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Why is `Appender._data` a pointer to its `Data`-store?
Oct 16, 2020
Per Nordlöw
Oct 17, 2020
Paul Backus
October 16, 2020
Gravatar of Per NordlöwPosted by Per NordlöwReply
Per Nordlöw
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Why is `Appender`'s store `Data` put directly as

    `private Data* _data;`

instead of

    `private Data _data;`

?

Removing the pointer indirection would give better locality.

If it's about optimizing for empty `Appender`s then a `Appender*` should be used in those cases instead.
October 16, 2020
Gravatar of Steven SchveighofferPosted by Steven Schveighoffer
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Steven Schveighoffer
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On 10/16/20 5:40 PM, Per Nordlöw wrote:
> Why is `Appender`'s store `Data` put directly as
> 
>      `private Data* _data;`
> 
> instead of
> 
>      `private Data _data;`
> 
> ?
> 
> Removing the pointer indirection would give better locality.
> 
> If it's about optimizing for empty `Appender`s then a `Appender*` should be used in those cases instead.

Appender is ref counted IIRC.

-Steve
October 17, 2020
Gravatar of Paul BackusPosted by Paul Backus
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Paul Backus
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On Saturday, 17 October 2020 at 00:06:31 UTC, Steven Schveighoffer wrote:
>
> Appender is ref counted IIRC.
>
> -Steve

It's not; it uses the GC.
October 17, 2020
Gravatar of Steven SchveighofferPosted by Steven Schveighoffer
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Steven Schveighoffer
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On 10/17/20 12:00 AM, Paul Backus wrote:
> On Saturday, 17 October 2020 at 00:06:31 UTC, Steven Schveighoffer wrote:
>>
>> Appender is ref counted IIRC.
>>
> 
> It's not; it uses the GC.

Oh yeah. In fact, it was me who did that (in 2010!).

My point should have been that the appender is a pImpl to avoid memory corruption. If you have multiple copies of an appender, and each has its own idea of what the capacity is, then you will get corruption.

See the original bug report here: https://issues.dlang.org/show_bug.cgi?id=4681

-Steve