Modify Your Base Class Interface in Derived Classes-4 : Page 4

Controlling Access Type
A using-declaration also enables you to change the access type of a base class's member in a derived class. Consider a ReadOnly class that enables users to read a file but not to change it. You may need to change the access type of Write() to private. This can be accomplished by adding a using-declaration to the private section of the class:

class ReadOnly: public File
{
private: //change the access of File::Write
 using File::Write;
};

Consequently, ReadOnly objects cannot invoke this member function:

ReadOnly ro;
ro.Write(mybuff);//error: Write() is not accessible

The Buck Stops Here
Each derived class can override a previous access type. For example, if you have a member function f() declared public in class A, and a derived class D which changes the access type of f() to private:

class A
{
public:
 int f();
protected:
 int g();
};
class D: public A
{
private
 using A::f(); //OK, f() is now private
};

A class derived from D may override the access type of f() once again:

class E : public D
{
public
 using A::f(); //OK, f() is public again
};

However, you cannot grant a more permissive access type than the one originally specified in the member's declaration. For example, the member function g() is declared protected in A. Class F that is derived from A can change g()'s access to private. A class derived from F may subsequently change g()'s access back to protected. However, none of them can change g()'s access type to public. This restriction ensures that the using-declaration facility doesn't violate the fundamental aspects of the C++ object model.

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