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C++核心——类中的权限

C++中权限分为三种等级public、protected、private。主要用于限定类中的成员和继承时继承的方式的选择。

类中成员的权限

• 代码1

#include <iostream>
using namespace std;

class Base
{
private:
    int a;
protected:
    int b; 
public:
    int c;
    Base(int a, int b, int c)
    {
        this->a = a;
        this->b = b;
        this->c = c;
    }
    void showInfo()
    {
        cout << this->a << this->b << this->c << endl;
    }
};

int main()
{
    Base base(1, 2, 3);
    base.showInfo();
    return 0;
}

*****************************输出结果*********************************
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ g++ 03_权限.cpp 
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ ./a.out 
123
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$  

1. 全部输出正常，在类中可以访问任意权限成员。

• 代码2

#include <iostream>
using namespace std;

class Base
{
private:
    int a;
protected:
    int b; 
public:
    int c;
    Base(int a, int b, int c)
    {
        this->a = a;
        this->b = b;
        this->c = c;
    }
};

int main()
{
    Base base(1, 2, 3);
    cout << base.c << endl;
    cout << base.b << endl;
    cout << base.a << endl;
    return 0;
}

*************************输出结果********************************
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ g++ 03_权限.cpp 
03_权限.cpp:34:18: error: 'b' is a protected member of 'Base'
    cout << base.b << endl;
                 ^
03_权限.cpp:9:9: note: declared protected here
    int b; 
        ^
03_权限.cpp:35:18: error: 'a' is a private member of 'Base'
    cout << base.a << endl;
                 ^
03_权限.cpp:7:9: note: declared private here
    int a;
        ^
2 errors generated.
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$   

2. 编译报错，提示了A和B的权限，如果这个代码main函数中后两行注释掉也能正常运行，可见类外只可以访问public权限的成员，protected和private权限的成员都是不可以访问的。

• 代码3-1

#include <iostream>
using namespace std;

class Base
{
private:
    int a;
protected:
    int b; 
public:
    int c;
    void showInfo()
    {
        cout << this->a << this->b << this->c << endl;
    }
};

class Son: public Base
{
protected:
    
public:
    Son(int a, int b, int c)
    {
        this->a = a;
        this->b = b;
        this->c = c;
    }
    void showInfo()
    {
        cout <<this->a << this->b << this->c << endl;
    }
};

int main()
{
    Son s(10, 20, 30);
    s.showInfo();
    return 0;
}

*****************************运行结果**************************
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ g++ 03_权限.cpp 
03_权限.cpp:25:15: error: 'a' is a private member of 'Base'
        this->a = a;
              ^
03_权限.cpp:7:9: note: declared private here
    int a;
        ^
03_权限.cpp:31:22: error: 'a' is a private member of 'Base'
        cout <<this->a << this->b << this->c << endl;
                     ^
03_权限.cpp:7:9: note: declared private here
    int a;
        ^
2 errors generated.
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ 

3. 编译阶段就报错，提示a 的权限是private。如果将a的代码主食掉，如下：

• 代码3-2

#include <iostream>
using namespace std;

class Base
{
private:
    int a;
protected:
    int b; 
public:
    int c;
    void showInfo()
    {
        cout << this->a << this->b << this->c << endl;
    }
};

class Son: public Base
{
protected:
    
public:
    Son(int a, int b, int c)
    {
        // this->a = a;
        this->b = b;
        this->c = c;
    }
    void showInfo()
    {
        cout <<this->b << this->c << endl;
    }
};

int main()
{
    Son s(10, 20, 30);
    s.showInfo();
    return 0;
}

*************************运行结果*********************************
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ g++ 03_权限.cpp 
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ ./a.out 
2030
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ 

运行结果正常，说明b和c都是可以被继承的，a是不可以被继承，这也就验证了开头给出的规律。

继承的方式

继承的时候在父类的前面也会加上权限，规律如下：

• 
  公共继承 属性不变
  
• 
  保护继承 公共变保护
  
• 
  私有继承 都变私有
  

代码3-2运行正常，但是如果在main函数改为如下

int main()
{
    Son s(10, 20, 30);
    s.showInfo();
    cout << s.c << s.b << endl;
    return 0;
}

*************************运行结果*********************************
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ g++ 03_权限.cpp 
03_权限.cpp:39:22: error: 'b' is a protected member of 'Base'
    cout << s.c << s.b << endl;
                     ^
03_权限.cpp:9:9: note: declared protected here
    int b; 
        ^
1 error generated.
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ 

1. 运行结果显示b的权限是protected，而c可以在类外使用肯定是public。所以使用public方式继承，protected还是protected，public还是public。当然private就不讨论，上面已经讨论过，因为private类型是不能继承的。

#include <iostream>
using namespace std;

class Base
{
private:
    int a;
protected:
    int b; 
public:
    int c;
    void showInfo()
    {
        cout << this->a << this->b << this->c << endl;
    }
};

class Son: protected Base
{
protected:
    
public:
    Son(int a, int b, int c)
    {
        // this->a = a;
        this->b = b;
        this->c = c;
    }
    void showInfo()
    {
        cout <<this->b << this->c << endl;
    }
};

int main()
{
    Son s(10, 20, 30);
    s.showInfo();
    cout << s.c << endl;
    return 0;
}

*************************运行结果*********************************
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ g++ 03_权限.cpp 
03_权限.cpp:39:13: error: cannot cast 'Son' to its protected base class 'Base'
    cout << s.c << endl;
            ^
03_权限.cpp:18:12: note: declared protected here
class Son: protected Base
           ^~~~~~~~~~~~~~
03_权限.cpp:39:15: error: 'c' is a protected member of 'Base'
    cout << s.c << endl;
              ^
03_权限.cpp:18:12: note: constrained by protected inheritance here
class Son: protected Base
           ^~~~~~~~~~~~~~
03_权限.cpp:11:9: note: member is declared here
    int c;
        ^
2 errors generated.
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ 

2. 从编译的提示信息中可以看出c已经变为了protected类型的了。

#include <iostream>
using namespace std;

class Base
{
private:
    int a;
protected:
    int b; 
public:
    int c;
    void showInfo()
    {
        cout << this->a << this->b << this->c << endl;
    }
};

class Son: private Base
{
protected:
    
public:
    Son(int a, int b, int c)
    {
        // this->a = a;
        this->b = b;
        this->c = c;
    }
    void showInfo()
    {
        cout <<this->b << this->c << endl;
    }
};

int main()
{
    Son s(10, 20, 30);
    s.showInfo();
    cout << s.c << s.b << endl;
    return 0;
}

*************************运行结果*********************************
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ g++ 03_权限.cpp 
03_权限.cpp:39:13: error: cannot cast 'Son' to its private base class 'Base'
    cout << s.c << s.b << endl;
            ^
03_权限.cpp:18:12: note: declared private here
class Son: private Base
           ^~~~~~~~~~~~
03_权限.cpp:39:15: error: 'c' is a private member of 'Base'
    cout << s.c << s.b << endl;
              ^
03_权限.cpp:18:12: note: constrained by private inheritance here
class Son: private Base
           ^~~~~~~~~~~~
03_权限.cpp:11:9: note: member is declared here
    int c;
        ^
03_权限.cpp:39:20: error: cannot cast 'Son' to its private base class 'Base'
    cout << s.c << s.b << endl;
                   ^
03_权限.cpp:18:12: note: declared private here
class Son: private Base
           ^~~~~~~~~~~~
03_权限.cpp:39:22: error: 'b' is a private member of 'Base'
    cout << s.c << s.b << endl;
                     ^
03_权限.cpp:18:12: note: constrained by private inheritance here
class Son: private Base
           ^~~~~~~~~~~~
03_权限.cpp:9:9: note: member is declared here
    int b; 
        ^
4 errors generated.
(base) BigfishdeMacBook-Pro:学习笔记 bigfish$ 

3. 从编译的提示信息中可以看出b和c的权限已经变为了private类型。