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Closures over temporary variables
Jun 14, 2022
Anonymouse
Jun 14, 2022
bauss
Jun 14, 2022
Ali Çehreli
June 14, 2022
Gravatar of AnonymousePosted by AnonymouseReply
Anonymouse
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What is the correct way of making this output 0 1 2?

void delegate()[] dgs;

foreach (immutable i; 0..3)
{
    dgs ~= () => writeln(i);
}

foreach (dg; dgs)
{
    dg();  // outputs: `2 2 2`
}
June 14, 2022
Gravatar of baussPosted by bauss
in reply to Anonymouse		Reply
bauss
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On Tuesday, 14 June 2022 at 08:26:53 UTC, Anonymouse wrote:

>

What is the correct way of making this output 0 1 2?

void delegate()[] dgs;

foreach (immutable i; 0..3)
{
    dgs ~= () => writeln(i);
}

foreach (dg; dgs)
{
    dg();  // outputs: `2 2 2`
}

You have to do it like this:

dgs ~= ( (n) => () { writeln(n); })(i);

Because D hasn't fixed their million dollar mistake after so many years:

https://issues.dlang.org/show_bug.cgi?id=2043

https://issues.dlang.org/show_bug.cgi?id=23136

June 14, 2022
Gravatar of Ali ÇehreliPosted by Ali Çehreli
in reply to bauss		Reply
Ali Çehreli
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On 6/14/22 02:04, bauss wrote:

> You have to do it like this:
>
> ```
> dgs ~= ( (n) => () { writeln(n); })(i);
> ```

The same thing with a named function as well as with iota():

import std.range;
import std.algorithm;
import std.stdio;

void main() {
  void delegate()[] dgs;

  auto makeDg(int i) {
    return () => writeln(i);
  }

  foreach (immutable i; 0 .. 3)
  {
    dgs ~= makeDg(i);
  }

  iota(3).each!(i => dgs ~= () => writeln(i));

  foreach (dg; dgs)
  {
    dg();
  }
}

Ali