Re: anyway to set a const object after the fact?

2018-10-30 Thread Jonathan M Davis via Digitalmars-d-learn
On Tuesday, October 30, 2018 2:18:15 AM MDT Laurent Tréguier via 
Digitalmars-d-learn wrote:
> On Monday, 29 October 2018 at 21:50:32 UTC, aliak wrote:
> > Hi, so if you have this piece of code:
> >
> > struct C {
> >
> >   void f() {
> >
> > string[] others;
> > const string[] restArgs;
> > foreach (i, arg; args) {
> >
> >   if (isValidArg(arg)) {
> >
> > restArgs = args[i + 1 .. $];
> > break;
> >
> >   }
> >   others ~= arg;
> >
> > }
> > // "others" is a list of args before the valid arg is
> >
> > encountered
> >
> > // "restArgs" is a list that is the args after the valid arg
> >
> >   }
> >
> > }
> >
> > Is there anyway to set a const object after declaring it in the
> > above context?
> >
> > Cheers,
> > - Ali
>
> It looks like there is a Rebindable type for that in std.typecons
>
> : https://dlang.org/phobos/std_typecons.html#Rebindable

Rebindable is specifically intentended for class references, since D doesn't
really distinguish between the reference and what it's pointing to, making
it impossible to under normal circumstances to have a mutable reference to a
const object. On the other hand, you can easily have mutable arrays of const
objects. Using Rebindable is a really a hack (albeit a very necessary one
for some circumstances), and I would very much advise _against_ using it if
you don't need it. Historically, it tends to run into compiler bugs, because
it's trying to hack it's way around the type system, and if you're not
dealing with class references, there's probably a better way to handle the
problem.

- Jonathan M Davis






Re: anyway to set a const object after the fact?

2018-10-30 Thread Laurent Tréguier via Digitalmars-d-learn

On Tuesday, 30 October 2018 at 11:23:48 UTC, aliak wrote:
Guess I could do that. But would there be a difference if I 
just declared the restArgs as non const then? Given the 
objective is "set this var to point to this thing and not allow 
it to be set to point to anything else".


The difference with const is that you wouldn't be able to modify 
the array itself (by adding or removing arguments for example). 
But yes, you can still re-assign it multiple times using 
Rebindable, so the lambda solution is a better idea indeed.


Re: anyway to set a const object after the fact?

2018-10-30 Thread aliak via Digitalmars-d-learn

On Monday, 29 October 2018 at 22:05:16 UTC, H. S. Teoh wrote:


What exactly are you trying to accomplish?  I.e., what 
semantics do you want from modifying restArgs?


Trying to set restArgs to point to some data but only set it 
once. Would require some sort of control flow analysis on the 
part of D though I guess. So meh.




If you're looking to rebind the array, just be a bit more 
explicit in how you spell out the type:


const(string)[] restArgs;

will allow you to rebind it to a different array / slice, but 
still not permit you to modify the array elements.



T


Ya, I was looking to bind once. Like what you can do this with a 
module constructor:


const int a;
static this() {
  a = 5;
}




Re: anyway to set a const object after the fact?

2018-10-30 Thread aliak via Digitalmars-d-learn
On Tuesday, 30 October 2018 at 08:18:15 UTC, Laurent Tréguier 
wrote:

On Monday, 29 October 2018 at 21:50:32 UTC, aliak wrote:

Hi, so if you have this piece of code:

struct C {

  void f() {
string[] others;
const string[] restArgs;
foreach (i, arg; args) {
  if (isValidArg(arg)) {
restArgs = args[i + 1 .. $];
break;
  }
  others ~= arg;
}
// "others" is a list of args before the valid arg is 
encountered
// "restArgs" is a list that is the args after the valid 
arg

  }
}

Is there anyway to set a const object after declaring it in 
the above context?


Cheers,
- Ali


It looks like there is a Rebindable type for that in 
std.typecons : 
https://dlang.org/phobos/std_typecons.html#Rebindable


Guess I could do that. But would there be a difference if I just 
declared the restArgs as non const then? Given the objective is 
"set this var to point to this thing and not allow it to be set 
to point to anything else".


Re: anyway to set a const object after the fact?

2018-10-30 Thread aliak via Digitalmars-d-learn

On Monday, 29 October 2018 at 22:12:24 UTC, Paul Backus wrote:

Use a lambda:

const string[] restArgs = () {
  foreach(i, arg; args) {
if (isValidArg(arg)) {
  return args[i+1 .. $];
}
others ~= arg;
  }
}();


That works.


Re: anyway to set a const object after the fact?

2018-10-30 Thread Laurent Tréguier via Digitalmars-d-learn

On Monday, 29 October 2018 at 21:50:32 UTC, aliak wrote:

Hi, so if you have this piece of code:

struct C {

  void f() {
string[] others;
const string[] restArgs;
foreach (i, arg; args) {
  if (isValidArg(arg)) {
restArgs = args[i + 1 .. $];
break;
  }
  others ~= arg;
}
// "others" is a list of args before the valid arg is 
encountered

// "restArgs" is a list that is the args after the valid arg
  }
}

Is there anyway to set a const object after declaring it in the 
above context?


Cheers,
- Ali


It looks like there is a Rebindable type for that in std.typecons 
: https://dlang.org/phobos/std_typecons.html#Rebindable


Re: anyway to set a const object after the fact?

2018-10-29 Thread Paul Backus via Digitalmars-d-learn

On Monday, 29 October 2018 at 21:50:32 UTC, aliak wrote:

Hi, so if you have this piece of code:

struct C {

  void f() {
string[] others;
const string[] restArgs;
foreach (i, arg; args) {
  if (isValidArg(arg)) {
restArgs = args[i + 1 .. $];
break;
  }
  others ~= arg;
}
// "others" is a list of args before the valid arg is 
encountered

// "restArgs" is a list that is the args after the valid arg
  }
}

Is there anyway to set a const object after declaring it in the 
above context?


Cheers,
- Ali


Use a lambda:

const string[] restArgs = () {
  foreach(i, arg; args) {
if (isValidArg(arg)) {
  return args[i+1 .. $];
}
others ~= arg;
  }
}();


Re: anyway to set a const object after the fact?

2018-10-29 Thread H. S. Teoh via Digitalmars-d-learn
On Mon, Oct 29, 2018 at 09:50:32PM +, aliak via Digitalmars-d-learn wrote:
> Hi, so if you have this piece of code:
> 
> struct C {
> 
>   void f() {
> string[] others;
> const string[] restArgs;
> foreach (i, arg; args) {
>   if (isValidArg(arg)) {
> restArgs = args[i + 1 .. $];
> break;
>   }
>   others ~= arg;
> }
> // "others" is a list of args before the valid arg is encountered
> // "restArgs" is a list that is the args after the valid arg
>   }
> }
> 
> Is there anyway to set a const object after declaring it in the above
> context?
[...]

What exactly are you trying to accomplish?  I.e., what semantics do you
want from modifying restArgs?

If you're looking to rebind the array, just be a bit more explicit in
how you spell out the type:

const(string)[] restArgs;

will allow you to rebind it to a different array / slice, but still not
permit you to modify the array elements.


T

-- 
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