C++ inheritance: Can't access private element of base class

Question

I have a base class:

class Base
{
public:
    functions...
private:
    std::vector<float> privateElement;
}

and a derived one:

class Derived : public Base
{
public:
    functions...
    Derived(anotherElement) : privateElement(anotherElement)
    {

    }
}

My problem now is that everytime I try to build my project, the compiler (gcc 4.7.2) always complains about not being able to access privateElement, like:

class Derived does not have any field named privateElement
std::vector<float> Base::privateElement is private

May anybody help me here?

And what? That's what's `private` is designed to do?!? – πάντα ῥεῖ Sep 05 '13 at 17:03 — πάντα ῥεῖ, Sep 05 '13 at 17:03

Nawaz · Accepted Answer · 2013-09-05T17:13:05.950

4

First of all, private members of base class are not accessible from derived class.

Now even if you fix that and make it protected (or public), then this will still be ill-formed, because you cannot initialize the base's members in the derived class's mem-init-list. It doesn't make sense, because by the time the derived class mem-init-list executes the base's members are already initialized, and the syntax : protectedElement(xyz) will make the compiler think that protectedElement is a member of the derived class, not of the base class!

See this error even after making it protected:

main.cpp: In constructor ‘Derived::Derived(std::vector<float>)’:
main.cpp:20:37: error: class ‘Derived’ does not have any field named ‘protectedElement’
     Derived(std::vector<float> v) : protectedElement(v)
                                     ^

Online demo

The right way to do this is to define a constructor for the base class, and call this from the derived class initialization-list.

Hope that helps.

edited Sep 05 '13 at 17:13

answered Sep 05 '13 at 17:01

Nawaz

353,942
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666
851

So when using the constructor from the base class, will privateElement be created in the derived class or will I actually be using the object of the base class? – BlackMamba Sep 05 '13 at 17:31
@BlackMamba: Try doing what you understand from my post. Then do let us know if you face any problem (then you also post the code). – Nawaz Sep 05 '13 at 17:37
Okay, this solved my inition problem, but now I face another issue with the constructor: I want to initialize another object with elements of privateElement, but when I try to use it like this: Derived(anotherElement) : Base(anotherElement), randomElement(privateElement[0], privateElement[1]) { } it complains about privateElement being private. How do I access this alement after initialization? – BlackMamba Sep 05 '13 at 17:58

score 1 · Answer 2 · answered Sep 05 '13 at 17:12

The right way to do it without breaking encapsulation would be for the base class to provide a constructor which we can call from derived class :

class Derived : public Base
{
public:
    functions...
    Derived(anotherElement) : Base(anotherElement)
    {

    }
}

class Base
{
public:
    Base(anotherElement):privateElement(anotherElement) { }
private:
    std::vector<float> privateElement;
}

score -1 · Answer 3 · answered Sep 05 '13 at 16:58

-1

Derived class cannot access private parts of the base class. That's because they are, well, private. Use protected if you want the derived class to have access to the base class' members without breaking the encapsulation.

answered Sep 05 '13 at 16:58

Marcin Łoś

3,226
1
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That's true, I haven't looked at the code in question carefully enough. – Marcin Łoś Sep 05 '13 at 19:23

C++ inheritance: Can't access private element of base class

3 Answers3