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Teh (tî'pô)
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 Posted: Mon Jan 26, 2004 12:04 am Post subject: Can types with address-of operator be used to create std::se |
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I'm working with a class (not mine) that overrides the address-of
operator (operator &) and I'm trying to construct a set of such
objects. CC and VS6 both croak on this sample but g++ compiles
successfully.
I've simplified the code to this:
#include <set>
class int_ptr {
int i;
public:
int * operator&() { return &i; }
};
std::set<int_ptr> s;
Is there a limitation on sets that prohibit the template argument from
defining operator& ? I couldn't find such a limitation in the
standard.
There's no problem in constructing a vector of the int_ptrs BTW the
problem seems to be in tree.
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Jeff Schwab
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| Posted: Mon Jan 26, 2004 10:45 am Post subject: Re: Can types with address-of operator be used to create std |
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Teh (tî'pô) wrote:
| Quote: | I'm working with a class (not mine) that overrides the address-of
operator (operator &) and I'm trying to construct a set of such
objects. CC and VS6 both croak on this sample but g++ compiles
successfully.
I've simplified the code to this:
#include <set
class int_ptr {
int i;
public:
int * operator&() { return &i; }
};
std::set
Is there a limitation on sets that prohibit the template argument from
defining operator& ? I couldn't find such a limitation in the
standard.
There's no problem in constructing a vector of the int_ptrs BTW the
problem seems to be in tree.
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What's the error message? My guess is that the implementation is trying
something like this:
T t; // "T" is int_ptr.
T* p = &t;
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Keith H Duggar
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| Posted: Mon Jan 26, 2004 8:20 pm Post subject: Re: Can types with address-of operator be used to create std |
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If it means anything, every C++ expert and master I've read highly
recommends against overloading the address-of operator. Try to find
another safer way.
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Gennaro Prota
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| Posted: Mon Jan 26, 2004 8:26 pm Post subject: Re: Can types with address-of operator be used to create std |
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On 25 Jan 2004 19:04:23 -0500, "Teh (tî'pô)" <teh (AT) mindless (DOT) com> wrote:
| Quote: | I'm working with a class (not mine) that overrides the address-of
operator (operator &) and I'm trying to construct a set of such
objects. CC and VS6 both croak on this sample but g++ compiles
successfully.
I've simplified the code to this:
#include <set
class int_ptr {
int i;
public:
int * operator&() { return &i; }
};
std::set
Is there a limitation [...]
|
Well, you would have troubles here without an adequate comparison
function, but I'll assume it exists in your "real" code.
Yes, there's an important requirement on the behavior of the &
operator, obfuscated under the (not so familiar) name of
"CopyConstructible". See also
http://std.dkuug.dk/jtc1/sc22/wg21/docs/lwg-closed.html#390
The issued is marked with "Future", but I wouldn't hold my breath.
*If* the address-of requirements will be dropped (but, anyway, why
weren't they put as a separate concept?) then library implementors
will need alternative ways to take the address of objects. Of course
it shouldn't be a problem to play unportable tricks here, but is there
a unique, portable solution? Incidentally, I've recently filed a DR
whose resolution could clear at least if the following
template <typename T> T* addressof(T& v)
{
return reinterpret_cast<T*>(
& const_cast<unsigned char&>(
reinterpret_cast<const volatile unsigned char &>(v)
)
);
}
is required to work on any conforming implementation.
Genny.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Allan W
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| Posted: Wed Jan 28, 2004 2:14 pm Post subject: Re: Can types with address-of operator be used to create std |
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| Quote: | "Teh (tî'pô)" <teh (AT) mindless (DOT) com> wrote:
I'm working with a class (not mine) that overrides the address-of
operator (operator &) and I'm trying to construct a set of such
objects. CC and VS6 both croak on this sample but g++ compiles
successfully.
#include <set
class int_ptr {
int i;
public:
int * operator&() { return &i; }
};
std::set
|
Gennaro Prota <gennaro_prota (AT) yahoo (DOT) com> wrote
| Quote: | Of course
it shouldn't be a problem to play unportable tricks here, but is there
a unique, portable solution?
|
I would imagine that wrapping int_ptr in a class that doesn't have
address-of, would help...
struct int_ptr_wrapper { int_ptr i; };
std::set<int_ptr_wrapper>s;
Notice that the int_ptr is public. Now the address-of operator gets
the address of the wrapper, and you can use it to access member i.
This has a drastic effect on the syntax of any code that uses the
set, but at least it should compile correctly...
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Gennaro Prota
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| Posted: Thu Jan 29, 2004 9:50 am Post subject: Re: Can types with address-of operator be used to create std |
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On 28 Jan 2004 09:14:13 -0500, [email]allan_w (AT) my-dejanews (DOT) com[/email] (Allan W)
wrote:
| Quote: | [...]
Gennaro Prota <gennaro_prota (AT) yahoo (DOT) com> wrote
Of course
it shouldn't be a problem to play unportable tricks here, but is there
a unique, portable solution?
I would imagine that wrapping int_ptr in a class that doesn't have
address-of, would help...
struct int_ptr_wrapper { int_ptr i; };
std::set<int_ptr_wrapper>s;
Notice that the int_ptr is public. Now the address-of operator gets
the address of the wrapper, and you can use it to access member i.
This has a drastic effect on the syntax of any code that uses the
set, but at least it should compile correctly...
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But I wasn't talking about client code. I was rather saying that,
since all contained types must be Assignable and CopyConstructible,
library writers are currently authorized to take the address of any
contained object by simply applying the & operator. Should the
CopyConstructible requirements be relaxed they would have to use
different artifacts. That said, I asked if there is (or there will be)
a consensus on a _portable_ way to do that.
PS: BTW, a strict interpretation of table 30 seems to say that you
can't overload operator & at all, since &t and &u are required to
*denote* the address of t and u. But I'm incline to think that as long
as it returns the same value as the built-in version, with the same
type, you can. Otherwise the requirements could have been stated much
more simply. Thoughts?
Genny.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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