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digitalmars.D - Re: What is the difference between...

reply "Janice Caron" <caron serenityfirefly.com> writes:
-----Original Message-----
From: digitalmars-d-bounces puremagic.com 
[mailto:digitalmars-d-bounces puremagic.com] On Behalf Of Daniel Keep
Sent: 07 September 2007 12:58
To: digitalmars-d puremagic.com
Subject: Re: What is the difference between...


 const(T) only has an effect on reference types.



O-Kaaaay.

Forgive me, but, that seems arbitrary and ... silly. If const(T) worked on all 
types, we'd have no need for const T, surely?

So, let's see if I've understood this right. Given

 struct S { int x; } /* a non-reference type */
 class C { int x; } /* a reference type */

 void f(const(S) s, const(C) c)
 {
     s.x = 5; /* allowed */
     c.x = 5; /* not allowed */
 }

Is that really, really right? It's going to take me a long time before that 
seems intuitive!
Sep 07 2007
parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
Janice Caron wrote:

 -----Original Message-----
 From: digitalmars-d-bounces puremagic.com
 [mailto:digitalmars-d-bounces puremagic.com] On Behalf Of Daniel Keep
 Sent: 07 September 2007 12:58
 To: digitalmars-d puremagic.com
 Subject: Re: What is the difference between...
 

 const(T) only has an effect on reference types.



 O-Kaaaay.
 
 Forgive me, but, that seems arbitrary and ... silly. If const(T) worked
 on all types, we'd have no need for const T, surely?
 
 So, let's see if I've understood this right. Given
 
 struct S { int x; } /* a non-reference type */
 class C { int x; } /* a reference type */
 
 void f(const(S) s, const(C) c)
 {
     s.x = 5; /* allowed */
     c.x = 5; /* not allowed */
 }
 
 Is that really, really right? It's going to take me a long time before
 that seems intuitive!



I get the feeling you haven't read the various articles on how const
works in D 2.0...

D 2.0 adds, basically, five kinds of const.

1. "final" (storage class) -- makes a variable's bits immutable after
the relevant constructor has finished (so this() for classes, static
this() for modules) or immediately after it's been assigned to.  Does
not produce a literal constant in ROM.

2. "const" (storage class) -- makes a variable's bits immutable at
compile time, and transitively adds const(T) to the stored type.

3. "invariant" (storage class) -- makes a variable's bits immutable at
compile time, and transitively adds invariant(T) to the stored type.

4. "const(T)" (type constructor) -- turns any reference type T into a
read-only view of the referenced data; has no effect on non-reference types.

5. "invariant(T)" (type constructor) -- turns any reference type T into
a *globally immutable* view of the referenced data.  This means that not
only can't you make changes, no one else can, either.  Has no effect on
non-reference types.

What you have to realise is that D 2.0's const system is *NOT* the same
as C's, C++'s, D 1.0's or anyone else's.  Yes, it's weird when you first
see it, but there are good reasons for it being the way it is.

Here's some links:

* http://www.digitalmars.com/d/const.html
* http://while-nan.blogspot.com/2007/06/you-cant-touch-this.html (note:
this was written before D 2.0 was released and doesn't contain the
storage class versions of "const" and "invariant", but should otherwise
be correct).

	-- Daniel
Sep 07 2007
parent Bruno Medeiros <brunodomedeiros+spam com.gmail> writes:
Daniel Keep wrote:

 
 
 4. "const(T)" (type constructor) -- turns any reference type T into a
 read-only view of the referenced data; has no effect on non-reference types.
 
 5. "invariant(T)" (type constructor) -- turns any reference type T into
 a *globally immutable* view of the referenced data.  This means that not
 only can't you make changes, no one else can, either.  Has no effect on
 non-reference types.
 



That's not quite the case, see my previous post.

-- 
Bruno Medeiros - MSc in CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D
Sep 07 2007