLists.txt. Also you don't have to specify Derived.cpp as library.