using base class methods that use private field

Question

I have a class, it has several public methods that use one private field that they all changed and use this field in their computations.

I need subclass from this class with few new methods, new methods won't use this field. I expect those methods from the base class behave the same as they did, but a little bit confused that they use private field.

Are private field accessible in the subclass? Should I also declare a field with same type and name in my subclass?

possible duplicate of [What are public, private and protected in object oriented programming?](http://stackoverflow.com/questions/1020749/what-are-public-private-and-protected-in-object-oriented-programming) — quetzalcoatl, Mar 11 '13 at 11:39
note to 'duplicate' vote: you should review the differences between 'private' and 'protected' — quetzalcoatl, Mar 11 '13 at 11:40
To add to all the existing answers: no, you should not declare a field with same type and name in your subclass. The one already declared in the base class is enough. — anatolyg, Mar 11 '13 at 11:55

mathematician1975 · Answer 1 · 2013-03-11T12:24:33.413

3

Private member data is not accessible to derived classes. If you want to access this data, change it to protected. Public member functions of the base class that are used to change the private member will still be available to your derived class but direct access of base class private data is not allowed. All public and protected fields of the base class will be available to your derived class (assuming you are using public inheritance)

edited Mar 11 '13 at 12:24

answered Mar 11 '13 at 11:35

mathematician1975

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I won't access it directly, I have inherited methods from base class that access it, I don't re-implement them but just use them. – Littlebitter Mar 11 '13 at 11:36

score 1 · Answer 2 · answered Mar 11 '13 at 11:36

1

Private fields are never accessible to derived classes. If you declared your methods which access the private field as public, the derived classes can use them.

answered Mar 11 '13 at 11:36

bash.d

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score 1 · Answer 3 · answered Mar 11 '13 at 11:45

Most posters seem to misunderstood your question.

YES, you can use those public methods. That is the whole point of private members - they can be accessed by any member of the same class be they public, protected or private

score 0 · Answer 4 · answered Mar 11 '13 at 11:46

In the derived class members, you cannot access the private field. However, the base class members, which you have inherited through the virtue of them being public, can of course still access it. The base member functions will be visible through the derived class if you used public inheritance, but only to the derived class if you used protected or private inheritance. protected and private inheritance is rarely used, though default if you are using the class keyword.

For further clarification, if you did want to access the field, you should define accessors in the base class with protected access. This would allow you to control the access to the private field.

However, you should ask yourself whether polymorphism is the correct solution here? Does the derived class pass the is-a test? Is the base class's destructor virtual?

score 0 · Accepted Answer · answered Mar 11 '13 at 12:43

You could use the public functions (which use a private field) in you drived class. You should not declare a field with same type!! But you could not direct access a private field from the base class in the drived class. Use for this a public function from your base class (e.g. setter and getter functions)

Sample:

//***********************************
//Base Class
//***********************************
class Base
{
public:
    Base();
    ~Base();

    void increment();
    void decrement();

    void get();

private:
    int id_;
};

Base::Base()
    : id_(0)
{
}

Base::~Base()
{
}

void Base::increment()
{
    ++id_;
}

void Base::decrement()
{
    --id_;
}

int Base::get()
{
    return id_;
}

void Base::set(id)
{
    id_ = id;
}

//***********************************
// Drived Class
//***********************************
class Drived() : public Base
{
public:
    Drived();
    ~Drived();

    void plus(int a);
    void minus(int b);
};

Drived::Drived()
    : Base()
{
}

Drived::~Drived()
{
}

void Drived::plus(int a)
{
    int i = get();
    set(i + a);             // <- OK
}

void Drived::minus(int b)
{
    id_ -= b;               // <- Not allowd; id is private
}

using base class methods that use private field

5 Answers5