std::bitset in union

[felixnielsen@hotmail.com]

I would wery much like this to work:

@code start
#include <iostream>
#include <bitset>
const unsigned short size = 2; // 2^<unsigned int> //
union V {
char c[(size*size*size )/8];
std::bitset<siz e*size*size> b;
} v;
@code end

The compiler wont accept it and i cant accept that ;-)
A far as im concersed a bitset is just a number of bits and it shouldnt
be a problem, however it is and im in great need of a solution.

compiler error: "member std::bitset<8u> V::b' with constuctor not
allowed in union"

[Thomas Tutone]

Re: std::bitset in union

felixniel...@ho tmail.com wrote:[color=blue]
> I would wery much like this to work:
>
> @code start
> #include <iostream>
> #include <bitset>
> const unsigned short size = 2; // 2^<unsigned int> //
> union V {
> char c[(size*size*size )/8];
> std::bitset<siz e*size*size> b;
> } v;
> @code end
>
> The compiler wont accept it and i cant accept that ;-)
> A far as im concersed a bitset is just a number of bits and it shouldnt
> be a problem, however it is and im in great need of a solution.
> compiler error: "member std::bitset<8u> V::b' with constuctor not
> allowed in union"[/color]

Why don't you explain why you need a union. Unions are rarely needed
in C++, in that generally you can substitute some C++ idiom instead.
So, let us know, and I bet we can find you a solution.

Best regards,

Tom

[Bo Persson]

Re: std::bitset in union


<felixnielsen@h otmail.com> skrev i meddelandet
news:1140034170 .027456.12410@g 14g2000cwa.goog legroups.com...[color=blue]
>I would wery much like this to work:
>
> @code start
> #include <iostream>
> #include <bitset>
> const unsigned short size = 2; // 2^<unsigned int> //
> union V {
> char c[(size*size*size )/8];
> std::bitset<siz e*size*size> b;
> } v;
> @code end
>
> The compiler wont accept it and i cant accept that ;-)[/color]

Too bad. :-)
[color=blue]
> A far as im concersed a bitset is just a number of bits and it
> shouldnt
> be a problem, however it is and im in great need of a solution.[/color]

It's an object. The internal implementation is not specified.
[color=blue]
>
> compiler error: "member std::bitset<8u> V::b' with constuctor not
> allowed in union"[/color]

Exactly.

However,
if you keep the size within moderate ranges, there is a constructor
bitset(unsigned long) that sets the bits from an unsigned long. There
is also the opposite conversion bitset::to_ulon g(), that gets you as
many bits as will fit.

As the size of an unsigned long is not fixed (but at least 32 bits) it
might solve just some of your problems.


Bo Persson

[Marcus Kwok]

Re: std::bitset in union

felixnielsen@ho tmail.com wrote:[color=blue]
> I would wery much like this to work:
>
> @code start
> #include <iostream>
> #include <bitset>
> const unsigned short size = 2; // 2^<unsigned int> //
> union V {
> char c[(size*size*size )/8];
> std::bitset<siz e*size*size> b;
> } v;
> @code end
>
> The compiler wont accept it and i cant accept that ;-)
> A far as im concersed a bitset is just a number of bits and it shouldnt
> be a problem, however it is and im in great need of a solution.
>
> compiler error: "member std::bitset<8u> V::b' with constuctor not
> allowed in union"[/color]

I found this thread:

http://groups.google.com/group/comp.lang.c++/browse_thread/thread/6dc8ce6c17701ea6/2f2a43450544df81?q=union+constructor&rnum=2#2f2a43450544df81

In Ron Ruble's post, he quotes the standard (section 9.5):

"An object of a class with a non-trivial constructor (12.1), a
non-trivial destructor (12.4), or a non-trivial copy constructor
(13.5.3, 12.8) cannot be a member of a union"

--
Marcus Kwok

[JetSnaiL]

Re: std::bitset in union

> felixnielsen@ho tmail.comwrote:
I would wery much like this to work:[color=blue]
>
> @code start
> #include <iostream>
> #include <bitset>
> const unsigned short size = 2; // 2^<unsigned int> //
> union V {
> char c[(size*size*size )/8];
> std::bitset<siz e*size*size> b;
> } v;
> @code end
>
> The compiler wont accept it and i cant accept that ;-)
> A far as im concersed a bitset is just a number of bits and it[/color]
shouldnt[color=blue]
> be a problem, however it is and im in great need of a solution.
>
> compiler error: "member std::bitset<8u> V::b' with constuctor[/color]
not[color=blue]
> allowed in union"[/color]

Dear Sir, C++ union is a little bit too useless because it supports
only POD types. In your case you have std::bitset which is not a POD
type. There are many solutions, but the best I can recommend is to
use "boost::variant ". Variant template in boost is a safe, generic,
stack-based discriminated union container, offering a simple solution
for manipulating an object from a heterogeneous set of types in a
uniform manner. That's what you need. Read more at

http://www.boost.org/doc/html/variant.html

Wish best regards,
Vladislav Lazarenko.

[JetSnaiL]

Re: std::bitset in union

> felixnielsen@ho tmail.comwrote:
I would wery much like this to work:[color=blue]
>
> @code start
> #include <iostream>
> #include <bitset>
> const unsigned short size = 2; // 2^<unsigned int> //
> union V {
> char c[(size*size*size )/8];
> std::bitset<siz e*size*size> b;
> } v;
> @code end
>
> The compiler wont accept it and i cant accept that ;-)
> A far as im concersed a bitset is just a number of bits and it[/color]
shouldnt[color=blue]
> be a problem, however it is and im in great need of a solution.
>
> compiler error: "member std::bitset<8u> V::b' with constuctor[/color]
not[color=blue]
> allowed in union"[/color]

Dear Sir, C++ union is a little bit too useless because it supports
only POD types. In your case you have std::bitset which is not a POD
type. There are many solutions, but the best I can recommend is to
use "boost::variant ". Variant template in boost is a safe, generic,
stack-based discriminated union container, offering a simple solution
for manipulating an object from a heterogeneous set of types in a
uniform manner. That's what you need. Read more at

http://www.boost.org/doc/html/variant.html

Wish best regards,
Vladislav Lazarenko.

[felixnielsen@hotmail.com]

Re: std::bitset in union

I have done some thingking and i now realize that i dont actually need
this, however i still think it could be usefull when manipulating bit
in wery large numbers.

Otherwise im thankfull for your help.

Regards
Zacariaz

[Jacek Dziedzic]

Re: std::bitset in union

felixnielsen@ho tmail.com wrote:[color=blue]
> I have done some thingking and i now realize that i dont actually need
> this, however i still think it could be usefull when manipulating bit
> in wery large numbers.
>
> Otherwise im thankfull for your help.[/color]

As a side note -- if you had planned to use this union
to 'convert' the bitset to char[] or vice versa, you'd
better be warned that it's illegal to do that using a
union.

HTH,
- J.