Наследование С++

class CPoint {
protected: 
    int x, y;
public:
    CPoint();
    CPoint(int X, int Y);
    ~CPoint();
    void set(int X, int Y);
    int get_x();
    int get_y();
 
};CPoint::CPoint() {
    x = y = 0;
}
 
CPoint::CPoint(int X, int Y){
    x = X; y = Y;
}
 
void CPoint::set(int X, int Y) {
    x = X; y = Y;
}
 
int CPoint::get_x() {
    return x;
}
 
int CPoint::get_y() {
    return y;
}

list<string>all;

class CPoint
{
public:
    int x, y;
    CPoint()
    {
    }
    ~CPoint()
    {
    }
    CPoint(int d, int s): x(d), y(s)
    {
    }
};

#include <iostream>
using namespace std;
 
class CPoint
{
public:
    int x, y;
    CPoint()
    {
    }
    ~CPoint()
    {
    }
    CPoint(int d, int s): x(d), y(s)
    {
    }
    
    void Set(int X, int Y)
    {
        x = X;
        y = Y;
    }
 
    int get_x()
    {
        return x;
    }
 
    int get_y()
    {
        return y;
    }
};
 
int main()
{
    CPoint p1;  
}

#include <list>
 
list<CPoint> Points;

class CLine : public CPoint {
    float d, s;
protected:
    int x1, y1, x2, y2;
public:
    CLine();
    CLine(int X1, int Y1);
    ~CLine();
    void set(int X1, int Y1);
    int get_x1();
};
 
______________________
 
CLine::CLine():CPoint() {
    x1 = y1 = x2 = y2 = 0;
}
 
CLine::CLine(int X1, int Y1) : CPoint(x1,y1){
    x1 = X1; y1 = Y1;
}
 
void CLine::set(int X1, int Y1) {
    x1 = X1; y1 = Y1;
}
 
int CLine::get_x1() {
    return x1;
}
 
CLine::~CLine() {
}

class CLine : public CPoint {
    float d, s;
protected:
    int x1, y1, x2, y2;
public:
    CLine();
    CLine(int X1, int Y1);
    ~CLine();
    void set(int X1, int Y1);
    int get_x1();
};
 
 
CLine::CLine(){
    x1 = y1 = x2 = y2 = 0;
}
 
CLine::CLine(int X1, int Y1) {
    x1 = X1; y1 = Y1;
}
 
void CLine::set(int X1, int Y1) {
    x1 = X1; y1 = Y1;
}
 
int CLine::get_x1() {
    return x1;
}
 
CLine::~CLine() {
}

CPoint p1;
CPoint p2;

class CPoint {
    int x, y;
public:
    CPoint();
    CPoint(int X, int Y);
    ~CPoint();
    void set(int X, int Y);
    
    int get_x() const;
    int get_y() const;
 
};
CPoint::CPoint() : x(), y() {
}
 
CPoint::CPoint(int X, int Y) : x(X), y(Y){
}
 
CPoint::~CPoint() {
}
 
void CPoint::set(int X, int Y) {
    x = X;
    y = Y;
}
 
int CPoint::get_x() const {
    return x;
}
 
int CPoint::get_y() const {
    return y;
}
//------------------------------------------------------
class CLine 
    : private CPoint 
{
    CPoint m_a, m_b;
private:
    static CPoint center(CPoint a, CPoint b);
        
public:
    CLine();
    CLine(CPoint a, CPoint b);
    ~CLine();
    void set(CPoint a, CPoint b);
        
    CPoint center() const;  
};
 
 
CLine::CLine() 
    : CPoint(), m_a(), m_b() {
}
 
CLine::CLine(CPoint a, CPoint b) // [ЗДЕСЬ]
    : CPoint(center(a, b)), m_a(a), m_b(b) {
}
 
CPoint CLine::center(CPoint a, CPoint b) {
    return CPoint((a.get_x() + b.get_x()) / 2, (a.get_y() + b.get_y()) / 2);
}
 
void CLine::set(CPoint a, CPoint b) {
    m_a = a;
    m_b = b;
    CPoint c = center(a, b);
    CPoint::set(c.get_x(), c.get_y());
}
 
CPoint CLine::center() const {
    return *this;
}
 
CLine::~CLine() {
}