When do you use protected visibility specifier to a class member in C?

Answer 1

a class member declared as private can only be accessed by member functions and friends of that class
a class member declared as protected can only be accessed by member functions and friends of that class,and by member functions and friends of derived classes

Answer 2

We use the protected access specifier and protected inheritance specifier whenever we design a base class and wish to expose some part of its private interface to all its derived classes (and their friends) without making that interface public.

Protected access and protected inheritance do different jobs regarding the visibility of base class members. First let's consider protected access:

struct base {

public: void publicMember();

protected: void protectedMember();

private: void privateMember();

};

• Everything that is aware of base can access base::publicMember().
• Derivatives of base and their friends can also access base::protectedMember().
• base and its friends can also access base::privateMember().

Note that protected access is ideally suited to member functions rather than to member data, although most technical examples will use member data to illustrate the concept. Ideally, all member data (the class representation) should be declared private to both enable encapsulation and to reduce the maintenance burden. That is, if we change the class representation in any way, only the class and its friends would be affected by those changes, thus localising the effects. So long as the public and protected interfaces remain unchanged, all consumers of our class (including existing code we cannot access and would otherwise break) would be unaffected by the changes.

Now let's consider protected inheritance:

struct publicDerived : public base {};

struct protectedDerived : protected base {};

struct privateDerived : private base {};

• Everything that is aware of publicDerived is also aware that publicDerived derives from base.
• Only protectedDerived, its friends, its derivatives and their friends are aware that protectedDerived derives from base.
• Only privateDerived and its friends are aware that privateDerived derives from base.

In addition, the protected and private inheritance specifiers also change the accessibility of non-private base class members with respect to the derived class. With protected inheritance, all public members of the base class become protected members of the derived class. With private inheritance, both the public and protected members of the base class become private members of the derived class. Note that private members of a base class can never be inherited so we cannot change their accessibility with respect to the derived class.

Inheritance specifiers can also be applied to the inherited protected and public members on a member-by-member basis. For example:

structPublicDerive : public base {

protected: void base::publicMember();

private: void base::protectedMember();

};

Here, everything that is aware of publicDerived is also aware that publicDerived derives from base, as before. However, publicDerived::base::publicMember() is now a protected member of publicDerived rather than a public member, while protectedDerived::base::protectedMember() is now a private member of publicDerived rather than a protected member. Note that we can only reduce or maintain the accessibility of an inherited member, never increase it. That is, we cannot make a protected member of a base class a public member of one of its derivatives.

Answer 3

When the class is expected to act as a base class and the member needs to remain private to the class, but must be accessible to the derived classes.

Answer 4

protected visibility specifier is used when don't want to give direct access of the members inside it to the user but we have to or might need to inherit those members in the child class.