Is it possible to append to a local buffer without reallocating?

[Simen Kjærås]

On Thursday, 11 January 2018 at 11:21:08 UTC, tipdbmp wrote:
> string push_stuff(char[] buf, int x) {
>     if (x == 1) {
>         buf ~= 'A';
>         buf ~= 'B';
>         buf ~= 'C';
>         return cast(string) buf[0 .. 3];
>     }
>     else {
>         buf ~= 'A';
>         buf ~= 'B';
>         return cast(string) buf[0 .. 2];
>     }
> }
>
> void foo() {
>     {
>         char[2] buf;
>         string result = push_stuff(buf, 1);
>         assert(buf.ptr != result.ptr);
>     }
>
>     {
>         char[2] buf;
>         string result = push_stuff(buf, 0);
>         assert(buf.ptr == result.ptr); // <-- this assert fails
>     }
> }

Have you checked what push_stuff actually returns with those 
inputs?

When you pass buf to push_stuff, it contains [255,255], and 
already has a length of 2. It's not possible to append to that 
without reallocating.

Now, that's not the whole problem. When you append to an array, 
the GC checks if there's any free space in the block that's been 
allocated. Since this is on the stack, the GC doesn't have a 
blockinfo for it, and even if it did, the layout would change so 
often that the overhead would be immense.

So, the answer is no, it's not possible.

--
   Simen