부모클래스에서 자식 클래스 멤버함수를 호출하는 방법

#include <stdio.h>
 
 
#include <sys/time.h>
#include <unistd.h>
 
//https://kldp.org/node/101730
//https://www.joinc.co.kr/w/man/2/gettimeofday
 
 
 
//실행 시간 구하는 함수
struct timeval val_old;
void getdoubledt(char *dt)
{
  struct timeval val;
  struct tm *ptm;
 
  gettimeofday(&val, NULL);
  ptm = localtime(&val.tv_sec);
 
  memset(dt , 0x00 , sizeof(dt));
 
  // format : YYMMDDhhmmssuuuuuu
  sprintf(dt, "%04d% 02d %02d %02d %02d %02d %06ld"
      , ptm->tm_year + 1900, ptm->tm_mon + 1, ptm->tm_mday
      , ptm->tm_hour, ptm->tm_min, ptm->tm_sec
      , val.tv_usec);
 
  printf("%06ld\t", val.tv_usec - val_old.tv_usec);
 
  val_old = val;
 
}
 
 
 
 
 
//----------------------
//https://csstudy.wordpress.com/2014/01/21/상속-오버라이딩-함수-vs-가상-함수-overriding-vs-pure-virtual-function/
 
class ParentClass
{
public:
    ParentClass(){}
    ~ParentClass(){}
 
    void func()
    {
        std::cout << "I am in ParentClass" << std::endl;
    }
 
    virtual void func2()
    {
        std::cout << "I am in ParentClass func2()" << std::endl;
    }
};
 
class ChildClass : public ParentClass
{
public:
    ChildClass(){}
    ~ChildClass(){}
 
    void func()
    {
        std::cout << "I am in ChildClass" << std::endl;
    }
 
    virtual void func2()
    {
        std::cout << "I am in ChildClass func2 ()" << std::endl;
    }
} ;
 
 
 
 
//----------------------
 
 
class CTest
{
private:
    int m_nVAL;
    friend class CChild;
 
public:
    CTest(){};
    ~CTest(){};
};
 
 
class CChild : private CTest
{
public:
    CChild(){};
    ~CChild(){};
 
    void fn()
    {
        m_nVAL = 123;
        printf("m_nVAL   %d\n", m_nVAL);
    }
};
 
 
 
//----------------------
 
class CTest2
{
public:
    int m_nVAL2;
    friend class CChild2;
 
    virtual void fn() = 0;
public:
    CTest2(){};
    ~CTest2(){};
};
 
 
class CChild2 : private CTest2
{
public:
    CChild2(){};
    ~CChild2(){};
 
    void fn()
    {
        m_nVAL2 = 123;
        printf("m_nVAL2   %d\n", m_nVAL2);
    }
};
 
 
 
 
int main()
{
    //----------------------
    {
    //자식 클래스 실행
    ChildClass* c = new ChildClass();
    c->func();
 
    //부모 클래스 실행
    ParentClass* p = new ParentClass();
    p->func();
 
    ParentClass* pc = new ChildClass();
    //부모 함수 실행
    pc->func();
    //자식 함수 실행
    pc->func2();
 
    delete c;
    delete p;
    delete pc;
 
    }
 
 
    //----------------------
    //자식 클래스 함수 사용하는 방법
    {
    CTest   t;
    CChild* c;
    c = (CChild*) &t;
    c->fn();
    }
 
 
    //----------------------
    //부모 클래스에서 순수 가상함수를 사용하는 방법
    {
    CTest2 *  t;
    CChild2 c;
    t = (CTest2 *) &c;
    t = (CTest2 *) new CChild2();
    t->fn();
    delete t;
    }
 
    return 0;
}
 
//출력
I am in ChildClass
I am in ParentClass
I am in ParentClass
I am in ChildClass func2 ()
m_nVAL   123
m_nVAL2   123