object.reserve() and array size
Lars T. Kyllingstad
public at kyllingen.NOSPAMnet
Wed Jul 14 08:55:00 PDT 2010
On Wed, 14 Jul 2010 10:35:19 -0400, Steven Schveighoffer wrote:
> On Wed, 14 Jul 2010 06:01:10 -0400, Lars T. Kyllingstad
> <public at 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.
Yeah, that was just an artificial example. :) The actual use case was
building a string from substrings.
Thanks!
-Lars
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