无法解决循环依赖

Question

Given following classes:

// myClass.hpp
class Child1;
class Child2;

class MyClass {
  public:
    virtual void doSomething (const MyClass&) const = 0;
    virtual void doSomething (const Child1&) const = 0;
    virtual void doSomething (const Child2&) const = 0;
};

// child1.hpp
#include "myClass.hpp"

class Child1 : public MyClass {
  public:
    virtual void doSomething (const MyClass&) const override;
    virtual void doSomething (const Child1&) const override;
    virtual void doSomething (const Child2&) const override;
};


// child2.hpp
#include "myClass.hpp"

class Child2 : public MyClass {
  public:
    virtual void doSomething (const MyClass&) const override;
    virtual void doSomething (const Child1&) const override;
    virtual void doSomething (const Child2&) const override;
};

The compiler gives me the errors:

undefined reference to 'Child1::doSomething(MyClass const&)'

The same error is printed for the other doSomething(..) functions.

I'm sure there is some mistake in including files (I use include guards for every header-file!!). My questions are: Where do I have to include the corresponding files and where do I need forward declaration?

Answer 1

"Undefined reference" looks like a linker problem. Ensure that you have a child1.cpp including child1.hpp and that you compile and with it; and that child1.cpp define the override function in the correct way.

Answer 2

The error tells you that there is no reference to the whole Method.

In other words you need to define the Method in a *.cpp File or directly in the header. See for example this overview or this similar question . The answers tell you that is actual not a Compiler error but a Linker error.

Edit: As Hans Olsson Answer suggests, it could also be that you need to include your cpp files .

If you do not exactly know how the implementation will look like but want to know if you header-concept works you can implement them empty

virtual void doSomething (const MyClass&) const override {}

Or compiler your code with gcc with the flag "-c" which tells gcc enter to just do the compiling not the linking .