Re: Restricting access should be illegal?
Walter Bright wrote:
Consider the following legal code:
-----------------------
#include <stdio.h>
class A {
public:
virtual void Member() { printf("A::Member\n"); }
};
class B : public A {
private:
virtual void Member() { printf("B::Member\n"); }
};
int main()
{
B *b = new B();
// b->Member(); // error, B::Member is private
A *a = b;
a->Member(); // calls B::Member
}
-------------------------
Shouldn't restricting access to an overriding virtual function be an
error? After all, we can get at it anyway via an implicit conversion.
Does anyone know of a legitimate design pattern that does this?
Certainly; it's a pain that Java doesn't allow this, but
that goes hand-in-hand with not allowing private derivation
from interfaces.
It's useful in C++ to be able to do
struct interface { virtual void foo() = 0; };
void use_interface(interface & i)
{
// Code including calls via the interface...
i.foo();
}
class concrete : private interface
{
public:
void bar()
{
use_interface(*this);
}
private:
void foo()
{
std::cout << "Only accessible to selected clients.\n";
}
};
I believe this is sometimes referred to as the "private
interface" idiom. It can come in handy when your class
finds it appropriate to implement an interface so that
it can work with some third-party code, but does not
want to expose that interface to its own clients.
-- James
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