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docs: updated gotchas section

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Andrzej Krzemienski
2016-09-17 01:25:12 +02:00
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doc/1A_on_performance.qbk
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@@ -23,7 +23,7 @@ Given type `optional<int>`, and assuming that `sizeof(int) == 4`, we will get `s
[$images/opt_align1.png]
This means you can fit twice as many `int`s as `optional<int>`s into the same space of memory. Therefore, if the size of the objects is critical for your application (e.g., because you want to utilize your CPU cache in order to gain performance) and you have determined you are willing to trade the code clarity, it is recommended that you simply go with type `int` and use some 'magic value' to represent ['not-an-int].
This means you can fit twice as many `int`s as `optional<int>`s into the same space of memory. Therefore, if the size of the objects is critical for your application (e.g., because you want to utilize your CPU cache in order to gain performance) and you have determined you are willing to trade the code clarity, it is recommended that you simply go with type `int` and use some 'magic value' to represent ['not-an-int], or use something like [@https://github.com/akrzemi1/markable `markable`] library.
Even if you cannot spare any value of `int` to represent ['not-an-int] (e.g., because every value is useful, or you do want to signal ['not-an-int] explicitly), at least for `Trivial` types you should consider storing the value and the `bool` flag representing the ['null-state] separately. Consider the following class: