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FFI: Clarify scalar boxing behavior.

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Prevent misunderstandings like in #1216
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Mike Pall 2024-05-25 15:48:07 +02:00
parent 9398123383
commit a6386bdabe
1 changed files with 13 additions and 0 deletions
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doc/ext_ffi_semantics.html
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@ -440,6 +440,19 @@ If you don't do this, the default Lua number &rarr; <tt>double</tt>
conversion rule applies. A vararg C&nbsp;function expecting an integer conversion rule applies. A vararg C&nbsp;function expecting an integer
will see a garbled or uninitialized value. will see a garbled or uninitialized value.
</p> </p>
<p>
Note: this is the only place where creating a boxed scalar number type is
actually useful. <b>Never use <tt>ffi.new("int")</tt>, <tt>ffi.new("float")</tt>
etc. anywhere else!</b>
</p>
<p style="font-size: 8pt;">
Ditto for <tt>ffi.cast()</tt>. Explicitly boxing scalars <b>does not</b>
improve performance or force <tt>int</tt> or <tt>float</tt> arithmetic! It
just adds costly boxing, unboxing and conversions steps. And it may lead
to surprise results, because
<a href="#cdata_arith">cdata arithmetic on scalar numbers</a>
is always performed on 64 bit integers.
</p>
<h2 id="init">Initializers</h2> <h2 id="init">Initializers</h2>
<p> <p>