#include <cassert>
#include <type_traits>

class Base {
    int secret_ = 7;
protected:
    int count_ = 0;
public:
    int secret() const { return secret_; }
};

class PublicChild : public Base {
public:
    void set_peer(PublicChild& peer, int value) { peer.count_ = value; }
    int count() const { return count_; }
};

class PrivateChild : private Base {
public:
    int exposed_secret() const { return secret(); }
};

int main() {
    static_assert(std::is_base_of<Base, PrivateChild>::value, "base exists");
    static_assert(std::is_convertible<PublicChild*, Base*>::value, "public");
    static_assert(!std::is_convertible<PrivateChild*, Base*>::value, "private");
    PublicChild first, second;
    first.set_peer(second, 9);
    assert(second.count() == 9);
    PrivateChild hidden;
    assert(hidden.exposed_secret() == 7);
}