0. 問(wèn)題：子類是否可以直接訪問(wèn)父類的私有成員？

1. 繼承中的訪問(wèn)級(jí)別

面向?qū)ο笾械脑L問(wèn)級(jí)別不只是public和private，可以定義protected關(guān)鍵字來(lái)使得修飾的成員不能被外界直接訪問(wèn)，可以被子類直接訪問(wèn)。

編程說(shuō)明：protected初探

#include <iostream>
#include <string>

using namespace std;

class Father
{
protected:          // protected 關(guān)鍵字
     int mValue;
public:
    Father()
    {
        mValue = 100;
    }
    
    int value()
    {
        return mValue;
    }
};

class Child : public Father
{
public:
    int addvalue(int v)
    {
        mValue = mValue + v;
        return mValue;
    }
};

int main()
{
    Child c;
    
    cout << c.addvalue(50) << endl;
    
    
    return 0;
}

輸出結(jié)果：

150

2. 問(wèn)題：為什么面向?qū)ο笾行枰猵rotected?

protected的作用則是實(shí)現(xiàn)繼承。protected成員可以被派生類（也叫子類）對(duì)象訪問(wèn)，不能被用戶代碼類外的代碼訪問(wèn)。

3. 定義類時(shí)訪問(wèn)級(jí)別的選擇

4. 組合與繼承的綜合實(shí)例

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class Object
{
protected:
    string mName;
    string mInfo;
public:
    Object()
    {
        mName = "Object";
        mInfo = "";
    }
    
    string name()
    {
        return mName;
    }
    
    string info()
    {
        return mInfo;
    }
};

class Point : public Object
{
private:
    int mX;
    int mY;
public:
    Point(int x = 0, int y = 0)
    {
        ostringstream s;
        mX = x;
        mY = y;
        mName = "Point";
        
        s << "Point(" << mX << ", " << mY << ")";
        
        mInfo = s.str();        
    }
    
    int x()
    {
        return mX;
    }
    
    int y()
    {
        return mY;
    }
};

class Line : public Object
{
private:
    Point mP1;
    Point mP2;
public:
    Line(Point p1, Point p2)
    {
        ostringstream s;
        mP1 = p1;
        mP2 = p2;
        mName = "Line";
        
        s << "Line " << mP1.info() << ", " << mP2.info() << endl;
        mInfo = s.str();    
    }
};

int main()
{
    Point p(1,2);
    Point pn(5, 6);
    
    Line l(p, pn);
    
    cout << p.x() << ", " << p.y() << endl;
    cout << p.name() << endl;
    cout << p.info() << endl;
    
    cout << endl;
    
    cout << l.name() << endl;
    cout << l.info() << endl;

    return 0;
}

輸出結(jié)果：

1, 2
Point
Point(1, 2)

Line
Line Point(1, 2), Point(5, 6)

5. 小結(jié)

• 面向?qū)ο笾械脑L問(wèn)級(jí)別不只是public和private
• protected修飾的成員不能被外界所訪問(wèn)
• protected使得子類能夠訪問(wèn)父類的成員
• protected關(guān)鍵字是為了繼承而專門設(shè)計(jì)的
• 沒(méi)有protected就無(wú)法完成真正意義上的代碼復(fù)用