From:	Patrick Peck <peck AT kfs DOT oeaw DOT ac DOT at>
Newsgroups:	comp.lang.c++,comp.os.msdos.djgpp
Subject:	Re: How to program a set of bits?
Date:	Thu, 16 Oct 1997 19:20:48 +0200
Organization:	Academy of Science, Austria
Lines:	41
Message-ID:	<34464CEF.BB0DB2D3@kfs.oeaw.ac.at>
References:	<34462916 DOT 7762 AT oce DOT nl>
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To:	djgpp AT delorie DOT com
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A. Jans-Beken wrote:

> In a program i'am writing I have a class like this:
>
> class PROPS {
>    unsigned long set_of_32_bits;
>    ...
>    }
>
> Now I want to have an operator '[]' so I can program:
>
> PROPS flags;
> flags[2] = FALSE;
> flags[4] = flags[31] = TRUE;
> flags[6] = flags[3];
>
> Please note that this is not the famous BITVECT example that can be
> found in many books about c++. I do not want to use a pointer to a bit
> set because the pointer takes up 4 bytes extra and i'am planning to use
> a huge array of PROPS (and related classes).
>
> please respond via e-mail
> thanx

Hi,

if you constrained yourself to a usage like
    bool b=flags[5];
you could easily define bool PROPS::operator[](int);

If you absolutely need to have flags[5] on the left side
of an assignment, things get more complicated;
take a look at the std-library class 'bitset', where they
use a helper class called 'reference' to do this;
while notationally convenient, it sure lacks performance;

Patrick

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