Summary of C++ Review Key Points No. 10 - Polymorphism (1)

Understanding of polymorphism:

is a new object-oriented requirement:

Judge the call of the rewritten function based on the actual object type

If the parent class pointer points to If it is a parent class object, call the function defined in the parent class

If the parent class pointer points to a subclass object, call the overridden function defined in the subclass

Solution:

Ø C++ supports polymorphism through the virtual keyword

Ø Functions declared using virtual can be rewritten to exhibit polymorphic features

//Three major concepts of object-oriented

Encapsulation

breaks through the concept of C language functions. .

Inheritance

Code reuse. . . . I reuse the code I originally wrote. . .

Polymorphism

Polymorphism can use the future. . . . . A framework was written in the 1980s. . . . . . Code written by people in the 1990s

Polymorphism is a goal pursued by our software industry. . .

//Written a framework that can call on the ability of code written by later generations

Further understanding

//Three conditions for indirect assignment to be established:

//1 Define two variables. . .

//2 Establish association. . . .

//3 *p

//Three conditions for polymorphism to be established:

//1 There must be inheritance

//2 There must be Function rewriting. . . C virtual function

//3 There must be a parent class pointer (parent class reference) pointing to the subclass object

//Polymorphism is the basis of the design pattern, and polymorphism is the basis of the framework

Knowledge point 1 Virtual destructor

The destructor is to release resources.

When there is a need to release resources and the object cannot be released directly, such as:

C *myC = new C; //C inherits B, and B inherits class A

delete myC; //Release resources directly through subclass objects. In this case, there is no need to write the virtual keyword

Need to execute the destructors of all subclasses and objects through the parent class pointer

If you want to release all subclass resources through the parent class pointer (you need to add it in the final parent class destructor virtual keyword)

void howtodelete(A *base)
{
delete base; //这句话不会表现成多态 这种属性
}

Knowledge point 2: The difference between overloading and rewriting

Function overloading

Must be done in the same class

Subclasses cannot overload functions of the parent class. Functions of the same name in the parent class will be overwritten by name (for example: there is function a() in the parent class, and there is also function a() in the subclass, but there is also function a(). int b) This function generates an error by overloading the parent class function! )

Overloading determines the function call based on the parameter type and number during compilation

Function rewriting

Must occur between the parent class and the subclass

And the functions in the parent class and the subclass must have exactly the same prototype

Using virtual declaration can produce polymorphism ( If virtual is not used, it is called redefinition)

Polymorphism determines function calls based on the type of specific objects during runtime

Example analysis:

//1 C++编译器 看到func名字 ,因子类中func名字已经存在了(名称覆盖).所以c++编译器不会去找父类的4个参数的func函数
//2 c++编译器只会在子类中,查找func函数,找到了两个func,一个是2个参数的,一个是3个参数的.
//3 C++编译器开始报错..... error C2661: “Child::func”: 没有重载函数接受 4 个参数
//4 若想调用父类的func,只能加上父类的域名..这样去调用..
c1.func(1, 3, 4, 5);
//c1.func();
//func函数的名字，在子类中发生了名称覆盖；子类的函数的名字，占用了父类的函数的名字的位置
//因为子类中已经有了func名字的重载形式。。。。
//编译器开始在子类中找func函数。。。。但是没有0个参数的func函数

3 Key points:

The implementation principle of polymorphism in C++

When a virtual function is declared in a class, the compiler will generate a virtual function table in the class

The virtual function table is A data structure that stores class member function pointers

The virtual function table is automatically generated and maintained by the compiler

Virtual member functions will be placed in the virtual function table by the compiler

When there is a virtual function, each object has a pointer to the virtual function table (vptr pointer)

Note 1:

Calling the rewritten function through the virtual function table pointer VPTR is performed when the program is running, so addressing operation is required to determine the function that should actually be called. . For ordinary member functions, the function to be called is determined at compile time. In terms of efficiency, virtual functions are much less efficient.

Note 2:

For efficiency reasons, it is not necessary to declare all member functions as virtual functions

Note 3: The C++ compiler executes the HowToPrint function, which is not required Distinguish whether it is a subclass object or a parent class object

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