Re: Containers of pointers and const-correctness

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Thu, 03 Sep 2009 03:07:38 +0200

Message-ID:

<h7n4st$ol$1@news.eternal-september.org>

* Stuart Golodetz:

#include <vector>

struct B {};
struct D1 : B {};
struct D2 : B {};

int main()
{
    std::vector<D1*> vD1;

    // Doesn't compile (shouldn't)
    //std::vector<B*>& vB = vD1;

    // Doesn't compile (shouldn't)
    //std::vector<const D1*>& vCD1C = vD1;

    // Doesn't compile (but why would it be a bad thing if it did?)
    const std::vector<const D1*>& CvCD1 = vD1;

    return 0;
}

Assume the vector has 1 element, which is a pointer.

A reference to a vector of 1 element is (with respect to what counts here)
functionally equivalent to a pointer to a vector of 1 element, which is
functionally equivalent to a pointer to a pointer.

So you're asking why you can't do

   T* p = ...
   T** pp = &p;
   T const** PP = pp;

It would break const correctness, allowing you to modify an original const thing.

See the FAQ item titled "Why am I getting an error converting a Foo** to const
Foo**", currently 18.17 and available at e.g. <url:
http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.17>.

There is, however, also another issue hinted at by your code, having to do with
upcast/downcast of a collection.

The answer for that other issue is that the standard library containers do not
support all that they in principle could support within type safety.

Cheers & hth.,

- Alf