Reserve on dynamic arrays

[Jonathan M Davis via Digitalmars-d-learn]

On Thursday, July 14, 2016 07:20:46 cym13 via Digitalmars-d-learn wrote:
> As one can see there seem to be absolutely no difference in
> allocations wether we reserve or not. I've ran more experiences,
> reserving way less or more than I appened to no avail:
> allocations seem to just ignore it.
>
> Isn't the point of .reserve to perform preallocation?

Whatever the profiler is telling you, it's clearly not actually telling you
whether an allocation took place, and that's easy to test. For instance,
let's change your example to

void main()
{
     import std.stdio;

     int[] arr;
     arr.reserve(50);
     writefln("capacity: %s", arr.capacity);

     immutable limit = cast(int)(arr.capacity + 2);
     foreach (i ; 0 .. limit)
     {
         if(append(arr, i))
             writefln("reallocation occurred: %s", i);
     }
}

bool append(ref int[] arr, int value)
{
    auto before = arr.ptr;
    arr ~= value;
    return arr.ptr !is before;
}

When I run it on my machine, I get

capacity: 63
reallocation occurred: 63

But if I change the code to

void main()
{
     import std.stdio;

     int[] arr;
     writefln("capacity: %s", arr.capacity);

     foreach (i ; 0 .. 63)
     {
         if(append(arr, i))
             writefln("reallocation occurred: %s", i);
     }
}

bool append(ref int[] arr, int value)
{
    auto before = arr.ptr;
    arr ~= value;
    return arr.ptr !is before;
}

so that it doesn't call reserve, and it just goes to 50 (since capacity is 0
to begin with), then it prints

capacity: 0
reallocation occurred: 0
reallocation occurred: 3
reallocation occurred: 7
reallocation occurred: 15
reallocation occurred: 31

So, reserve is definitely doing its job. My guess is that a GC function
related to allocation occurs, which is why the GC profiler decides that
allocation has occurred. I don't know. But if you look at the array itself,
reserve is clearly doing its job. So, the profiler may need a bug report (I
don't know much of anything about how the GC profiler works, so I don't know
if this behavior is expected or not, though it certainly seems wrong), but
from what I can see, reserve is not buggy.

- Jonathan M Davis