Solved

Virtual functions

Posted on 2003-11-01
6
293 Views
Last Modified: 2013-11-18
I was under the impression that virtual function would not do this, perhaps someone will knwo why they do and how to get around it.

class A {
public:
virtual void foo() { cout << "A's foo"; }
}

class B : public A
{
void foo() { cout << "B's foo"; }
}

void test(A& bar)
{
  bar.foo();
}

this prints "A's foo". How do I get it to use B's foo?

Thanks,
-Sandra
0
Comment
Question by:Sandra-24
6 Comments
 
LVL 48

Expert Comment

by:AlexFM
ID: 9665106
This code fragment is not full. How do you call test function?
0
 
LVL 8

Expert Comment

by:Exceter
ID: 9665108
Try this,

You did not terminate your class declarations with a semicolon and B's foo was declared as private so I'm not even sure why that compiled.

#include <iostream>
#include <iomanip>

using namespace std;

class A
{
      public:
            virtual void foo() { cout << "A's foo"; }
};

class B : public A
{
      public:
            void foo() { cout << "B's foo"; }
};

int main()
{
      B test;
      test.foo();

    return 0;
}

Cheers!
Exceter
0
 
LVL 30

Accepted Solution

by:
Axter earned 250 total points
ID: 9665127
The following code does print out "B's foo"

class A
{
public:
      virtual void foo() { cout << "A's foo\n"; }
};

class B : public A
{
public:
      void foo() { cout << "B's foo\n"; }
};

void test(A& bar)
{
      bar.foo();
}


int main(int argc, char* argv[])
{
    B b_test;
    test(b_test);
0
U.S. Department of Agriculture and Acronis Access

With the new era of mobile computing, smartphones and tablets, wireless communications and cloud services, the USDA sought to take advantage of a mobilized workforce and the blurring lines between personal and corporate computing resources.

 
LVL 30

Assisted Solution

by:Axter
Axter earned 250 total points
ID: 9665131
However, the following code prints out "A's foo"

void test(A bar)
{
      bar.foo();
}


Are you sure your code is using a reference argument?
0
 
LVL 48

Expert Comment

by:AlexFM
ID: 9665143
class A
{
public:
virtual void foo() { cout << "A's foo\n"; }
};

class B : public A
{
public:
void foo() { cout << "B's foo\n"; }
};

void test(A& bar)
{
 bar.foo();
}

int main(int argc, char* argv[])
{
    int i;
    A a;
    test(a);

    B b;
    test(b);

    return 0;
}

Result is:
A's foo
B's foo

However, this is not so interesting. Virtual functions are used for array of the class instances. You can create array of A and fill it with A abd B instances. Now you can call virtual function for each array element, and required function is called:

class A
{
public:
virtual void foo() { cout << "A's foo\n"; }
};

class B : public A
{
public:
void foo() { cout << "B's foo\n"; }
};


void test1(A* bar)
{
 bar->foo();
}

int main(int argc, char* argv[])
{
    int i;

    A* array[2];
    array[0] = new A();
    array[1] = new B();

    for ( i = 0; i < 2; i++ )
        test1(array[i]);

    for ( i = 0; i < 2; i++ )
        delete array[i];
   

    return 0;
}

Result is:
A's foo
B's foo


Make foo non-virtual and you will get
A's foo
A's foo

in the both cases.
0
 
LVL 3

Author Comment

by:Sandra-24
ID: 9666576
Thanks axter, that was it exactly. I passsed B into test() by value. Sorry guys about the bad code example, I was rushed when I posted.
0

Featured Post

Free Tool: Subnet Calculator

The subnet calculator helps you design networks by taking an IP address and network mask and returning information such as network, broadcast address, and host range.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

JavaScript has plenty of pieces of code people often just copy/paste from somewhere but never quite fully understand. Self-Executing functions are just one good example that I'll try to demystify here.
The article will include the best Data Recovery Tools along with their Features, Capabilities, and their Download Links. Hope you’ll enjoy it and will choose the one as required by you.
The viewer will the learn the benefit of plain text editors and code an HTML5 based template for use in further tutorials.
HTML5 has deprecated a few of the older ways of showing media as well as offering up a new way to create games and animations. Audio, video, and canvas are just a few of the adjustments made between XHTML and HTML5. As we learned in our last micr…

809 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question