Using enforce or assert to check for fullness when appending to fixed length array container

[Jonathan M Davis]

On Friday, October 4, 2019 4:00:08 AM MDT Per Nordlöw via Digitalmars-d-
learn wrote:
> I have a wrapper container FixedArray at
>
> https://github.com/nordlow/phobos-next/blob/25f4a4ee7347427cebd5cd375c9990
> b44108d2ef/src/fixed_array.d
>
> on top of a static array store that provides the member
>
>      void insertBack(Es...)(Es es) @trusted
>      if (Es.length <= capacity) // TODO use `isAssignable`
>      {
>          import std.exception : enforce;
>          enforce(_length + Es.length <= capacity, `Arguments don't
> fit in array`); // TODO use assert insteead?
>
>          foreach (immutable i, ref e; es)
>          {
>              moveEmplace(e, _store[_length + i]); // TODO remove
> `move` when compiler does it for us
>          }
>          _length = cast(Length)(_length + Es.length); // TODO
> better?
>      }
>
> Is the usage of `enforce` to check for out of bounds (fullness)
> idiomatic D or should an `assert()` be used instead?

It depends entirely on how you intend for it to be used. Is it a bug if
anyone attempts to append when there's no space, or is that normal program
behavior that should be handled and recovered from? Most typically, with D,
it would be considered a bug, and the caller should be sure that there's
room before attempting to append, but depending on your use case, using
exceptions may be more appropriate. However, if you don't have a really good
reason to treat it as a recoverable error condition rather than a bug, I'd
suggest that you treat it as a bug.

Also, you could treat it as a RangeError instead of an AssertError (the main
difference being that with assert, the checks go away when -release is
used). However, if your code would trigger a RangeError when accessing the
static array anyway, then you could just forgo all checks entirely and let
druntime throw a RangeError.

- Jonathan M Davis