On quarta-feira, 28 de março de 2012 15.37.46, Marc Mutz wrote: > So I'd like to propose to extend the scope of Q_PRIMITIVE_TYPE to cover such > non-POD classes that are nevertheless close enough to PODs. > > That means: > 1a. memset(0, &t, sizeof(T)) constructs a valid object, and that object is > equal to T(), if T has a default constructor.
That crashes, but I'm sure you meant well :-)
> -alternatively-
> 1b Any bit pattern represents a valid object (one that the dtor can be
> called on).
> 2. memcpy() creates a valid independent copy of the object
> 3. The dtor doesn't need to be run.
>
> With 1a+2+3, primitiveness is a strict superset of PODness.
>
> If Jędrzej is right, though, and the intent was for primitive types to not
> require initialisation, ie. the 1b alternative is used, PODness is no longer
> sufficient for primitiveness. And cups_option_t must be downgraded to
> movable.
>
> Looking at existing usage, I think 1a+2+3 comes closest to what everyone
> thinks primitive types mean, so I'd like to update the docs accordingly.
>
> Opinions?
Note that #2 is the difference between those simple types and the regular,
movable types. QString is movable, but a memcpy does not create a new
independent object. With that, the traits that I think are interesting to us
are:
- trivial default constructor ( = can be initialised by memset)
- trivially copyable ( = memcpy creates independent object)
[note: implies the next two]
- movable ( = memcpy can be performed if the old object is discarded) - not a
C++11 definition
- trivial destructor ( = no destructor need be run)
We currently only care about two combinations:
- Q_PRIMITIVE_TYPE = all of them
- Q_MOVABLE_TYPE = just the trivially-movable trait
Note that when all four traits are grouped together, C++11 defines them as
"trivial type".
The only thing I ask is that we keep Q_PRIMITIVE_TYPE for *real* primitive
types and create a new macro, say Q_TRIVIAL_TYPE, for everything that matches
the constraints but is not an actual primitive (fundamental). I don't know if
we'll ever need the difference, but I'd rather we had it proper if it doesn't
cost us much.
The closest definition that C++11 has for our Q_PRIMITIVE_TYPE is either
"scalar type" (arithmetic, enums, pointers) or "fundamental type" (arithmetic,
void).
--
Thiago Macieira - thiago.macieira (AT) intel.com
Software Architect - Intel Open Source Technology Center
Intel Sweden AB - Registration Number: 556189-6027
Knarrarnäsgatan 15, 164 40 Kista, Stockholm, Sweden
signature.asc
Description: This is a digitally signed message part.
_______________________________________________ Development mailing list [email protected] http://lists.qt-project.org/mailman/listinfo/development
