I denna handledning lär vi oss om C ++ virtuell funktion och dess användning med hjälp av exempel.
En virtuell funktion är en medlemsfunktion i basklassen som vi förväntar oss att omdefiniera i härledda klasser.
I grund och botten används en virtuell funktion i basklassen för att säkerställa att funktionen åsidosätts . Detta gäller särskilt fall där en pekare av basklass pekar på ett objekt från en härledd klass.
Tänk till exempel på koden nedan:
class Base ( public: void print() ( // code ) ); class Derived : public Base ( public: void print() ( // code ) );
Senare, om vi skapar en pekare av Base
typ för att peka på ett objekt av Derived
klass och kallar print()
funktionen, kallar den print()
funktionen för Base
klassen.
Med andra ord Base
åsidosätts inte medlemsfunktionen.
int main() ( Derived derived1; Base* base1 = &derived1; // calls function of Base class base1->print(); return 0; )
För att undvika detta förklarar vi klassens print()
funktion Base
som virtuell med hjälp av det virtuella nyckelordet.
class Base ( public: virtual void print() ( // code ) );
Virtuella funktioner är en integrerad del av polymorfism i C ++. För att lära dig mer, se vår handledning om C ++ polymorfism.
Exempel 1: C ++ virtuell 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:
getType()
to return the value of type
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:
When print(animal1)
is called, the pointer points to an Animal
object. So, the virtual function in Animal
class is executed inside of print()
.
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()
.
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()
.