On Wednesday, 30 March 2022 at 12:46:07 UTC, Vijay Nayar wrote:
Consider the following code example:
```d
import std.stdio;
void main()
{
alias DelegateT = string delegate();
// An array of delegates, each has their own scope.
DelegateT[] funcs;
foreach (i; ["ham", "cheese"]) {
// Deliberately create a copy to keep in delegate scope.
string myStr = i.dup;
// The delegate will hopefully carry this copy around in
its own scope.
funcs ~= (() => myStr ~ " sandwich");
}
foreach (f; funcs) {
writeln(f());
}
}
```
The expected output is: "ham sandwich" and then "cheese
sandwich".
The actual output is: "cheese sandwich" and then "cheese
sandwich".
It seems that the variable `myStr` is in a sort of shared scope
for both functions in the array, and the last value written to
it dominates.
How do I create a delegate that acts like a
[closure](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures), that is, it carries with it the environment in which it was created?
You can also use `static foreach` and `alias`, although that will
restrict that code to compile time usage only :/
```d
import std;
import std.stdio;
void main()
{
alias DelegateT = string delegate();
// An array of delegates, each has their own scope.
DelegateT[] funcs;
static foreach (alias i; ["ham", "cheese"]) {
// Deliberately create a copy to keep in delegate scope.
//string myStr = i.dup;
// The delegate will hopefully carry this copy around in its
own scope.
funcs ~= (() => /*myStr*/ i ~ " sandwich");
}
foreach (f; funcs) {
writeln(f());
}
}
```
