Re: What is the static type of 'this'

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 17 Jun 2009 20:37:49 CST

Message-ID:

<79sqcoF1pc87jU1@mid.individual.net>

Kris Prad wrote:

In the following code, I make a call to func() that accepts Base,
but I pass DerivedFromPrivateBase which privately inherits Base.
The call is accepted (compiles) when made from a member of
DerivedFromPrivateBase f2.
Does not compile if called from elsewhere, say from
TestPrivateInheritance(). Is this OK?
Compiled on Visual C++ 2008 express and Comeau
---------------------------------

#include <iostream>
using namespace std;

struct Base
{
virtual void f() = 0;
};

void func(Base* b)
{
b->f(); // calls DerivedFromPrivateBase ::f() in the code
}

struct DerivedFromPrivateBase : private Base
{
void f()
{
cout << "DerivedFromPrivateBase:f() called" << endl;
}
void f2()
{
func(this); // Compiles: this is DerivedFromPrivateBase but calls
func that accepts Base
}

The class has access to its own direct base, whether that one is
private or not. That makes a derived to base conversion possible here
(inside the class).

};

void TestPrivateInheritance()
{
DerivedFromPrivateBase d;
d.f2();
// func (&d); // this gives compile error about inaccessible base
}

Yes, the base is private so it is not accessible outside of the class.
By providing the f2 functions, the class explicitly gives you access
to whatever it wants to. You cannot do this from the outside.

Bo Persson

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