#define versus const

I have never really understood the difference between

1.) #define NUMBER 1.653

vs.

2.) const double NUMBER = 1.653

I know that the #define is taken care of by the pre-processor and the
compiler is used in the second case but why would one be chosen over the
other.

One book I read suggested that your should prefer the compiler to the
preprocessor, but why does it really matter?

Thanks in advance.

johny smith

"johny smith" <> wrote in
news::

> I have never really understood the difference between
>
> 1.) #define NUMBER 1.653
>
> vs.
>
> 2.) const double NUMBER = 1.653
>
> I know that the #define is taken care of by the pre-processor and the
> compiler is used in the second case but why would one be chosen over
the
> other.
>
> One book I read suggested that your should prefer the compiler to the
> preprocessor, but why does it really matter?
>
> Thanks in advance.
>
>
>

One reason is that you will typically get more meaningful diagnostics if
you misuse the symbol.

But a more important reason is that, unlike #define, a const symbol obeys
scoping rules, so it won't have the potential for nasty side-effects,
substituting itself where it doesn't belong.

Gregg

Gregg

"johny smith" <> schrieb im Newsbeitrag news:...
> I have never really understood the difference between
>
> 1.) #define NUMBER 1.653
>
> vs.
>
> 2.) const double NUMBER = 1.653
>
> I know that the #define is taken care of by the pre-processor and the
> compiler is used in the second case but why would one be chosen over the
> other.
>
> One book I read suggested that your should prefer the compiler to the
> preprocessor, but why does it really matter?

One reason, among many others:

const int NUMBER = -42;
int main()
{
int x = -NUMBER;
}

Now replace the const by a #define and let a novice try to understand the compiler's message.

Heinz

Heinz Ozwirk

johny smith wrote:
> I have never really understood the difference between
>
> 1.) #define NUMBER 1.653
>
> vs.
>
> 2.) const double NUMBER = 1.653
>
> I know that the #define is taken care of by the pre-processor and the
> compiler is used in the second case but why would one be chosen over
> the other.
>
> One book I read suggested that your should prefer the compiler to the
> preprocessor, but why does it really matter?

For one thing #define is not typesafe, while const is.

Another thing to keep in mind is that #define is text substitution, not a
variable.
That means that if you do this:
#define NUMBER 5+2;

int x = 3 * NUMBER;

The value of x is now 17, while with a const it would've been 21.

--
Unforgiven

Unforgiven

"Heinz Ozwirk" <> wrote in message
news:c99h3v$sph$01$
>
> One reason, among many others:
>
> const int NUMBER = -42;
> int main()
> {
> int x = -NUMBER;
> }
>
> Now replace the const by a #define and let a novice try to understand
> the compiler's message.
>
> Heinz

I assume that you expect that it will expand to

int x = --42;

and hence an error. Interestingly enough, neither VC++ 7.1 (which assigns 42
to x) nor Comeau online give errors.


--
John Carson
1. To reply to email address, remove donald
2. Don't reply to email address (post here instead)

John Carson

Unforgiven wrote:
<snip>
>
> #define NUMBER 5+2;
>
> int x = 3 * NUMBER;
>
> The value of x is now 17, while with a const it would've been 21.

And with a semicolon on the end of your #define, if you do this:
int x = NUMBER * 3;

You will get a compile error.

- Pete

Petec

> int x = --42;
>

What's wrong with this? It seems perfectly legal to me.
I would expect any compiler to perform double negation.

RM

Unforgiven posted:

> #define NUMBER 5+2;


If you have #define statements like that then enclose them in brackets.


#define NUMBER (5+2)

JKop

JKop wrote:
>
> Unforgiven posted:
>
> > #define NUMBER 5+2;
>
> If you have #define statements like that then enclose them in brackets.
>
> #define NUMBER (5+2)

You mean enclose them in parentheses.

<nitpick>

( is a parenthesis, plural parentheses

[ is a bracket

{ is a brace

< is an angle bracket

</nitpick>

Julie

"RM" <I_am_not_@_home.com> wrote in message
news:XM2uc.582962$Pk3.139545@pd7tw1no
>> int x = --42;
>>
>
> What's wrong with this? It seems perfectly legal to me.
> I would expect any compiler to perform double negation.

-- is interpreted as the prefix decrement operator, not as a double minus.
You cannot apply the decrement operator to a constant (it must be a
modifiable lvalue).

To get double negation, you need a space between the two minus signs.


--
John Carson
1. To reply to email address, remove donald
2. Don't reply to email address (post here instead)

John Carson