항목 9] 객체 생성 및 소멸 과정 중, 절대로 가상 함수를 호출하지 말자

객체 생성 및 소멸 과정 중, 절대로 가상 함수를 호출하지 말자

class Transaction
{
public: 
    Transaction()
    {
        logTransaction();
    }
    virtual void logTransaction() const=0;
};
 
 
class butTransaction: public Transaction
{
public:
    butTransaction(){};
    virtual void logTransaction() const;
};
 
void mian()
{
    BuyTransaction b;
}

상속 관계에서 생성자에 쓰이는 virtual 키워드의 사용시 주의 사항

Transaction 생성자의 마지막 줄을 보면 가상 함수인 logTransaction을 호출하는 문장이 보이는
부모 클래스에서 순수 가상 함수로 virtual 을 처리해 줘도 이것은 아직 파생클래스가 생성 되기 이전이기 때문에 곳 자신의 함수를 호출하게 됩니다.
이유 : 가상 함수는 절대로 기본클래스의 생성자가(자식클래스)가 호출될 동안은 절대 자식 클래스 쪽으로 내려가지 않습니다. 그 대신 객체 자신이 기본 클래스 타입인 것처럼 동작합니다.

 

예제 1

 
#include <iostream>
using namespace std;
 
class AAA
{
public:
    AAA(char *_pstr) 
        : m_pstr1(null)
    {
         m_pstr1 = new char[strlen(_pstr)+1];
        strcpy(m_pstr1,_str1);
    }
 
/*virtual*/ 
    ~AAA()
    {
        cout << "~AAA() 소멸자" <<endl;
        if( null != m_pstr1 ) 
        {
            delete [] m_pstr1;
            m_pstr1 = null;
        }
    }
    virtual void Showstring()
    {
        if( null != m_pstr1 ) 
        {
            cout << m_pstr1 <<endl;
            return;
        }    
        cout << " m_pstr1  Error " <<endl;
    }
 
private:
    char * m_pstr1;
};
 
class BBB : public AAA
{
private:
    char * m_pstr2;
 
public:
    BBB(char *_pstr1, char *_pstr2)
        :AAA(_pstr1)
        ,m_pstr2(null)
    {
        m_pstr2 = new char[strlen(_pstr2)+1];
        strcpy(m_pstr2,_pstr2);
    }
    ~BBB()
    {
        cout<<"~BBB() 소멸자"<<endl;
        if( null != m_pstr2) 
        {
            delete [] m_pstr2;
            m_pstr2= null;
        }
    }
    virtual void Showstring()
    {
        AAA::Showstring();
        if( null != m_pstr2 ) 
        {
            cout << m_pstr2 <<endl;
            return;
        }    
        cout << " m_pstr2  Error " <<endl;
     }
 
};
 
void main()
{
    AAA* pA = new BBB("Good","evening");
    BBB* pB = new BBB("Good","morning");
    
    pA->Showstring();
    pB->Showstring();
    
    cout<<"------------객체 소멸 직전---------"<<endl;
    delete pA ;
    delete pB ;
 
}

이번 예제에서는 소멸자시에 업캐스팅에 의해서 *a의 반영범위가 부모 클래스의 부분에 제한이 됨으로 자식 소멸에의 호출이 이루어지지 않는다.