Why do we need virtual functions in C++?

Question

I'm learning C++ and I'm just getting into virtual functions.

From what I've read (in the book and online), virtual functions are functions in the base class that you can override in derived classes.

But earlier in the book, when learning about basic inheritance, I was able to override base functions in derived classes without using virtual.

So what am I missing here? I know there is more to virtual functions, and it seems to be important so I want to be clear on what it is exactly. I just can't find a straight answer online.

Answer 1

Virtual Functions are used to support Runtime Polymorphism.

That is, virtual keyword tells the compiler not to make the decision (of function binding) at compile time, rather postpone it for runtime".

• You can make a function virtual by preceding the keyword virtual in its base class declaration. For example,

   class Base
   {
      virtual void func();
   }
  

• When a Base Class has a virtual member function, any class that inherits from the Base Class can redefine the function with exactly the same prototype i.e. only functionality can be redefined, not the interface of the function.

   class Derive : public Base
   {
      void func();
   }
  

• A Base class pointer can be used to point to Base class object as well as a Derived class object.

• When the virtual function is called by using a Base class pointer, the compiler decides at run-time which version of the function - i.e. the Base class version or the overridden Derived class version - is to be called. This is called Runtime Polymorphism.

Answer 2

You need at least 1 level of inheritance and a downcast to demonstrate it. Here is a very simple example:

class Animal
{        
    public: 
      // turn the following virtual modifier on/off to see what happens
      //virtual   
      std::string Says() { return "?"; }  
};

class Dog: public Animal
{
    public: std::string Says() { return "Woof"; }
};

void test()
{
    Dog* d = new Dog();
    Animal* a = d;       // refer to Dog instance with Animal pointer

    cout << d->Says();   // always Woof
    cout << a->Says();   // Woof or ?, depends on virtual
}

Answer 3

Why do we need Virtual Methods in C++?

Quick Answer:

1. It provides us with one of the needed "ingredients"¹ for object oriented programming.

In Bjarne Stroustrup C++ Programming: Principles and Practice, (14.3):

The virtual function provides the ability to define a function in a base class and have a function of the same name and type in a derived class called when a user calls the base class function. That is often called run-time polymorphism, dynamic dispatch, or run-time dispatch because the function called is determined at run time based on the type of the object used.

2. It is the fastest more efficient implementation if you need a virtual function call ².

To handle a virtual call, one needs one or more pieces of data related to the derived object ³. The way that is usually done is to add the address of table of functions. This table is usually referred to as virtual table or virtual function table and its address is often called the virtual pointer. Each virtual function gets a slot in the virtual table. Depending of the caller's object (derived) type, the virtual function, in its turn, invokes the respective override.

---

^{1.The use of inheritance, run-time polymorphism, and encapsulation is the most common definition of object-oriented programming.}

^{2. You can't code functionality to be any faster or to use less memory using other language features to select among alternatives at run time. Bjarne Stroustrup C++ Programming: Principles and Practice.(14.3.1)}.

^{3. Something to tell which function is really invoked when we call the base class containing the virtual function.}

Answer 4

Here is a merged version of the C++ code for the first two answers.

#include        <iostream>
#include        <string>

using   namespace       std;

class   Animal
{
        public:
#ifdef  VIRTUAL
                virtual string  says()  {       return  "??";   }
#else
                string  says()  {       return  "??";   }
#endif
};

class   Dog:    public Animal
{
        public:
                string  says()  {       return  "woof"; }
};

string  func(Animal *a)
{
        return  a->says();
}

int     main()
{
        Animal  *a = new Animal();
        Dog     *d = new Dog();
        Animal  *ad = d;

        cout << "Animal a says\t\t" << a->says() << endl;
        cout << "Dog d says\t\t" << d->says() << endl;
        cout << "Animal dog ad says\t" << ad->says() << endl;

        cout << "func(a) :\t\t" <<      func(a) <<      endl;
        cout << "func(d) :\t\t" <<      func(d) <<      endl;
        cout << "func(ad):\t\t" <<      func(ad)<<      endl;
}

Two different results are:

Without #define virtual, it binds at compile time. Animal *ad and func(Animal *) all point to the Animal's says() method.

$ g++ virtual.cpp -o virtual
$ ./virtual 
Animal a says       ??
Dog d says      woof
Animal dog ad says  ??
func(a) :       ??
func(d) :       ??
func(ad):       ??

With #define virtual, it binds at run time. Dog *d, Animal *ad and func(Animal *) point/refer to the Dog's says() method as Dog is their object type. Unless [Dog's says() "woof"] method is not defined, it will be the one searched first in the class tree, i.e. derived classes may override methods of their base classes [Animal's says()].

$ g++ virtual.cpp -D VIRTUAL -o virtual
$ ./virtual 
Animal a says       ??
Dog d says      woof
Animal dog ad says  woof
func(a) :       ??
func(d) :       woof
func(ad):       woof

It is interesting to note that all class attributes (data and methods) in Python are effectively virtual. Since all objects are dynamically created at runtime, there is no type declaration or a need for keyword virtual. Below is Python's version of code:

class   Animal:
        def     says(self):
                return  "??"

class   Dog(Animal):
        def     says(self):
                return  "woof"

def     func(a):
        return  a.says()

if      __name__ == "__main__":

        a = Animal()
        d = Dog()
        ad = d  #       dynamic typing by assignment

        print("Animal a says\t\t{}".format(a.says()))
        print("Dog d says\t\t{}".format(d.says()))
        print("Animal dog ad says\t{}".format(ad.says()))

        print("func(a) :\t\t{}".format(func(a)))
        print("func(d) :\t\t{}".format(func(d)))
        print("func(ad):\t\t{}".format(func(ad)))

The output is:

Animal a says       ??
Dog d says      woof
Animal dog ad says  woof
func(a) :       ??
func(d) :       woof
func(ad):       woof

which is identical to C++'s virtual define. Note that d and ad are two different pointer variables referring/pointing to the same Dog instance. The expression (ad is d) returns True and their values are the same <main.Dog object at 0xb79f72cc>.

Answer 5

You need virtual methods for safe downcasting, simplicity and conciseness.

That’s what virtual methods do: they downcast safely, with apparently simple and concise code, avoiding the unsafe manual casts in the more complex and verbose code that you otherwise would have.

---

Non-virtual method ⇒ static binding

The following code is intentionally “incorrect”. It doesn’t declare the value method as virtual, and therefore produces an unintended “wrong” result, namely 0:

#include <iostream>
using namespace std;

class Expression
{
public:
    auto value() const
        -> double
    { return 0.0; }         // This should never be invoked, really.
};

class Number
    : public Expression
{
private:
    double  number_;

public:
    auto value() const
        -> double
    { return number_; }     // This is OK.

    Number( double const number )
        : Expression()
        , number_( number )
    {}
};

class Sum
    : public Expression
{
private:
    Expression const*   a_;
    Expression const*   b_;

public:
    auto value() const
        -> double
    { return a_->value() + b_->value(); }       // Uhm, bad! Very bad!

    Sum( Expression const* const a, Expression const* const b )
        : Expression()
        , a_( a )
        , b_( b )
    {}
};

auto main() -> int
{
    Number const    a( 3.14 );
    Number const    b( 2.72 );
    Number const    c( 1.0 );

    Sum const       sum_ab( &a, &b );
    Sum const       sum( &sum_ab, &c );

    cout << sum.value() << endl;
}

In the line commented as “bad” the Expression::value method is called, because the statically known type (the type known at compile time) is Expression, and the value method is not virtual.

---

Virtual method ⇒ dynamic binding.

Declaring value as virtual in the statically known type Expression ensures that the each call will check what actual type of object this is, and call the relevant implementation of value for that dynamic type:

#include <iostream>
using namespace std;

class Expression
{
public:
    virtual
    auto value() const -> double
        = 0;
};

class Number
    : public Expression
{
private:
    double  number_;

public:
    auto value() const -> double
        override
    { return number_; }

    Number( double const number )
        : Expression()
        , number_( number )
    {}
};

class Sum
    : public Expression
{
private:
    Expression const*   a_;
    Expression const*   b_;

public:
    auto value() const -> double
        override
    { return a_->value() + b_->value(); }    // Dynamic binding, OK!

    Sum( Expression const* const a, Expression const* const b )
        : Expression()
        , a_( a )
        , b_( b )
    {}
};

auto main() -> int
{
    Number const    a( 3.14 );
    Number const    b( 2.72 );
    Number const    c( 1.0 );

    Sum const       sum_ab( &a, &b );
    Sum const       sum( &sum_ab, &c );

    cout << sum.value() << endl;
}

Here the output is 6.86 as it should be, since the virtual method is called virtually. This is also called dynamic binding of the calls. A little check is performed, finding the actual dynamic type of object, and the relevant method implementation for that dynamic type, is called.

The relevant implementation is the one in the most specific (most derived) class.

Note that method implementations in derived classes here are not marked virtual, but are instead marked override. They could be marked virtual but they’re automatically virtual. The override keyword ensures that if there is not such a virtual method in some base class, then you’ll get an error (which is desirable).

---

The ugliness of doing this without virtual methods

Without virtual one would have to implement some Do It Yourself version of the dynamic binding. It’s this that generally involves unsafe manual downcasting, complexity and verbosity.

For the case of a single function, as here, it suffices to store a function pointer in the object and call via that function pointer, but even so it involves some unsafe downcasts, complexity and verbosity, to wit:

#include <iostream>
using namespace std;

class Expression
{
protected:
    typedef auto Value_func( Expression const* ) -> double;

    Value_func* value_func_;

public:
    auto value() const
        -> double
    { return value_func_( this ); }

    Expression(): value_func_( nullptr ) {}     // Like a pure virtual.
};

class Number
    : public Expression
{
private:
    double  number_;

    static
    auto specific_value_func( Expression const* expr )
        -> double
    { return static_cast<Number const*>( expr )->number_; }

public:
    Number( double const number )
        : Expression()
        , number_( number )
    { value_func_ = &Number::specific_value_func; }
};

class Sum
    : public Expression
{
private:
    Expression const*   a_;
    Expression const*   b_;

    static
    auto specific_value_func( Expression const* expr )
        -> double
    {
        auto const p_self  = static_cast<Sum const*>( expr );
        return p_self->a_->value() + p_self->b_->value();
    }

public:
    Sum( Expression const* const a, Expression const* const b )
        : Expression()
        , a_( a )
        , b_( b )
    { value_func_ = &Sum::specific_value_func; }
};


auto main() -> int
{
    Number const    a( 3.14 );
    Number const    b( 2.72 );
    Number const    c( 1.0 );

    Sum const       sum_ab( &a, &b );
    Sum const       sum( &sum_ab, &c );

    cout << sum.value() << endl;
}

One positive way of looking at this is, if you encounter unsafe downcasting, complexity and verbosity as above, then often a virtual method or methods can really help.

Answer 6

If the base class is Base, and a derived class is Der, you can have a Base *p pointer which actually points to an instance of Der. When you call p->foo();, if foo is not virtual, then Base's version of it executes, ignoring the fact that p actually points to a Der. If foo is virtual, p->foo() executes the "leafmost" override of foo, fully taking into account the actual class of the pointed-to item. So the difference between virtual and non-virtual is actually pretty crucial: the former allow runtime polymorphism, the core concept of OO programming, while the latter don't.