Virtual functions

Question

1.26/5 (5 votes)

See more:

C++

C#

What are the advantages and disadvantages of virtual functions?

Posted 22-Jul-10 17:45pm

umesh_tk85 Jun2010

Updated 26-Sep-19 1:09am

Johnny J.

v2

Add a Solution

Comments

Arun Jacob 23-Jul-10 0:38am

Reason for my vote of 1
Try some online resources/books.

6 solutions

Solution 5

Virtual fucntion are a must to support runtime polymorphism.

C++

class A
{
public:
   virtual ~A() {}
   virtual void hallo()
   { std::cout << "I'm A[" << this << "]" << std::endl; }
};

class B: public A;
{
public:
   virtual void hallo()
   { std::cout << "I'm B[" << this << "]" << std::endl; }
};

int main()
{
    static const size_t N = 4;
    A* v[N];

    for(size_t i=0; i<N; i+=2)
    {
       v[i]   = new A;
       v[i+1] = new B;
    }

    for(size_t i=0; i<N; ++i)
       v[i]->hallo();

    for(size_t i=0; i<N; ++i)
    { delete v[i]; v[i]=0;  }

    return 0;
}

Even if main manages just A* (and new B returns a B* that automatically convert into A* being B derived from A), making hallo and the destructor virtual makes v[i] (that is an A*) when calling hallo orwhen under delete to call the most derived method.

This is not possible without virtual functions, unless storing tales of function pointers that are noting more than the compiler generate v-tables.

The program output should be:

txt

I'm A[0xxxxxxx]
I'm B[0yyyyyyy]
I'm A[0zzzzzzz]
I'm B[0wwwwwww]

Posted 23-Jul-10 6:49am

Emilio Garavaglia

Solution 2

You might want to go through some books on OOPS concept while you are reading about virtual methods.

Posted 22-Jul-10 18:43pm

Abhinav S

Solution 3

>If a class is inherited from a class containing a virtual function, then the function definition can be redefined in the inherited class.
If a function is virtual and a new definition of the function is given in a derived class, then for any object of the derived class, that object will always use the new definition. Even if it is invoked in the inherited function.
Virtual functions takes much space as virtual function table is created by compiler which contains pointers for each virtual member function which points to the memory location of code for that function.
Programs containing Virtual functions run slow.

Posted 22-Jul-10 18:43pm

Shining Legend

Comments

Emilio Garavaglia 23-Jul-10 12:30pm

Reason for my vote of 1
The most of the issues about space and speed are resolved by compiler optimization.
In 2010 be afraid of virtual functions is simply a non-sense.

Solution 6

virtual functions plays an important role in all framework designs.

Posted 23-Jul-10 8:29am

g_shanmuga

Comments

Emilio Garavaglia 23-Jul-10 15:42pm

Reason for my vote of 2
irrelevant respect to the question

Aescleal 23-Jul-10 16:52pm

Reason for my vote of 1
Not necessarily - at least one framework use the Curious Recurring Template Pattern to avoid using virtual functions.

Solution 1

This is a very basic question, Better you learn some basics.

Virtual keyword allows a method to be overridden in the inherited class.

Virtual C#[^]

Posted 22-Jul-10 18:38pm

Arun Jacob

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Aescleal · Accepted Answer · 2010-07-22T22:03:00

The main advantage of virtual functions are that they directly support object oriented programming. When you declare a function as virtual you're saying that exactly what code is executed depends on the type of the object you call it against.

The main disadvantage of virtual functions is the converse of the above: It can be hard to reason about what your code is doing just by reading it. So if you see a function:

void foo( interface_ptr *p )
{
    p->a();
    p->b();
}

you can't tell exactly what code path it's going to follow.

If you're concerned about performance then don't be. Even luddite games programmers are now using virtual functions and finding the performance isn't that bad and usually beats hand rolling polymorphism other ways.

Cheers,

Ash

Virtual functions

6 solutions

Solution 4

Solution 5

Solution 2

Solution 3

Solution 6

Solution 1

Add your solution here

Preview 0

Existing Members

...or Join us