Re: [Dev-C++] 'extern const' and std::vector

[David M. <ci...@ya...>]

I see.

So const extern is out of the question then. No problem
just need to
hope these functions don't try to write to the varible.

Actually 2^32 is the upper limit using the current world
size we have not
passed 10000 entries. We have some serious expansion in
mind thus I am
planning for some of the heaviest loads. The current
implementation uses a
doubly-linked list (links point both to the next entry and
the previous
entry).
As you gan guess this can be problematic when the number of
entries begins
to really start growing. Thats why I am looking for
alternatives.


----- Original Message ----- 
From: "Per Westermark" <pw...@ia...>
To: "David McKen" <ci...@ya...>
Cc: <dev...@li...>
Sent: Thursday, May 01, 2003 10:23 AM
Subject: Re: [Dev-C++] 'extern const' and std::vector


I'm not too good at the latest updates of the standards,
but to the best
of my knowledge, there is no const extern declarations.

Extern is just a keyword to inform the compiler that
somewhere in another
linking unit, the specified variable or function exists.
However, you can
not just lie, because when you reference the symbol, the
compiler will
insert a name-mangled reference and the linker will then
fail if an
exactly identical reference isn't available somewhere in
one of the
included object or library files.

Because c++ uses name-mangling, you can't just change the
declaration of a
variable in your extern statement. There might be C++
compilers that
doesn't change the name-mangling for a const contra
non-const variable. In
that case, this scheme would work for that compiler.
However, I'm pretty
sure that you would have to search a lot for a compiler
with such a
name-mangling.

If, on the other hand, you declare the variable as "C", you
can do just
about anything, i.e. you might declare the variable as char
in one source
file and as unsigned long in another. Then it's up to you
to figure out if
the program will survive, or if it will go up in a big
explosion when you
try and run it.

About 2^32 elements in a vector - I hope you are running a
64-bit
operating system. Even a vector of 2^32 characters would
fill the full
address space of a 32-bit processor. Ever wondered about
the big need of
64-bit processors and operating systems for large database
applications?

If I assume that you don't need 2^32 elements, just an
address range of
2^32, I would suggest a slower (log n) solution using some
form of btree
or similar. Then you might traverse down the tree until you
find the node
containing the range, or no match is found. If you don't
have to support
ranges, but just want to either find an element, or know
that it isn't
there, go for a hash solution instead. It will perform
insert and lookup
operations in constant time.

/Per W

On Thu, 1 May 2003, David McKen wrote:

> My first question concerns the extern keyword. If I have
a varible in one
file (it is not a constant) but that varible needs to
accessed from the
functions in another file. However these other functions do
not need
(actually I don't want them to have) write access to this
variable, can I do
something similar to 'const <var_type> <var_name>?'. For
those of you who
are wondering why I don;t just try it myself I would like
to know what EVERY
ANSI compatible compiler will do. The program I am writing
must remain
portable.  My other question concerns the std::vector
container class of the
STD. As far as I have seen the std:vector class stores it's
contents
contiguously in memory (just like an array). Does this make
vector classes
reach their maximum size once there is no block of memory
big enough to hold
it's data? I plan to use this class to act as the index for
world system
with up to 2^32 rooms in it (even if I store only pointers
to each structure
it still gets pretty big). Would any of the other
containers be better
suited to this purpose? I chose the vector bacause of it's
speed (4 billion
possible rooms, gonna need the speed). The requirements for
the container is
to allow for a numbering system for eah of the rooms with
rooms being
grouped within ranges.
>
> ---------------------------------
> Do you Yahoo!?
> The New Yahoo! Search - Faster. Easier. Bingo.


__________________________________
Do you Yahoo!?
The New Yahoo! Search - Faster. Easier. Bingo.
http://search.yahoo.com