Access in cpp classes

When a base class is inherited as private, all public and protected members of that class
become private members of the derived class. However, in certain circumstances, you
may want to restore one or more inherited members to their original access
specification. For example, you might want to grant certain public members of the base
class public status in the derived class even though the base class is inherited as private. 

 

In Standard C++, you have two ways to accomplish this. First, you can use a
using statement, which is the preferred way. The second way to restore an inherited 

member's access specification is to employ an access declaration
within the derived class. Access declarations are currently supported by Standard C++,
but they are deprecated. This means that they should not be used for new code. Since
there are still many, many existing programs that use access declarations, they will be
examined here.
An access declaration takes this general form:
base-class::member;
The access declaration is put under the appropriate access heading in the derived class'
declaration. Notice that no type declaration is required (or, indeed, allowed) in an
access declaration.
To see how an access declaration works, let's begin with this short fragment:
class base {
public:
int j; // public in base
};
// Inherit base as private.
class derived: private base {
public:
// here is access declaration
base::j; // make j public again
.
.
.
};
Because base is inherited as private by derived, the public member j is made a private
member of derived. However, by including
base::j;
as the access declaration under derived's public heading, j is restored to its public status.
You can use an access declaration to restore the access rights of public and
protected members. However, you cannot use an access declaration to raise or lower a
member's access status. For example, a member declared as private in a base class
cannot be made public by a derived class. (If C++ allowed this to occur, it would
destroy its encapsulation mechanism!)
The following program illustrates the access declaration; notice how it uses access
declarations to restore j, seti(), and geti() to public status.
#include
using namespace std;
class base {
int i; // private to base
public:
int j, k;
void seti(int x) { i = x; }
int geti() { return i; }
};
// Inherit base as private.
class derived: private base {
public:
/* The next three statements override
base's inheritance as private and restore j,
seti(), and geti() to public access. */
base::j; // make j public again - but not k
base::seti; // make seti() public
base::geti; // make geti() public
// base::i; // illegal, you cannot elevate access
int a; // public
};
int main()
{
derived ob;
//ob.i = 10; // illegal because i is private in derived
ob.j = 20; // legal because j is made public in derived
//ob.k = 30; // illegal because k is private in derived
ob.a = 40; // legal because a is public in derived
ob.seti(10);
cout << ob.geti() << " " << ob.j << " " << ob.a;
return 0;
}
Access declarations are supported in C++ to accommodate those situations in
which most of an inherited class is intended to be made private, but a few members are
to retain their public or protected status.
While Standard C++ still supports access declarations, they are deprecated. This
means that they are allowed for now, but they might not be supported in the future.
Instead, the standard suggests achieving the same effect by applying the using
keyword.