Re: doubt in basic c++ code

From:

Rolf Magnus <ramagnus@t-online.de>

Newsgroups:

comp.lang.c++

Date:

Wed, 14 Jun 2006 18:29:39 +0200

Message-ID:

<e6pdhj$49s$03$1@news.t-online.com>

jackdorse wrote:

Also to save execution time, because all calculations that are based on
constants can be done at compile time.

compiler will nt hv to allocate memory for such const variables.

That depends.

bt when we are dealing with the address of such const variables(as we
are in this case), it should allocate memory and thereby it should not
replace the occurences. (Theoretically).

It doesn't matter if the constant has a memory location or not. The
compiler can (and usually will) still choose to use the value directly
wherever it wants.

I hv tried to run the code on several compilers, bt same problem.

Also when u use const or u use #define, the VC6 compiler generates same
assembly code for both. which is very confusing.

Why?

In case of #define .. the compiler, simply replaces the occurences ...
tht is quite understood. But when it is const int i; there shud be some
memory location reserved, instead of merely replacing the occurences.
(Atleast when we are taking the address of it).

Yes, when you are taking the address of it, the data might actually be put
into a memory location, dedpending on what you use the pointer for and on
how you defined the constant.

(pls refer the assembly code generated, u ll get a better idea)

What I know from having done so is that the compiler tends to do many
optimizations I didn't expect and not do optimizations I would have
expected. It's not wise to depend on the optimzation behavior of the
compiler.

i also tried printing the value using *(&i) --> bt still the compiler
replaces it with the value. (compilers can get a bit oversmart
sometimes ... :) )

If you want a real memory access, try to define the constant as volatile.
However, I don't think that this kind of working around your own
limitations (making an object that you plan to modify const) is something
to be ever considered.