Can you cast a pointer to a pointer to the first member?

Asked Jan 08 '20 at 17:25

Active Jan 08 '20 at 17:30

Viewed 73 times

Given:

struct Foo { int a; int b; };
struct Bar { Foo foo; int c; };

Is this well-defined?

void f(Bar *bar) {
    auto foo = (Foo*)bar;
    cout << foo->a << ',' << foo->b;
}

Clang translates code using inheritance, to the equivalent of the above, but I want to make sure it's defined by the standard.

edited Jan 08 '20 at 17:30

asked Jan 08 '20 at 17:25

rwallace

3

Why do You need to do this? – Robert Andrzejuk Jan 08 '20 at 17:29
1

You might want to add `language-lawyer` tag, especially if you want chapter and verse from the standard. – Eljay Jan 08 '20 at 17:30
@Eljay Good idea, thanks! – rwallace Jan 08 '20 at 17:31
1

I was going to point to http://www.cplusplus.com/reference/cstddef/offsetof/, but then the question was closed, and at least one answer in the link provided does the same thing. – Enno Jan 08 '20 at 17:34
But as @RobertAndrzejuk asks, "why do you want this?" when you could just use &bar->foo ? – Enno Jan 08 '20 at 17:35
As per the other Q&A, it seems that "standard layout" requirement is rather easily/inadvertently violated. – Eljay Jan 08 '20 at 17:36
@RobertAndrzejuk Because there might be several Bar's each with a Foo as the first member. – rwallace Jan 08 '20 at 17:37

0 Answers0