set of pointers

[REH]

Is it well defined to use a std::set of pointers? I ask because of
the restrictions on using relational operators with pointers. Does a
set of pointers suffer from the same restrictions, or does the
standard require it to be supported. I can't find any information one
way or the other in the standard or from other sources.

Thanks,
REH

[Fei Liu]

REH wrote:

Is it well defined to use a std::set of pointers? I ask because of
the restrictions on using relational operators with pointers. Does a
set of pointers suffer from the same restrictions, or does the
standard require it to be supported. I can't find any information one
way or the other in the standard or from other sources.

Thanks,
REH

std::set of pointers are well supported, pointers have value semantic,
comparable.

The relational operators you refer work fine because pointers can be
compared by value.

Fei

[Lance Diduck]

On Feb 26, 10:31*am, REH <[e-mail]:

Is it well defined to use a std::set of pointers? *I ask because of
the restrictions on using relational operators with pointers. *Does a
set of pointers suffer from the same restrictions, or does the
standard require it to be supported. *I can't find any information one
way or the other in the standard or from other sources.

Thanks,
REH

Since pointers implement operator< as required by std::set, then it is
possible to use pointers.
Note that what is being compared is the actual address being stored in
the pointer, and not what it points to.

Lance

[red floyd]

REH wrote:

Is it well defined to use a std::set of pointers? I ask because of
the restrictions on using relational operators with pointers. Does a
set of pointers suffer from the same restrictions, or does the
standard require it to be supported. I can't find any information one
way or the other in the standard or from other sources.

In addition to what everyone here has discussed so far, you can provide
a custom comparison functor for your set, so you can sort it based on
what the pointers refer to, rather than the raw pointers themselves.

[REH]

On Feb 26, 10:45 am, Lance Diduck <[e-mail]:

Since pointers implement operator< as required by std::set, then it is
possible to use pointers.
Note that what is being compared is the actual address being stored in
the pointer, and not what it points to.

Lance

Thanks for replying, but you are not correct. Pointers are not
necessarily addresses, and the standard only allows pointers to be
relationally compared if they come from the same object (i.e., the
same struct or array).

REH

[REH]

On Feb 26, 10:44 am, Fei Liu <[e-mail]:

std::set of pointers are well supported, pointers have value semantic,
comparable.

The relational operators you refer work fine because pointers can be
compared by value.

Fei

Your logic doesn't hold. Pointers can only be relationally compared
if they are from the same struct, array, etc.

REH

[James Kanze]

On Feb 26, 4:44 pm, Fei Liu <[e-mail]:

REH wrote:

Is it well defined to use a std::set of pointers? I ask
because of the restrictions on using relational operators
with pointers. Does a set of pointers suffer from the same
restrictions, or does the standard require it to be
supported. I can't find any information one way or the
other in the standard or from other sources.

std::set of pointers are well supported, pointers have value
semantic, comparable.

The relational operators you refer work fine because pointers
can be compared by value.

Pointers are only comparable for inequality (e.g. <) if they
point into the same object or array. You can't expect to
compare two arbitrary pointers and get anything meaningful. (It
does actually work on a lot of machines, but not always on Intel
architectures.)

You can use pointers in std::set, however, since std::set
doesn't use the < operator, but rather std::less, and the
standard requires the implementation to make this work, even if
the < operator doesn't work.

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Fei Liu]

REH wrote:

On Feb 26, 10:44 am, Fei Liu <[e-mail]:

>std::set of pointers are well supported, pointers have value semantic,
comparable.

The relational operators you refer work fine because pointers can be
compared by value.

Fei

Your logic doesn't hold. Pointers can only be relationally compared
if they are from the same struct, array, etc.

REH

It's implied that your 'pointers' are of the same type. Otherwise your
question is ill formed. I was going to mention this but it's good that
you understand that you must use same type of pointers in a single set.

Fei

[Victor Bazarov]

Fei Liu wrote:

REH wrote:

>On Feb 26, 10:44 am, Fei Liu <[e-mail]:

>>std::set of pointers are well supported, pointers have value
semantic, comparable.

The relational operators you refer work fine because pointers can be
compared by value.

Fei

Your logic doesn't hold. Pointers can only be relationally compared
if they are from the same struct, array, etc.

REH

It's implied that your 'pointers' are of the same type. Otherwise your
question is ill formed. I was going to mention this but it's good that
you understand that you must use same type of pointers in a single
set.

Pointers can only be compared using relational operators if the objects
they point to belong the same [larger] object, for example, an array.
If they pointers have the same type but aren't part of another, larger,
object, using relational operators on them is undefined.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[REH]

On Feb 26, 4:21 pm, Fei Liu <[e-mail]:

It's implied that your 'pointers' are of the same type. Otherwise your
question is ill formed. I was going to mention this but it's good that
you understand that you must use same type of pointers in a single set.

Fei

No, it has nothing to do with being the same type. Pointers must
point to elements or fields of the same OBJECT to be relationally
comparable.

REH

[James Kanze]

On Feb 26, 10:33 pm, "Victor Bazarov" <[e-mail]:

Fei Liu wrote:

REH wrote:

On Feb 26, 10:44 am, Fei Liu <[e-mail]:
std::set of pointers are well supported, pointers have value
semantic, comparable.

>The relational operators you refer work fine because
pointers can be compared by value.

Your logic doesn't hold. Pointers can only be relationally
compared if they are from the same struct, array, etc.

It's implied that your 'pointers' are of the same type.
Otherwise your question is ill formed. I was going to
mention this but it's good that you understand that you must
use same type of pointers in a single set.

Pointers can only be compared using relational operators if
the objects they point to belong the same [larger] object, for
example, an array. If they pointers have the same type but
aren't part of another, larger, object, using relational
operators on them is undefined.

And if they're of different types, the compiler will try to find
a common type, and use that. (I'm not sure of the exact rules,
because I can't think of a general case where it would be
useful. But cv qualifiers can definitely be discarded---you can
compare a char const* with a char* without any problems, as long
as they point into the same object.)

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]