On Wed, 14 Jul 2010 06:01:10 -0400, Lars T. Kyllingstad
<pub...@kyllingen.nospamnet> wrote:
Is object.reserve() only useful for large arrays, or should I always use
it if I intend to append to an array?
Let's say I want to read some data, and I expect there to be ~100 bytes
of data. Is there any point in using reserve() first, or will there
always be that much memory available to an array?
byte[] buffer;
buffer.reserve(100);
foreach(byte b; dataSource) buffer ~= b;
Yes, you should always reserve if you know how much data is coming. If
you do not here is what happens:
Upon adding the first byte, a block of 16 bytes is allocated
Upon adding the 16th byte (there is one byte for padding), a *new* block
of 32 bytes is allocated, and the 15 previous bytes are copied to the
32-byte block. The original 16 byte block is left allocated because there
could be other aliases to the data.
Upon adding the 32nd byte, a block of 64 bytes is allocated, rinse and
repeat.
Upon adding the 64th byte, a block of 128 bytes is allocated, same deal.
Then the block will hold 100 bytes.
If you reserve 100 bytes, then a block of 128 bytes is allocated, and your
data all goes in there.
OT, I assume you realize that reading data in this way is horribly
inefficient :) You should read a block at once if possible.
-Steve
