CODEWITHSUNDEEP
c++typescasting
lironz
lironz

Reputation: 43

c++ automatic 'type cast' conversion

Can somebody explain why below code does not compile on error:

Error   1   error C2243: 'type cast' : conversion from 'Der *' to 'Base *' exists, but is inaccessible  d:\users\lzizva\documents\mta\c++\projects\temp1\17022014.cpp   50  1   temp1
Error   2   error C2243: 'type cast' : conversion from 'Der *' to 'Base *' exists, but is inaccessible  d:\users\lzizva\documents\mta\c++\projects\temp1\17022014.cpp   51  1   temp1
    3   IntelliSense: conversion to inaccessible base class "Base" is not allowed   d:\users\lzizva\documents\mta\c++\projects\temp1\17022014.cpp   50  12  temp1
    4   IntelliSense: conversion to inaccessible base class "Base" is not allowed   d:\users\lzizva\documents\mta\c++\projects\temp1\17022014.cpp   51  14  temp1

I thought that when there is private inheritance the child gets all the properties and methods and set them as private and it only should affect the subclasses of the child. What am I missing here? What does actually the compiler do ?

Thanks in advance, Liron

#include <iostream>

using namespace std;

class Base
{
int n;
Base* next;
public:
Base(int n, Base* next = NULL) : n(n), next(next)
{}

virtual void print() const
{
cout << n << endl;

if (next != NULL)
{
next->print();
}
}

virtual ~Base()
{
cout << "Base" << endl;
}
};

class Der : private Base
{
int counter;
public:
Der(int n, Base* next = NULL) : Base(n, next), counter(n){}

void print() const
{
cout << counter << endl;
Base::print();
}

~Der()
{
cout << "Der" << endl;
}
};

void main()
{
Der one(1);
Der two(2, &one);
Der three(3, &two);

three.print();
}

Upvotes: 2

Views: 664

Answers (2)

juanchopanza
juanchopanza

Reputation: 227370

Despite the tight coupling between base and derived types, private inheritance is a "has-a" relationship, rather than an "is-a". It really is suited for inheriting implementations, not interfaces, and, as you have discovered, it cannot be used for polymorphism since references or pointers to the base type cannot bind to instances of the derived one.

When you make this call

Der two(2, &one);

you are attempting to bind &one, a Derived* to a Base*.

See the uses and abuses of inheritance GOTW for more on private inheritance.

Upvotes: 3

James McNellis
James McNellis

Reputation: 354979

The problem is in the construction of two and three: The Der constructor takes a Base*, but you are passing Der* pointers.

Because Der privately derives from Base, the Der -> Base conversion is inaccessible in main(), thus the errors.

Upvotes: 5

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