[Faq-20.6] What is a "virtual constructor"?

[Alex Vinokur]

http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.6

Faq-20.6 contains function userCode(Shape&) :

void userCode(Shape& s)
{
Shape* s2 = s.clone();
Shape* s3 = s.create();
...
// stuff; for instance
// s2->draw();
delete s2;
delete s3;
}

Why don't use userCode2(Shape*) ?
void userCode2(Shape* s)
{
//stuff; for instance
// s->draw();
}

 

[Rolf Magnus]

http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.6

Faq-20.6 contains function userCode(Shape&) :

void userCode(Shape& s)
{
Shape* s2 = s.clone();
Shape* s3 = s.create();
...
// stuff; for instance
// s2->draw();
delete s2;
delete s3;
}

Why don't use userCode2(Shape*) ?
void userCode2(Shape* s)
{
//stuff; for instance
// s->draw();
}

Looks to me just like an example that isn't chosen too well. The idea
seems to be that you work on a copy, not the original object.

 

[Alex Vinokur]

Looks to me just like an example that isn't chosen too well. The idea
seems to be that you work on a copy, not the original object.

Why should we work on a copy?

 

[Rolf Magnus]

Why should we work on a copy?

That's why I think the example is bad. It doesn't give a reason.

 

[Bill Seurer]

Why should we work on a copy?

The example may not show it but you often want to take something and
manipulate it without changing the original. Thus you need a copy.