vector and bool

[Ioannis Vranos]

Re: vector and bool

Nonenix (astalavista.ne t) wrote:[color=blue]
> hmm not sure but i think somethink lieke that can be done with structures i
> mena this:
>
>
> struct item {
>
> unsigned int IsTrue:1;
> unsigned int watever:1;
> unsigned int number:14;
>
> }
>
> some cpu dosen't support that or the compiler doesen't use it because of
> speed loss
>
> but if it work is should looks like this in the ram:
>
> 0 | 1 | 2.............1 5 |
> IsTrue | waterver | number |
>
>
> you could use it like an int but IMPORTANT
>
> if you forgot the unsignet the int will always be 0[/color]


Why?

[color=blue]
> gnu says this:
>
> ..cpp: In function 'int main()':
> ...cpp:13: warning: comparison is always 0 due to width of bitfield[/color]


May you provide that code?


Actually it could be even like this:


struct item
{
bool a:1;
bool b:1;
bool c:1;
bool d:1;
};



--
Ioannis Vranos

http://www23.brinkster.com/noicys

[Richard Herring]

Re: vector and bool

In message <1097606565.479 989@athnrd02>, Ioannis Vranos
<ivr@guesswh.at .grad.com> writes[color=blue]
>Richard Herring wrote:
>[color=green]
>> For a weak enough meaning of "should", I agree that that's the
>>intention. But I don't think anything in the Standard rules out a
>>naive implementation that makes no attempt to optimise, and simply
>>implements vector<bool> in terms of an underlying vector<some_int _type>.[/color]
>
>
>There is not an explicit prohibition on this, however explicit
>prohibitions are rare in the standard.[/color]

Just so. The standard specifies an interface and complexity (speed)
requirements, not the mechanism used to implement them. But it says
nothing prescriptive about the memory requirements for containers.[color=blue]
>
>Also the implementer has to implement this particular reference:
>
>
>// bit reference:
>class reference {
>friend class vector;
>reference();
>public:
>Ëœreference( );
>operator bool() const;
>reference& operator=(const bool x);
>reference& operator=(const reference& x);
>void flip(); // flips the bit
>};
>[/color]
Yes, and the fact that this is what operator[] is mandated to return is
what makes it *possible* for the implementor to provide access to data
packed into something smaller than an addressable unit.[color=blue]
>
>with the note:
>
>reference is a class that simulates the behavior of references of a
>single bit in vector<bool>.[/color]

Yes; note the word "simulates" .[color=blue]
>
>and this particular member function:
>
>void flip(); // flips all bits
>
>Also vector<bool> is mentioned as a separate case in the standard, and
>thus I think the legislator intended this to be implemented only with
>bits.[/color]

I'd say "intended it to be _possible_ to implement with bits".
[color=blue]
>:-)[/color]

--
Richard Herring

[Jeff Flinn]

Re: vector and bool


"daniel" <danielheiserer @netscape.net> wrote in message
news:b385c098.0 410121314.5b5c0 715@posting.goo gle.com...[color=blue]
> Well I found something:
> http://www.sgi.com/tech/stl/bit_vector.html states
> ----------------
> A bit_vector is essentially a vector<bool>: it is a Sequence that has
> the same interface as vector. The main difference is that bit_vector
> is optimized for space efficiency. A vector always requires at least
> one byte per element, but a bit_vector only requires one bit per
> element.
>
> Warning: The name bit_vector will be removed in a future release of
> the STL. The only reason that bit_vector is a separate class, instead
> of a template specialization of vector<bool>, is that this would
> require partial specialization of templates. On compilers that support
> partial specialization, bit_vector is a specialization of
> vector<bool>. The name bit_vector is a typedef. This typedef is not
> defined in the C++ standard, and is retained only for backward
> compatibility.
> ----------------
> The std::bitset seems not to be useful because I cannot change its
> size during runtime (no ->resize()).[/color]


Which is why I also suggested boost::dynamic_ bitset. See

http://www.boost.org/libs/dynamic_bitset/dynamic_bitset.html

Jeff F


[color=blue]
> So that should answer my first question.
>
> Thanks for the help, daniel
>
>
>
> "Jeff Flinn" <NONONE@nowhere .com> wrote in message[/color]
news:<ckgnot$q0 h$1@bluegill.ad i.com>...[color=blue][color=green]
> > "daniel" <danielheiserer @netscape.net> wrote in message
> > news:b385c098.0 410112357.408a0 c8e@posting.goo gle.com...[color=darkred]
> > > 1)
> > > is C++ smart enough to automatically use "bits" for bool or will
> > > a bool have the size of a charcter (byte).
> > > 2) The index of a vector is it an integer (4 byte) or a "long long"
> > > with 8 bytes or something else?
> > >
> > > I ask because I want to use
> > >
> > > vector<bool> with a few billion entries exceeding the int32 range for
> > > indexing and I want to use as few memory as possible for the whole
> > > vector<bool>[/color]
> >
> > Use either std::bitset or boost::dynamic_ bitset. Both "pack" the bits[/color][/color]
for[color=blue][color=green]
> > better memory usage. You'll most likely have to modify these classes to
> > account for indices > 32 bit.
> >
> > Jeff F[/color][/color]

[Ioannis Vranos]

Re: vector and bool

Richard Herring wrote:
[color=blue]
> I'd say "intended it to be _possible_ to implement with bits".[/color]


Well in the reference type definition



// bit reference:
class reference {
friend class vector;
reference();
public:
Ëœreference();
operator bool() const;
reference& operator=(const bool x);
reference& operator=(const reference& x);
void flip(); // flips the bit
};



the remark for flip() is "flips the bit". Which bit?



--
Ioannis Vranos

http://www23.brinkster.com/noicys

[Richard Herring]

Re: vector and bool

In message <1097678253.422 286@athnrd02>, Ioannis Vranos
<ivr@guesswh.at .grad.com> writes[color=blue]
>Richard Herring wrote:
>[color=green]
>> I'd say "intended it to be _possible_ to implement with bits".[/color]
>
>
>Well in the reference type definition
>
>
>// bit reference:
>class reference {
>friend class vector;
>reference();
>public:
>Ëœreference( );
>operator bool() const;
>reference& operator=(const bool x);
>reference& operator=(const reference& x);
>void flip(); // flips the bit
>};
>
>
>the remark for flip() is "flips the bit". Which bit?[/color]

The one "simulated" by the reference in the note you quoted earlier:
[color=blue]
>reference is a class that simulates the behavior of references of a
>single bit in vector<bool>.[/color]


--
Richard Herring