[Development] What's Q_PRIMITIVE_TYPE for?

[Giuseppe D'Angelo]

Il 12/11/20 15:13, Lars Knoll ha scritto:
> 
> Is_trivial() explicitly states that you can memcpy/memmove objects and I 
> really want to keep this. With your change, those will by default fall 
> back to the complex case, slowing down operations on that list.

The right detection for copies and copy assignments isn't is_trivial, 
but is_trivially_copyiable. At the beginning of the thread, I was indeed 
confused by this; we are conflating multiple independent semantics into 
the "primitive" / "isComplex" case.


Should they be split in *independent* semantics? At least 1/2 are different:


1) Can be value constructed with memset(0)
   * Primitive: yes by definition
   * Trivial(ly constructible): not in general, see the PDM example
   * Relocatable: not in general

2) Can be copy/move constructed/assigned with memcpy
   * Primitive: yes by definition
   * Trivial(ly copy constructible): yes
   * Relocatable: not in general

3) Can be relocated with memcpy
   * Primitive: yes by definition
   * Trivial(ly copy constructible, destructible): yes
   * Relocatable: yes by definition

4) Can be destroyed by just freeing memory
   * Primitive: yes by definition
   * Trivial(ly destructible): yes
   * Relocatable: not in general


(Note: I'm completely ignoring the lifetime issues at just blessing a 
block of memory and saying "there are N objects here". Thiago is right, 
we shouldn't ignore that. But I'm not sure how, except by biting the 
bullet and calling constructors/destructors, then relying on the 
compiler to do the right thing.)

--

Language Lawyer Hat: is_trivial is the wrong type trait when it comes to 
detect trivial copiability anyhow, example:

   struct S { int i; S operator=(const S &)=delete; };

is trivial and not copy assignable. Isn't C++ a fun language to work 
with... :P

-- 
Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - The Qt, C++ and OpenGL Experts

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