CODEWITHSUNDEEP
c++pointers
St.Antario
St.Antario

Reputation: 27385

Base class pointer conversion

The following code I've written to check pointer conversion:

#include <iostream>
#include <cstdlib>

using std::cout;
using std::endl;

class A
{ 
    friend class B;
};

class B : private A { };

int main()
{
    B *b = new B;
    A *a = b;
    cout << b << endl;
    cout << a;
}

Section 11.2/4 N3797 says:

A base class B of N is accessible at R, if 
— an invented public member of B would be a public member of N, or 
— R occurs in a member or friend of class N, and an invented public member of B would be a private or protected member of N, or 
— R occurs in a member or friend of a class P derived from N, and an invented public member of B would be a private or protected member of P, or 
— there exists a class S such that B is a base class of S accessible at R and S is a base class of N accessible at R

I think A is accessible base class of B.

A prvalue of type “pointer to cv D”, where D is a class type, can be converted to a prvalue of type “pointer to cv B”, where B is a base class (Clause 10) of D. If B is an inaccessible (Clause 11) or ambiguous (10.2) base class of D, a program that necessitates this conversion is ill-formed

Why do I recieve a compile-time error?

main.cpp:17:12: error: cannot cast 'B' to its private base class 'A'

demo

Upvotes: 3

Views: 105

Answers (1)

Sneftel
Sneftel

Reputation: 41474

In order for the friend declaration to make the conversion valid, it needs to take place in a scope where the friendship has effect -- that is, in B's own code. The fact that A declares B as a friend has no effect outside B's definition.

Upvotes: 4

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