동적바인딩을 이용한 Stack, Queue 구현

#include <iostream>
using namespace std;
 
class Memory {
 
protected:int *M;
          int count;
public:
 
    Memory() {
        M = new int[20];
        count = 0;
    }
    virtual void push() = 0;
    virtual void pop() = 0;
 
};
class MyStack : public Memory {
    int S_pushNum;
    int S_popNum;
    int last = 20;//19
public:
    void push() {
        if (count < last) {
            cout << "숫자를 입력하세요" << endl << "==>" << endl;
            cin >> S_pushNum;
            M[count++] = S_pushNum;
            cout << count - 1 << "번째 값입니다 : " << M[count - 1] << endl;
        }
        else { cout << "가득찼어요" << endl; }
    }
    void pop() {
        if (count > 0) {
            cout << "뽑은 값은 " << M[--count] << endl;
        }else { cout << "더이상 뺄 값이 없습니다" << endl; }
 
    }
};
class MyQueue : public Memory {
    int Q_pushNum;
    int Q_popNum;
    int front = 0;
public:
    void push() {
        if (count < 20) {
        cout << "숫자를 입력하세요" << endl<<"==>"<<endl;
        cin >> Q_pushNum;
        M[count++] = Q_pushNum;
        cout << count - 1 << "번째 값입니다 :" << M[count-1] << endl;
        }
        else { cout << "가득찼어요" << endl; }
    }
    void pop() {
        if (front == count) {
        cout << "더이상 뺄 값이 없습니다" << endl; }
        else { cout <<"뽑은 값은"<< M[front++] << endl; }
    }
};
void main() {
    Memory *M = 0;
    MyStack S;
    MyQueue Q;
    int SelNum; //스택 큐선택
    int num;    //푸쉬 팝선택
    int i;
 
    do {
        cout << "어떤 것을 쓸래?" << endl;
        cout << "1. Stack  2. Queue 3. Exit" << endl;
        cin >> SelNum;
        if (SelNum == 1) {
            M = &S;
            system("cls");
        }
        else if (SelNum == 2) {
            M = &Q;
            system("cls");
        }
        else if (SelNum == 3) {
            exit(-1);
        }
        do {
            cout << "1. Push  2. Pop  3. Menu" << endl;
            cin >> num;
            switch (num) {
            case 1: M->push(); break;
            case 2: M->pop(); break;
            }
        } while (num != 3);  {system("cls"); }
 
    } while (SelNum != 3); { exit(-1); }
}