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#include <assert.h>
 
struct Parent {
  Parent(int number) : number_{number}{}
 
  int number_{-1};
};
struct Child1 : private Parent { Child1() : Parent(1) {} };
struct Child2 : private Parent { Child2() : Parent(2) {} };
 
void f(Child1& child1)
{
  Parent& parent = child1;
  parent = Child2();
}
 
int main()
{
  Child1 child1;
  assert(child1.number_ == 1); // Ok
  f(child1);
  assert(child1.number_ == 2); // Ok :-) (Surprise for lamers) :-)
 
  return 0;
}

prog.cpp: In function 'void f(Child1&)':
prog.cpp:13:20: error: 'Parent' is an inaccessible base of 'Child1'
   Parent& parent = child1;
                    ^
prog.cpp:14:10: error: 'Parent' is an inaccessible base of 'Child2'
   parent = Child2();
          ^
prog.cpp: In function 'int main()':
prog.cpp:6:17: error: 'int Parent::number_' is inaccessible
   int number_{-1};
                 ^
In file included from prog.cpp:1:0:
prog.cpp:20:17: error: within this context
   assert(child1.number_ == 1); // Ok
                 ^
prog.cpp:6:17: error: 'int Parent::number_' is inaccessible
   int number_{-1};
                 ^
In file included from prog.cpp:1:0:
prog.cpp:22:17: error: within this context
   assert(child1.number_ == 2); // Ok :-) (Surprise for lamers) :-)
                 ^

Standard output is empty