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digitalmars.D.learn - Read Once then reset/init value?

reply =?iso-8859-1?Q?Robert_M._M=FCnch?= <robert.muench saphirion.com> writes:
I quite often have the pattern where a value should be read just once 
and after this reset itself. The idea is to avoid that others read the 
value by accident and get an older state, instead they get an 
"invalid/reset" value.

Is there a library function that can mimic such a behaviour?

-- 
Robert M. Münch
http://www.saphirion.com
smarter | better | faster
Oct 29 2019
parent reply Simen =?UTF-8?B?S2rDpnLDpXM=?= <simen.kjaras gmail.com> writes:
On Tuesday, 29 October 2019 at 22:24:20 UTC, Robert M. Münch 
wrote:

 I quite often have the pattern where a value should be read 
 just once and after this reset itself. The idea is to avoid 
 that others read the value by accident and get an older state, 
 instead they get an "invalid/reset" value.

 Is there a library function that can mimic such a behaviour?


Something like this?

T readOnce(T)(ref T value) {
     auto tmp = value;
     value = T.init;
     return tmp;
} unittest {
     int i = 3;
     assert(i.readOnce == 3);
     assert(i == 0);
}

If so, no, there is no library function for it, but feel free to 
use the above. You may very well have to change T.init to 
something more fitting for your use case, of course.

If this is not what you need, feel free to explain further, as 
I'm not sure I understood you correctly. :)

--
   Simen
Oct 29 2019
next sibling parent reply Jacob Carlborg <doob me.com> writes:
On 2019-10-30 00:28, Simen Kjærås wrote:


 Something like this?
 
 T readOnce(T)(ref T value) {
      auto tmp = value;
      value = T.init;
      return tmp;
 } unittest {
      int i = 3;
      assert(i.readOnce == 3);
      assert(i == 0);
 }


Perhaps better to encapsulate it in a struct to avoid someone accessing 
the value directly.

-- 
/Jacob Carlborg
Oct 30 2019
parent Simen =?UTF-8?B?S2rDpnLDpXM=?= <simen.kjaras gmail.com> writes:
On Wednesday, 30 October 2019 at 11:53:42 UTC, Jacob Carlborg 
wrote:

 On 2019-10-30 00:28, Simen Kjærås wrote:


 Something like this?
 
 T readOnce(T)(ref T value) {
      auto tmp = value;
      value = T.init;
      return tmp;
 } unittest {
      int i = 3;
      assert(i.readOnce == 3);
      assert(i == 0);
 }


 Perhaps better to encapsulate it in a struct to avoid someone 
 accessing the value directly.


Quite possibly, but the post was somewhat low on details, and 
encapsulating it like that does put certain limits on how it can 
be used, so it's not necessarily the best idea.

FWIW, here's one possible way to do it with a struct:

struct Readonce(T, T defaultValue = T.init) {
     private T value;

     alias get this;

     T get() {
         auto tmp = value;
         value = defaultValue;
         return tmp;
     }

     void get(T newValue) {
         value = newValue;
     }

     this(T newValue) {
         value = newValue;
     }
}

unittest {
     Readonce!(int, -1) a = 3;
     assert(a == 3);
     assert(a == -1);

     a = 3;
     assert(a == 3);
     assert(a == -1);
}

--
   Simen
Oct 30 2019
prev sibling parent =?iso-8859-1?Q?Robert_M._M=FCnch?= <robert.muench saphirion.com> writes:
On 2019-10-29 23:28:35 +0000, Simen Kjærås said:


 On Tuesday, 29 October 2019 at 22:24:20 UTC, Robert M. Münch wrote:

 I quite often have the pattern where a value should be read just once 
 and after this reset itself. The idea is to avoid that others read the 
 value by accident and get an older state, instead they get an 
 "invalid/reset" value.
 
 Is there a library function that can mimic such a behaviour?


 
 Something like this?
 
 T readOnce(T)(ref T value) {
      auto tmp = value;
      value = T.init;
      return tmp;
 } unittest {
      int i = 3;
      assert(i.readOnce == 3);
      assert(i == 0);
 }
 
 If so, no, there is no library function for it, but feel free to use 
 the above. You may very well have to change T.init to something more 
 fitting for your use case, of course.


Hi, that looks very good. I forgot about the UFCS possibility. That's 
very good because it works on basic types too. Thanks :-)

-- 
Robert M. Münch
http://www.saphirion.com
smarter | better | faster
Nov 03 2019