Private inheritance causing problem in c++ [duplicate] - c++

Question

This question already has answers here:
private inheritance
(6 answers)
What is the difference between public, private, and protected inheritance in C++?
(16 answers)
Closed 3 years ago.
Why would the following code run into error of ‘A’ is an inaccessible base of ‘B’? Here's my thoughts:
whenever we call function foo(), it will execute new B(5), which will first call the constructor of its base struct A.
struct A's constructor function is a public method, hence it should be accessible by its derived struct B (as protected if i'm not wrong).
then struct B's constructor function will be call to create a vector with five 0s.
then deleting object a will call destructor B, then destructor A.
Is there anything wrong with my logic? Your answer will be greatly appreciated
#include <iostream>
#include <vector>
using namespace std;
struct A
{
A() { cout << "Constructor A called"<< endl;}
virtual ~A() { cout << "Denstructor A called"<< endl;}
};
struct B : private A
{
vector<double> v;
B(int n) : v(n) { cout << "Constructor B called"<< endl;}
~ B() { cout << "Denstructor B called"<< endl;}
};
int main()
{
const A *a = new B(5);
delete a;
return 0;
}

Answer 1

There's nothing wrong with your logic, except that it's missing one point:
private inheritance basically means that only the inheriting class (B in this case) knows that it inherits from the base A.
That in turn means that only B can make use of all the privileges that come with this inheritance. One of these privileges is to be able to cast B* to A*. The function foo() doesn't know about B's inheritance, so it cannot perform that cast.

Answer 2

TL;DR
You are deriving B as 'private' from A. You must change it to
struct B : public A{
vector<double> v;
B(int n): v(n) {std::cout << "B Constructor" << std::endl};
~B() {std::cout << "B Destruktor" << std:.endl;};
};
Extended explanation
By using private inheritance you define B has a A instead of B is a A. With a has-a dependency you cannot up- and downcast between both classes (Apple cannot become a worm and vice versa. Even if an apple has a worm).
You mainly use private inheritance if you want to take advante of features implemented by another class, without exposing the public Inteface of the used class. For e.g. You could use features of a parser inside your class without being a parser on your own.