Question

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.

Answer 1

$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