Polymorphism->error: 'no known conversion from derived class to base class'

Question

How can I possibly do this:

class Base
{
public:
    int a, b, c;
    void function();
    ...
};

class Derived1 : 
    private Base
{
public:
    int d, e, f;
    ...
};

class Derived2 :
    private Base
{
public:
    void addObject(const Base* obj);
    ...
};

Note that I inherit it as private

---

Then I would like to do like:

Base* d1 = new Derived1();
Base* d2 = new Derived2();

d2->addObject(d1);

And here is the relevant issue I am getting from the compiler:

C:\test\test.cpp||In function 'void init()':|
C:\test\test.cpp|423|error: no matching function for call to 'Derived2::addObject(const Base*)'|
C:\test\test.cpp|423|note: candidate is:|
C:\test\test.h|29|note: void Derived2::addObject(const Base*)|
C:\test\test.h|29|note:   no known conversion for argument 1 from 'Derived1 {aka ... }' to 'const Base* {aka ... }'|
||=== Build finished: 1 errors, 0 warnings (0 minutes, 9 seconds) ===|

The reason I want to inherit the Base as private by some derived classes is to hide some Base 's members/functions in public scope.

I do not want to directly access some Base::functions() in a public scope but else, used a Derived::function() instead to manipulate its data members and let the derived class decide what action it would perform.

However, I could think of overriding each function from base class that I do not want to modify directly in public scope but its not flexible and too many in my case.

What I mean is:

class Base
{
public:
    //I must think in advance deciding whether if it'll be virtual or not.
    [virtual] void f1(int);
    [virtual] void f2(int);
    [virtual] void f3(int);
    //and many more...
};

class Derivedn :
    private Base
{
public:
    //hmm, I should not neglect to override certain base' member functions
    //that I don't want to access in public scope.
    void f1(int);
};

I want to avoid this but...

Is there any other way I can do, similar to this one, without overriding each Base::function() s?

Answer 1

The reason this does not work is that inheriting privately does not constitute an "is a" relationship. In fact, is a lot closer to composition than inheritance, because the derived class acquires the implementation of the base without acquiring its interface.

The reason I want to inherit the Base as private by some derived classes is to hide some Base's members/functions in public scope.

That is a wrong reason to inherit privately. Here are some good answers to the question when you should inherit privately.

Is there any other way I can do, similar to this one, without overriding each Base::function()s?

You are not required to override every single one of the Base's functions, unless they are pure virtual (also known as "abstract") ones. Your base class can provide empty implementations for its virtual functions, so that the derived classes could decide which functionality to provide.

Answer 2

I think your code should read:

d2.addObject(&d1); // Note the "&"

As the compiler says, there is no possible conversion from an instance of Derived1 to a const pointer to an instance of Base .

Answer 3

Not sure what you wanna, but polymorphism do exactly that, allow you to override a base function in Derived class:

class Base
{
public:
    int a, b, c;
    virtual void function();
    ^^^^^^^
    ...
};

class Derived1 : 
    public Base
{
public:
    int d, e, f;
    virtual void function(); // do derived version of Base::function
};

and now you can access Derived version of function from Base objects