inheritance - How to properly delegate interface to sister class in C++?

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Let's assume I have classes Child and Parent, where Child extends the Parent:

class Parent 
 void foo() cout << "foon";
 void bar() cout << "barn";
 void jar() cout << "jarn";
;

class Child: public Parent 
 void child_method1();
 void child_method2();
;

Now I want to export some of the Child functionality using the following interface:

class Interface 
public:
 virtual ~Interface()

 virtual void foo() = 0;
 virtual void bar() = 0;

protected:
 Interface()
;

As you can see the interface methods are all already implemented in Parent. How do I properly define Child so that the Interface is delegated to the Parent ?

I'm trying to do it like:

class Child: public Parent, public Interface 
 void child_method1();
 void child_method2();

But the compiler complains that the virtual methods foo() and bar() are not implemented in Child.

If I write an explicit delegation like this:

class Child: public Parent, public Interface 
 void foo() Parent::foo();
 void bar() Parent::bar();

 void child_method1();
 void child_method2();

the compiler doesn't complain.
Is it necessary to write the explicit delegation? What is the proper way to delegate the virtual methods to a sister class?

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