----- Original Message -----
Sent: Sunday,
August 18, 2002 1:03 PM
Subject: Re: [boost]
request for comment on possible submission to library
I, for one, would like to
see it.
The topic kind of came up recently with respect to the "relational
operators" and dynamic_bitset.
I hope you will also add the operators +=, -=, *= and /=
since the C/C++ operators | (for union) and & (for intersection)
I would suggest that perhaps:
b += red;
should be legal, in addition to the more clumsy
b += ColorSet(red);
I think if (red == b) will cause more confusion than it's
worth. If we allow b += red;
then (b >= red) would mean the same thing as (b.contains(red)) and would
give the notational convenience we're searching for. If you are careful
with the helper functions, we would also be able to write if (red
<= b) which the cognoscenti would pronounce "if red in b".
We should both resist and _strongly_ discourage any suggestion that
"#define in <=" be used :)
in order to not get things confused with std::set, I recommend the word
powerset as the name of the class.
some further comments embedded in your example
At Saturday 2002/08/17 09:17, you wrote:
I was wondering if
anyone would be interested in a Set Library, similar to Pascal's set type for
ordinal types. I want to make sure someone would fine it useful (and not
trivial, done-to-death, etc.) before I take the time to "Boostify" it
and get a release from my employer.
A while ago I needed a set type that was compact,
fast, and avoided heap allocation. The number of elements was constant for a
particular set type, and the element type could be any integral type (primarily
enums). Similar types from STL are set, vector<bool>, and bitset,
but none quite suited the application. My set type (Set) stores elements at the
bit level, like vector<bool> and bitset, but it allows the creation of
compile-time set constants. Also, with a good optimizing
compiler, operations on constant sets will be done
at compile-time, though as of yet, there is no direct support for guaranteeing
this in the code. The class provides the usual set operations.
For efficiency, set operations are done in
parallel groups of the machine's word size. So, set types whose max. number of
elements is less than or equal to the machine's word size, will have most of
its operations done in constant time (usually one machine instruction per
operation). Sets whose max. number of elements is greater than the machine's
word size will be done in linear time, usually in E / Wb cycles, where E is the
max. number of elements and Wb is the machine's word size in bits. Though, with
a good compile (through inlining and loop unrolling), this linear time would
only be E / Wb instructions. On an average 32-bit processor, that's just
8 instructions to perform the union or intersection of a set of 256 elements.
Originally, I avoided loops and used metatemplates to inline the operations,
but this resulted in very long compile-times without any gains in performance!
And sometimes it resulted in worse performance. That will teach me not to try
and second guess today's optimizing compilers!
As I stated above, the class template may be
instantiated with any integral type, but it was primarily meant for
enumerations with will give you the added bonuses of type-safely and symbolic
names for the elements. The class assumes that the range given is not disjoint
(i.e., 0-63 and not 0 - 16, 19, 63). You may use disjoint ranges, but your sets
will contain wasted space. The only requirement is that the range of values is
greater than or equal to zero, and that the element type has a representation
for -1 (used as a (hidden) sentinel in argument lists of elements). You could
even conceivably use a class that was "castable" to int.
For example:
// element type
enum Color {red, orange, yellow, green, blue,
violet, NumColors,
NilColor = -1};
it certainly is a shame that we "need" to add NumColors to the enum.
BTW, are you suggesting here that NilColor is part of the enum?
// type for set of colors
typedef Set<Color, NumColors> ColorSet;
typedef Set<Color> ColorSet;
this would certainly be nicer, but.... C/C++ doesn't "know" how many
elements are in an enum or clearly won't let the programmer discover it.
// set constant
const ColorSet rgb = ColorSet::SetOf<red,
green, blue>::value(); // this is done at compile-time
void foo()
{
// construction
ColorSet s =
ColorSet::SetOf<orange, yellow, violet>::value(); // this is done at
compile-time
ColorSet s1 = ColorSet(a); //
this *may* be at compile-time
ColorSet s2 = ColorSet(orange,
yellow, violet); // this is NOT done at compile-time
I trust that ColorSet(orange) uses an explicit constructor.
// (but allows non-constant elements)
// operations, some of which
*may* be done at compile-time by a smart compiler
ColorSet a = s; // assignment
ColorSet b = s + rgb; // union
ColorSet bx(s);
bx += rgb;
assert (b ==
bx);
ColorSet c
= s * rgb; // intersection
ColorSet d = s - rgb; //
difference
ColorSet e = s / rgb; //
symmetric difference
bool b1 = s == rgb; // equality
bool b1 = s != rgb; //
unequality
bool b3 = s >= rgb; //
superset
bool b4 = s > rgb; // proper
superset
bool b5 = s < rgb; // subset
bool b6 = s > rgb; // proper
subset
if (b.contains(red)) // test
for set membership
std::cout << "b contains the element red" << std::endl;
if (red == b) // same as above.
can anyone suggest a better operator?
// maybe use red -> b and use NULL for false?
// or maybe remove and just use b.contains(red)?
baaaaaad idea.... see start of message
std::cout << "b contains the element red" << std::endl;
}
What do you think?
Thanks,
Rich
(spamjunk@stny.rr.com)
PGP RSA fingerprint = 4D20
EBF6 0101 B069 3817 8DBF C846 E47A
PGP D-H fingerprint = 98BC
65E3 1A19 43EC 3908 65B9 F755 E6F4 63BB 9D93
The five most dangerous
words in the English language:
"There oughta be a law"