On Monday, 6 May 2024 at 06:29:49 UTC, Liam McGillivray wrote:
Delegates can be a pain, as they often have results different
from what one would intuitively expect. This can easily result
in bugs.
Here's a line that caused a bug that took me awhile to find:
```
foreach(card; unitCards) card.submitted = delegate() =>
selectUnit(card.unit);
```
Each `UnitInfoCard` object (which `card` is a member of)
contains a `Unit` object called `unit`. The intention of this
line was that each object in `unitCards` would call
`selectUnit` with it's own `unit` every time it calls
`submitted`. Instead, every card calls `submitted` with the
*last* value of `card`.
Yes, this is because the foreach loop reuses the same memory slot
for `card`.
Even though this is allocated as a closure, it still only
allocates the frame stack of the *enclosing function*, and does
not allocate a new slot for each loop iteration.
You can force this by using a lambda which allocates the closure:
```d
foreach(card; unitCards)
card.submitted = (c2) { return () => selectUnit(c2.unit);
}(card);
```
This is a lambda which accepts `card` as a parameter, and returns
an appropriate delegate. It is important to use a parameter,
because if you just use card inside there, it's still using the
single stack frame of the calling function!
I renamed the inner parameter `c2` to avoid confusion, but you
could name it `card` also. Essentially, the stack frame of the
inner function is now allocated a closure, and it has it's own
reference to `card` as a parameter.
This is a very old issue:
https://issues.dlang.org/show_bug.cgi?id=2043 since "moved" to
https://issues.dlang.org/show_bug.cgi?id=23136
I would love to see a solution, but the workaround at least
exists!
-Steve
