On Wednesday, 21 June 2023 at 15:48:56 UTC, H. S. Teoh wrote:
On Wed, Jun 21, 2023 at 02:09:26AM +0000, Cecil Ward via
Digitalmars-d-learn wrote:
First is an easy one:
1.) I have a large array and a sub-slice which I want to set
up to be pointing into a sub-range of it. What do I write if I
know the start and end indices ? Concerned about an off-by-one
error, I have start_index and past_end_index (exclusive).
array[start_idx .. one_past_end_idx]
2.) I have a dynamic array and I wish to preinitialise its
alloc cell to be a certain large size so that I don’t need to
reallocate often initially. I tell myself that I can set the
.length property. Is that true?
You can use `array.reserve(preinitSize);`.
2a.) And what happens when the cell is extended, is the
remainder zero-filled or remaining full of garbage, or is the
size of the alloc cell something separate from the dynamic
array’s knowledge of the number of valid elements in it ?
The size of the allocated cell is managed by druntime. On the
user code side, all you know is the slice (start pointer +
length). The allocated region outside the current array length
is not initialized. Assigning to array.length initializes the
area that the array occupies after the length has been extended.
T
Is .reserve()’s argument scaled by the entry size after it is
supplied, that is it is quoted in elements or is it in bytes? I’m
not sure whether the runtime has a knowledge of the element type
so maybe it doesn’t know anything about scale factors, not sure.
