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c++privateradixderivedaccessible

Why would the conversion between derived* to base* fails with private inheritance?

Here is my code -

#include<iostream>
using namespace std;

class base
{
public:
    void sid()
    {
    }  
};

class derived : private base
{
public:
    void sid()
    {
    }
};

int main()
{
    base * ptr;
    ptr = new derived; // error: 'base' is an inaccessible base of 'derived'
    ptr->sid();
    return 0;
}

This gives a compile time error. 

error: 'base' is an inaccessible base of 'derived'

Since the compiler will try and call the base class sid() why do I get this error? Can someone please explain this.

Solution

  • $11.2/4 states-

    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."

    Here 'B' is 'Base', 'N' is 'Derived' and 'R' is main.

    1. Consider the 2nd bullet- 'R occurs in a member or friend of a class N,...'. This clause does not apply as 'R'(main) is neither a member nor friend of 'N'(Derived)

    2. Consider the 3rd bullet- 'R occurs in a member or friend of a class P....'. This claus also does not apply for the same reasons as above

    3. Consider the 4th bullet- Once again this clause does not apply

    Thus we can conclude that 'Base' is not an accessible class of 'Derived'.

    $11.2/5 states -

    If a base class is accessible, one can implicitly convert a pointer to a derived class to a pointer to that base class (4.10, 4.11). [ Note: it follows that members and friends of a class X can implicitly convert an X* to a pointer to a private or protected immediate base class of X. —end note ]

    Since Base is not an accessible class of Derived when accessed in main, the Standard conversion from Derived class to Base class is ill-formed. Hence the error.

    EDIT 2:

    Study the error messages of some popular compilers and that should help you get a better understanding. Note how the word 'inaccessible' pops up so frequently and consistently across all the error messages

    The references are from the draft standard N3000. I am yet to download the latest draft :)

    GCC prog.cpp: In function ‘int main()’: prog.cpp:27: error: ‘base’ is an inaccessible base of ‘derived’

    Comeau Online "ComeauTest.c", line 26: error: conversion to inaccessible base class "base" is not allowed ptr = new derived;

    VS2010 error C2243: 'type cast' : conversion from 'derived *' to 'base *' exists, but is inaccessible