Re: A Better Choice?

From:

Jorgen Grahn <grahn+nntp@snipabacken.se>

Newsgroups:

comp.lang.c++

Date:

3 Oct 2013 22:36:31 GMT

Message-ID:

<slrnl4rsbe.2m9.grahn+nntp@frailea.sa.invalid>

On Wed, 2013-10-02, Victor Bazarov wrote:

On 10/2/2013 5:38 PM, Jorgen Grahn wrote:

On Wed, 2013-10-02, Victor Bazarov wrote:

On 10/2/2013 2:48 PM, Gert-Jan de Vos wrote:

On Sunday, 29 September 2013 00:15:35 UTC+2, Mike Copeland wrote:

I have the following data declaration that is causing compiler
warnings:

char DBI[60001] = {'\0'} ;

The purpose of this data is to store a character value ('a'..'I') for
each corresponding record that has been read from a database. The
records are identified by a value that ranges from 1-60000. I can't use
a bitset here, as I need to store the character value associated with
each record. I don't want to be limited by the current supported range
(1...60000).

For mapping a number to a character with any number of entries, this would
be my first choice:

std::map<int, char> dbi;

I wonder what the difference with std::vector<char> would be if *all*
sixty thousand and one numbers in the range need a character. I mean,
the total size of a vector is 60001+sizeof(std::vector<char>) plus the
overhead of allocating the block, which is ~16 bytes. For a map it
would be 60000 * (sizeof(__node_type) + overhead), yes? Whatever
__node_type is, that is. And given that it usually needs to keep at
least three pointers, the value, and [as in VC++] a couple of chars,
that's like 27 bytes... So, is that, like, 40 times the memory? I
don't think it's worth considering unless the array is *really* sparse.

Yes, using a std::map would be a waste of memory. On the other hand:
(a) it would not be noticeable on a typical system!

I am guessing it depends very much on the definition of "noticeable" and
on the definition of "typical".

Well, I assume he's not working on a small embedded system, and not
using special debugging tools to measure memory usage.

(b) using std::map (or unordered_map) ought to make for more readable
code, if this is indeed a value->value mapping problem

Really? More readable, how? Instead of writing

mytable[recordnum] = thischar;

you will write

mytable[recordnum] = thischar;

?

He didn't say, but I suspect he will do more things to his map than
write to it. Perhaps one day he will want more than 60,001 entries,
too.

If you want a mapping, use a mapping container. Makes sense to me,
and I'm surprised you seem to argue against it. Or is it some kind
of Devil's Advocate thing?

/Jorgen

--
// Jorgen Grahn <grahn@ Oo o. . .
\X/ snipabacken.se> O o .