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digitalmars.D.learn - Proper way to protect (lock) a struct field after initialization ??

reply james.p.leblanc <james.p.leblanc gmail.com> writes:
Hello All.

Is there a standard way to protect a field of a struct after
the struct has been initialized?

Is this possible with a struct?

If not, I suppose a class (object) would be needed?  If so,
are there any simple pointers to an example of this?

Thanks in advance,

James
Aug 08 2021
next sibling parent reply jfondren <julian.fondren gmail.com> writes:
On Sunday, 8 August 2021 at 10:11:37 UTC, james.p.leblanc wrote:

 Hello All.

 Is there a standard way to protect a field of a struct after
 the struct has been initialized?

 Is this possible with a struct?

 If not, I suppose a class (object) would be needed?  If so,
 are there any simple pointers to an example of this?

 Thanks in advance,

 James


`private` works for structs the same as it does for classes.
https://dlang.org/spec/attribute.html#visibility_attributes

Perhaps you tried it, realized you could still access it within 
the same module, and concluded that it didn't work? Consider note 

accessed from within the same module." Import the struct into 
another module and test the visibility there and you'll get the 
behavior you're looking for.
Aug 08 2021
parent james.p.leblanc <james.p.leblanc gmail.com> writes:
On Sunday, 8 August 2021 at 10:19:46 UTC, jfondren wrote:

 On Sunday, 8 August 2021 at 10:11:37 UTC, james.p.leblanc wrote:

 Hello All.

 Is there a standard way to protect a field of a struct after
 the struct has been initialized?

 Is this possible with a struct?

 If not, I suppose a class (object) would be needed?  If so,
 are there any simple pointers to an example of this?

 Thanks in advance,

 James


 `private` works for structs the same as it does for classes.
 https://dlang.org/spec/attribute.html#visibility_attributes

 Perhaps you tried it, realized you could still access it within 
 the same module, and concluded that it didn't work? Consider 

 be accessed from within the same module." Import the struct 
 into another module and test the visibility there and you'll 
 get the behavior you're looking for.


Hej JFondren,

Wow, thanks for the quick response.  I had read that about the 
modules ...
but as my test example had failed, I thought that I had 
misunderstood
the larger picture.

Based on you kind reply, I went back over my example and found 
that I
had been deceiving myself.

With a quick fix-up edit, it indeed is working as your 
explanation.

Now, I proceed onto the trickier part of my endeavor ...

Thanks again, and Best Regards,
James

(Sorry for the noise...)
Aug 08 2021
prev sibling parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 8/8/21 3:11 AM, james.p.leblanc wrote:

 Hello All.
=20
 Is there a standard way to protect a field of a struct after
 the struct has been initialized?
=20
 Is this possible with a struct?
=20
 If not, I suppose a class (object) would be needed?=C2=A0 If so,
 are there any simple pointers to an example of this?
=20
 Thanks in advance,
=20
 James
=20


I understand your question differently from jfondren. You may be looking =

for a 'const' (or 'immutable') member:

struct S {
   const int i;

   this(int i) {
     // This will work because "first assignment is initialization"
     this.i =3D i;
   }
}

void main() {
   auto s =3D S(42);

   // This won't work
   s.i =3D 43;

   // This won't work either
   s =3D S(44);
}

Ali
Aug 08 2021
parent james.p.leblanc <james.p.leblanc gmail.com> writes:
On Sunday, 8 August 2021 at 10:40:51 UTC, Ali Çehreli wrote:


 I understand your question differently from jfondren. You may 
 be looking for a 'const' (or 'immutable') member:

 struct S {
   const int i;

   this(int i) {
     // This will work because "first assignment is 
 initialization"
     this.i = i;
   }
 }

 void main() {
   auto s = S(42);

   // This won't work
   s.i = 43;

   // This won't work either
   s = S(44);
 }

 Ali


Hello Again Ali,

Excellent!  I had tried (an erroneous) variant of this idea 
earlier ... but
also failed with my attempt.

I am appreciating very much the example you have provided.  I 
will try
this approach as well for the problem I am working on.  (Some 
details on
my path forward remain unclear ...)

Best Regards,
James
Aug 08 2021