Re: private virtual functions - accessibility legality
On Oct 19, 2:34 am, Vijay Meena <vijay.me...@gmail.com> wrote:
Private functions can be virtual in C++ ( unlike Java) ? What
is the use of private virtual functions ?
The usual pattern is to define the contractual interface in
public functions, then implement it in private functions and
data. The contractual interface remains the same (or should
remain the same) in all derived classes; you shouldn't be able
to override it. So any virtual functions are usually private
(although there are a number of exceptions---e.g. if there is an
inversion of the call sequence, and the interface doesn't define
any contract).
Below is the code -
#include <iostream>
class Foo {
public:
virtual void test() { std::cout << "Foo::test()" << std::=
endl; }
};
class Bar : public Foo {
private:
void test() { std::cout << "Bar::test()" << std::endl; }
friend void friendToBar(Bar *);
};
void notFriendToBar (Foo *pFoo) {
pFoo->test();
}
void friendToBar (Bar *pBar) {
pBar->test();
}
int main(int argc, char *argv[]) {
Bar b;
friendToBar(&b);
notFriendToBar(&b);
}
Function Bar::test() is private to Bar. but stil we can access
it outside Bar in notFriendToBar function. Is it legal ?
It's legal. It's a consequence of the other rules; this
particular use is not useful. (To date, I've found no useful
reason for restricting the overriding function more than the
function in the base class, but there's no real reason to ban it
either. In the end, most virtual functions are private to begin
with.)
Is not private functions are meant to be either accessible to
friend or to its class only ?
Yes, but there are ways of subverting this.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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