CODEWITHSUNDEEP
c++inheritancereadonlymember
danijar
danijar

Reputation: 34215

How to make a member readonly for derived classes?

Given an abstract base class with a protected member, how can I provide read access only to derived classes?

To illustrate my intention I provide a minimal example. This is the base class.

class Base
{
public:
    virtual ~Base() = 0;
    void Foo()
    {
        Readonly = 42;
    }
protected:
    int Readonly;                 // insert the magic here
};

This is the derived class.

class Derived : public Base
{
    void Function()
    {
        cout << Readonly << endl; // this should work
        Readonly = 43;            // but this should fail
    }
};

Unfortunately I cannot use a const member since it have to be modifiable by the base class. How can I produce the intended behavior?

Upvotes: 1

Views: 887

Answers (3)

emonkey
emonkey

Reputation: 1

Best-practice guidelines for inheritance should be to always make member variables private and accessor functions public. If you have public functions that you only want called from derived classes it means you are writing spaghetti code. (source: Meyer's Effective C++ item 22)

Upvotes: 0

syam
syam

Reputation: 15079

The usual way to do it is to make your member private in the base class, and provide a protected accessor:

class Base
{
public:
    virtual ~Base() = 0;
    void Foo()
    {
        m_Readonly = 42;
    }
protected:
    int Readonly() const { return m_Readonly; }
private:
    int m_Readonly;
};

Upvotes: 8

TieDad
TieDad

Reputation: 9929

As protected member is visible in derived class, if you want the member to be readonly in derived class, you can make it private, and provide a getter function.

class Base {
public:
    Base();
    virtual Base();

    public:
         int getValue() {return value;}

    private:
         int value;
}

This way you can still change the value in base class, and it's readonly in children class.

Upvotes: 4

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