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📜  C++多态性

📅  最后修改于: 2020-10-16 07:04:31             🧑  作者: Mango

C++多态

术语“多态”是“多” +“变形”的组合，表示多种形式。这是一个希腊词。在面向对象的编程中，我们使用3个主要概念：继承，封装和多态。

多态性的现实例子

让我们考虑一个现实生活中的多态性示例。一位女士的行为就像教室里的老师，家里的母亲或女儿以及市场上的客户一样。在这里，一个人的行为会根据情况而有所不同。

C++中有两种类型的多态性：

  • 编译时多态性：通过匹配参数的类型和数量来调用重载函数。该信息在编译时可用，因此，编译器在编译时选择适当的函数。它是通过函数重载和运算符重载来实现的，也称为静态绑定或早期绑定。现在，让我们考虑一下函数名称和原型相同的情况。
   class A                                  //  base class declaration.
  {
       int a;
       public:
       void display()
       { 
             cout<< "Class A ";
        }
  };
class B : public A                       //  derived class declaration.
{
    int b;
    public:
   void display()
  {
        cout<<"Class B";
  }
};

在上述情况下，在基类和派生类中，display()函数的原型都是相同的。因此，不能应用静态绑定。如果在运行时选择了适当的函数，那就太好了。这称为运行时多态。

  • 运行时多态性：运行时多态性是在运行时而不是编译时调用对象的方法时实现的。它是通过方法覆盖(也称为动态绑定或后期绑定)来实现的。

黑白差异编译时间和运行时多态性。

Compile time polymorphism Run time polymorphism
The function to be invoked is known at the compile time. The function to be invoked is known at the run time.
It is also known as overloading, early binding and static binding. It is also known as overriding, Dynamic binding and late binding.
Overloading is a compile time polymorphism where more than one method is having the same name but with the different number of parameters or the type of the parameters. Overriding is a run time polymorphism where more than one method is having the same name, number of parameters and the type of the parameters.
It is achieved by function overloading and operator overloading. It is achieved by virtual functions and pointers.
It provides fast execution as it is known at the compile time. It provides slow execution as it is known at the run time.
It is less flexible as mainly all the things execute at the compile time. It is more flexible as all the things execute at the run time.

C++运行时多态示例

让我们看一个简单的C++运行时多态性示例。

//没有virtual关键字的示例。

#include   
using namespace std;  
class Animal {  
    public:  
void eat(){    
cout<<"Eating...";    
    }      
};   
class Dog: public Animal    
{    
 public:  
 void eat()    
    {           cout<<"Eating bread...";    
    }    
};  
int main(void) {  
   Dog d = Dog();    
   d.eat();  
   return 0;  
}

输出：

Eating bread...

C++运行时多态示例：通过使用两个派生类

让我们看一下C++中运行时多态的另一个示例，其中有两个派生类。

//一个带有虚拟关键字的示例。

#include   
using namespace std;  
class Shape {                                        //  base class
    public:  
virtual void draw(){                             // virtual function
cout<<"drawing..."<draw();   
        s=&rec;  
     s->draw();    
    s=?  
    s->draw();   
}

输出：

drawing...
drawing rectangle...
drawing circle...

数据成员的运行时多态

运行时多态可以通过C++中的数据成员来实现。让我们看一个例子，其中我们通过引用变量访问字段，引用变量引用派生类的实例。

#include   
using namespace std;  
class Animal {                                          //  base class declaration.
    public:  
    string color = "Black";    
};   
class Dog: public Animal                       // inheriting Animal class.
{    
 public:  
    string color = "Grey";    
};  
int main(void) {  
     Animal d= Dog();    
    cout<

输出：

Black