Visual Studio 2015: is a set of const pointers allowed?

Question

We will be upgrading to VS2015 soon, and I found this in the breaking changes list:

const elements

The C++ standard has always forbidden containers of const elements (such as vector or set). Visual C++ 2013 and earlier accepted such containers. In the current version, such containers fail to compile.

source

I was wondering if anyone knows if this also applies to a set. I know a map can still contain const pointers as keys, since they are const anyway.

An example:

std::set<const QObject*>

Can I still do this? I would think not, according to the post on the site of Microsoft.

The quote literally says that it applies to set. However, your code doesn't have much to do with the quote so you're safe :) — Lightness Races in Orbit, May 09 '16 at 12:09
The confusion is reinforced by the nobly-intended-but-misleading style that omits the space before the asterisk (or ampersand on a reference). Writing `const QObject*` visually suggests that the `*` binds to QObject tighter than the `const` does. Moving the `const` doesn't help: `QObject const*`. It's like writing `return a * x+b * y;`. — Adrian McCarthy, May 09 '16 at 16:33

cadaniluk · Accepted Answer · 2016-05-09T08:46:53.320

26

const QObject* is a pointer to a const QObject. The pointer itself is still modifiable. const QObject* const would make the pointer itself const.

Since the Microsoft article talks about const objects, which const QObject* is not, your example is fine.

edited May 09 '16 at 08:46

answered May 09 '16 at 08:38

cadaniluk

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score 4 · Answer 2 · answered May 09 '16 at 09:14

I know a map can still contain const pointers as keys, since they are const anyway.

std::set<T* const> has always been invalid, and std::map<int* const, int* const> s; has always been valid. The reason is because the allocator for std::set is std::allocator<Key>, whereas the allocator for std::map is std::allocator<std::pair<const Key, T>>. By definition, a std::allocator<const T> is ill-formed. If you want the map code to fail, you'll have to specify a custom allocator like this:

int i = 42;
int* const j = &i;
std::map<int* const, int* const,
    std::allocator<const std::pair<int* const, int* const>>> s{{j, j}};

Visual Studio 2015: is a set of const pointers allowed?

2 Answers2