C ++ virtuelle funktioner

I denne vejledning lærer vi om C ++ virtuel funktion og dens anvendelse ved hjælp af eksempler.

En virtuel funktion er en medlemsfunktion i basisklassen, som vi forventer at omdefinere i afledte klasser.

Dybest set bruges en virtuel funktion i baseklassen for at sikre, at funktionen tilsidesættes . Dette gælder især tilfælde, hvor en markør af baseklassen peger på et objekt fra en afledt klasse.

Overvej f.eks. Koden nedenfor:

 class Base ( public: void print() ( // code ) ); class Derived : public Base ( public: void print() ( // code ) );

Senere, hvis vi opretter en markør af Basetypen for at pege på et objekt i Derivedklassen og kalde print()funktionen, kalder print()den Baseklassens funktion.

Med andre ord Basetilsidesættes medlemsfunktionen ikke.

 int main() ( Derived derived1; Base* base1 = &derived1; // calls function of Base class base1->print(); return 0; )

For at undgå dette, erklærer vi den print()funktion Baseklassen som virtuel ved hjælp af det virtuelle søgeord.

 class Base ( public: virtual void print() ( // code ) );

Virtuelle funktioner er en integreret del af polymorfisme i C ++. For at lære mere, se vores vejledning om C ++ polymorfisme.

Eksempel 1: C ++ virtuel funktion

 #include using namespace std; class Base ( public: virtual void print() ( cout << "Base Function" << endl; ) ); class Derived : public Base ( public: void print() ( cout << "Derived Function"

Output

 Derived Function

Here, we have declared the print() function of Base as virtual.

So, this function is overridden even when we use a pointer of Base type that points to the Derived object derived1.

Working of virtual functions in C++

C++ override Identifier

C++ 11 has given us a new identifier override that is very useful to avoid bugs while using virtual functions.

This identifier specifies the member functions of the derived classes that override the member function of the base class.

For example,

 class Base ( public: virtual void print() ( // code ) ); class Derived : public Base ( public: void print() override ( // code ) );

If we use a function prototype in Derived class and define that function outside of the class, then we use the following code:

 class Derived : public Base ( public: // function prototype void print() override; ); // function definition void Derived::print() ( // code )

Use of C++ override

When using virtual functions. it is possible to make mistakes while declaring the member functions of the derived classes.

Using the override identifier prompts the compiler to display error messages when these mistakes are made.

Otherwise, the program will simply compile but the virtual function will not be overridden.

Some of these possible mistakes are:

  • Functions with incorrect names: For example, if the virtual function in the base class is named print(), but we accidentally name the overriding function in the derived class as pint().
  • Functions with different return types: If the virtual function is, say, of void type but the function in the derived class is of int type.
  • Functions with different parameters: If the parameters of the virtual function and the functions in the derived classes don't match.
  • No virtual function is declared in the base class.

Use of C++ Virtual Functions

Suppose we have a base class Animal and derived classes Dog and Cat.

Suppose each class has a data member named type. Suppose these variables are initialized through their respective constructors.

 class Animal ( private: string type;… public: Animal(): type("Animal") ()… ); class Dog : public Animal ( private: string type;… public: Animal(): type("Dog") ()… ); class Cat : public Animal ( private: string type;… public: Animal(): type("Cat") ()… );

Now, let us suppose that our program requires us to create two public functions for each class:

  1. getType() to return the value of type
  2. print() to print the value of type

We could create both these functions in each class separately and override them, which will be long and tedious.

Or we could make getType() virtual in the Animal class, then create a single, separate print() function that accepts a pointer of Animal type as its argument. We can then use this single function to override the virtual function.

 class Animal (… public:… virtual string getType (… ) );… … void print(Animal* ani) ( cout << "Animal: "

This will make the code shorter, cleaner, and less repetitive.

Example 2: C++ virtual Function Demonstration

 // C++ program to demonstrate the use of virtual function #include #include using namespace std; class Animal ( private: string type; public: // constructor to initialize type Animal() : type("Animal") () // declare virtual function virtual string getType() ( return type; ) ); class Dog : public Animal ( private: string type; public: // constructor to initialize type Dog() : type("Dog") () string getType() override ( return type; ) ); class Cat : public Animal ( private: string type; public: // constructor to initialize type Cat() : type("Cat") () string getType() override ( return type; ) ); void print(Animal* ani) ( cout << "Animal: "

Output

 Animal: Animal Animal: Dog Animal: Cat

Here, we have used the virtual function getType() and an Animal pointer ani in order to avoid repeating the print() function in every class.

 void print(Animal* ani) ( cout << "Animal: "

In main(), we have created 3 Animal pointers to dynamically create objects of Animal, Dog and Cat classes.

 // dynamically create objects using Animal pointers Animal* animal1 = new Animal(); Animal* dog1 = new Dog(); Animal* cat1 = new Cat();

We then call the print() function using these pointers:

  1. When print(animal1) is called, the pointer points to an Animal object. So, the virtual function in Animal class is executed inside of print().
  2. When print(dog1) is called, the pointer points to a Dog object. So, the virtual function is overridden and the function of Dog is executed inside of print().
  3. When print(cat1) is called, the pointer points to a Cat object. So, the virtual function is overridden and the function of Cat is executed inside of print().

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