Compilation error when using multiple inheritance

Question

Can't get my program compile. For historical reason I have a bit "complicated" hierarchy of the classes in my application. And now I faced with another problem: it won't compile because of this hierarchy. My project is too big to show it here, but below you can see the example that demonstrates the problem. Is there is a simple and elegant way to fix it? Every interface/class has really big amount of methods. Thanks in advance.

struct ITest
{
    virtual ~ITest() {}
    virtual void a() = 0;
    // and many other methods
};

struct Test1 : public ITest
{
    virtual ~Test1() {}
    virtual void a() override {}
    // and many other methods overrides from ITest
    // plus a lot of other logic
};

struct ExtendedTest1 : public Test1
{
    virtual ~ExtendedTest1() {}
    // a lot of some other stuff
};

struct ITest2 : public ITest
{
    virtual ~ITest2(){}
    // and big count of the its methods and logic
};

struct MainClass : public ExtendedTest1, public ITest2
{
     virtual ~MainClass(){}
     // a lot of logic
};

int main()
{
    MainClass mainClassObj;
    return 0;
}

And the errors:

main.cpp: In function ‘int main()’:
main.cpp:36:15: error: cannot declare variable ‘mainClassObj’ to be of abstract type ‘MainClass’
     MainClass mainClassObj;
               ^~~~~~~~~~~~
main.cpp:28:8: note:   because the following virtual functions are pure within ‘MainClass’:
 struct MainClass : public ExtendedTest1, public ITest2
        ^~~~~~~~~
main.cpp:4:18: note:    virtual void ITest::a()
     virtual void a() = 0;
                  ^

Do not judge strictly :)

UPD: before asking this question I really tried virtual inheritance to solve my problem but it didn't work. So after suggestion to try it again, it works) So, replacing these lines solved my problem:

struct Test1 : public ITest  --->  struct Test1 : virtual public ITest
struct ITest2 : public ITest ---> struct ITest2 : virtual public ITest

I know, we must avoid virtual inheritance, but we can't 'cause of historical reason and very big amount of code

Thanks everybody for help!

Answer 1

You need Virtual Inheritance.

Virtual inheritance can get rid of redundant functions:

Instead, if classes B and C inherit virtually from class A, then objects of class D will contain only one set of the member variables from class A.

If you inherit virtually, you will get rid of your duplicate abstract function.