asked on

Virtual functions

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

AlexFM

This code fragment is not full. How do you call test function?

Exceter

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

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Axter

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SOLUTION

Axter

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AlexFM

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.

Sandra-24

ASKER

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.