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📜  C++编程中的公共，受保护和私有继承

📅  最后修改于: 2020-09-25 05:07:39             🧑  作者: Mango

在本教程中，我们将借助示例来学习在C++中使用公共继承，受保护继承和私有继承。

在C++继承中，我们可以在不同的访问模式下从基类派生一个子类。例如， 

class Base {
.... ... ....
};

class Derived : public Base {
.... ... ....
};

注意代码中的关键字public 

class Derived : public Base

这意味着我们已经在公共模式下从基类创建了派生类。另外，我们还可以在受保护或私有模式下派生类。

这3个关键字( public ， protected和private )在C++继承中称为访问说明符 。

C++中的公共，受保护和私有继承

public ， protected和private继承具有以下功能：

注意：派生类无法访问基类的private成员。 

class Base {
    public:
        int x;
    protected:
        int y;
    private:
        int z;
};

class PublicDerived: public Base {
    // x is public
    // y is protected
    // z is not accessible from PublicDerived
};

class ProtectedDerived: protected Base {
    // x is protected
    // y is protected
    // z is not accessible from ProtectedDerived
};

class PrivateDerived: private Base {
    // x is private
    // y is private
    // z is not accessible from PrivateDerived
}

示例1：C++公共继承

// C++ program to demonstrate the working of public inheritance

#include 
using namespace std;

class Base {
   private:
    int pvt = 1;

   protected:
    int prot = 2;

   public:
    int pub = 3;

    // function to access private member
    int getPVT() {
        return pvt;
    }
};

class PublicDerived : public Base {
   public:
    // function to access protected member from Base
    int getProt() {
        return prot;
    }
};

int main() {
    PublicDerived object1;
    cout << "Private = " << object1.getPVT() << endl;
    cout << "Protected = " << object1.getProt() << endl;
    cout << "Public = " << object1.pub << endl;
    return 0;
}

输出 

Private = 1
Protected = 2
Public = 3

在这里，我们以public模式从Base派生PublicDerived 。

结果，在PublicDerived ：

由于私有成员和受保护成员不可访问，因此我们需要创建公共函数getPVT()和getProt()来访问它们： 

// Error: member "Base::pvt" is inaccessible
cout << "Private = " << object1.pvt;

// Error: member "Base::prot" is inaccessible
cout << "Protected = " << object1.prot;

公共继承中的可访问性

示例2：C++保护的继承

// C++ program to demonstrate the working of protected inheritance

#include 
using namespace std;

class Base {
   private:
    int pvt = 1;

   protected:
    int prot = 2;

   public:
    int pub = 3;

    // function to access private member
    int getPVT() {
        return pvt;
    }
};

class ProtectedDerived : protected Base {
   public:
    // function to access protected member from Base
    int getProt() {
        return prot;
    }

    // function to access public member from Base
    int getPub() {
        return pub;
    }
};

int main() {
    ProtectedDerived object1;
    cout << "Private cannot be accessed." << endl;
    cout << "Protected = " << object1.getProt() << endl;
    cout << "Public = " << object1.getPub() << endl;
    return 0;
}

输出

Private cannot be accessed.
Protected = 2
Public = 3

在这里，我们以保护模式从Base派生了ProtectedDerived 。

结果，在ProtectedDerived ：

我们知道，不能直接访问受保护的成员。

其结果是，我们不能使用getPVT()从ProtectedDerived 。这就是为什么我们需要在ProtectedDerived中创建getPub() 函数以访问pub变量的原因。 

// Error: member "Base::getPVT()" is inaccessible
cout << "Private = " << object1.getPVT();

// Error: member "Base::pub" is inaccessible
cout << "Public = " << object1.pub;

受保护继承中的可访问性

示例3：C++私有继承

// C++ program to demonstrate the working of private inheritance

#include 
using namespace std;

class Base {
   private:
    int pvt = 1;

   protected:
    int prot = 2;

   public:
    int pub = 3;

    // function to access private member
    int getPVT() {
        return pvt;
    }
};

class PrivateDerived : private Base {
   public:
    // function to access protected member from Base
    int getProt() {
        return prot;
    }

    // function to access private member
    int getPub() {
        return pub;
    }
};

int main() {
    PrivateDerived object1;
    cout << "Private cannot be accessed." << endl;
    cout << "Protected = " << object1.getProt() << endl;
    cout << "Public = " << object1.getPub() << endl;
    return 0;
}

输出 

Private cannot be accessed.
Protected = 2
Public = 3

在这里，我们以私有模式从Base派生PrivateDerived 。

结果，在PrivateDerived ：

众所周知，私有成员无法直接访问。

其结果是，我们不能使用getPVT()从PrivateDerived 。这就是为什么我们需要在PrivateDerived中创建getPub() 函数以访问pub变量的原因。 

// Error: member "Base::getPVT()" is inaccessible
cout << "Private = " << object1.getPVT();

// Error: member "Base::pub" is inaccessible
cout << "Public = " << object1.pub;

私有继承中的可访问性