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




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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.

Danny Kalev is a certified system analyst and software engineer specializing in C++. He was a member of the C++ standards committee between 1997 and 2000 and has since been involved informally in the C++0x standardization process. He is the author of "The ANSI/ISO Professional C++ Programmer's Handbook" and "The Informit C++ Reference Guide: Techniques, Insight, and Practical Advice on C++."
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