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From: Timothy Madden on 24 Jun 2010 09:50
Hello

My compiler complains if I try to declare

std::map<unsigned, RECT const &>::iterator it

in the argument list for a function declaration.

So I would like to know if it is legal to declare such a container.
I understand that operator [] can not possibly work on such a map,
because the operator creates a new pair when the requested index is
not found in the map, and there is no way to initialize the enclosed
refrence, but otherwise I think the container should be legal, despite
my compiler's error message when declaring the iterator.

Thank you,
Timothy Madden

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From: Goran on 24 Jun 2010 17:54
On Jun 25, 2:50 am, Timothy Madden <terminato...(a)gmail.com> wrote:
> Hello
>
> My compiler complains if I try to declare
>
> std::map<unsigned, RECT const &>::iterator it
>
> in the argument list for a function declaration.
>
> So I would like to know if it is legal to declare such a container.

I don't think so. The problem is, that would produce "references to
references" in quite a bit of places.

So pointers are in order. You might want to wrap the pointer in a
reference-like object and use that in your map ?

( NB: very limited usability; compiled with head-compiler and tested
with head-debugger ;-) )

class rect_holder
{
public:
rect_holder(const RECT& p) :_p(&p) {}
operator const RECT&() const { assert(p); return *p; }
private:
const RECT* _p;
};

std::map<unsigned, const rect_holder> mmm;

rect_holder gives you "copyability", and you can even do e.g.

const RECT& r = your_map[1];

But note also: with a map, if your value type is not const, you can't
use operator[] like so:

your_map[key] = value;

So IMO it's not a good idea to use const value type. You should
instead pass const map around.

Goran.


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From: T. Schroeder on 24 Jun 2010 18:01
Am 25.06.2010 02:50, schrieb Timothy Madden:
> Hello
>
> My compiler complains if I try to declare
>
> std::map<unsigned, RECT const &>::iterator it
>
> in the argument list for a function declaration.
> ...

no it isnt allowed... but you can use a boost::reference_wrapper

typedef boost::reference_wrapper<RECT> rect_ref;
typedef std::map<unsigned, rect_ref> rectmap;

RECT r;
rectmap m;
m.insert(rectmap::value_type(0, boost::ref(r)));

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From: red floyd on 25 Jun 2010 17:24
On Jun 25, 2:01 am, "T. Schroeder" <schroe...(a)ipe-chemnitz.de> wrote:
> Am 25.06.2010 02:50, schrieb Timothy Madden:
>
> > Hello
>
> > My compiler complains if I try to declare
>
> > std::map<unsigned, RECT const &>::iterator it
>
> > in the argument list for a function declaration.
> > ...
>
> no it isnt allowed... but you can use a boost::reference_wrapper
>
> typedef boost::reference_wrapper<RECT> rect_ref;
> typedef std::map<unsigned, rect_ref> rectmap;
>
> RECT r;
> rectmap m;
> m.insert(rectmap::value_type(0, boost::ref(r)));

Note that even if references to references weren't an issue,
the OP's container would be invalid, because it doesn't meet
the requirements in 23.1/4 that the type T stored in a container
(in this case, std::pair<const unsigned, const RECT&>) be
Assignable.


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