Tomas,
I am thinking of writing something for R-devel, and hope to have your
input first.
I get moderated on R-devel as I am now subscribed as
brian.rip...@r-project.org which of course I cannot send from. So I am
even more discouraged from posting there. (R-core is bad enough with
Luke discouraging all innovation except by him and Simon completely
misunderstanding the C23 status.)
Thanks,
Brian
----------------
There are several of these, and few guarantees for inter-working.
a) R's logical vectors, which include a value NA for its elements.
b) R's Rboolean type in C/C++
c) C++'s bool type
d) C23's bool type
e) C99's _Bool type to which bool is aliased if <stdbool.h> is included.
f) Fortran's LOGICAL type
a) is currently implemented as a C int (so 32-bit) type with NA as the C
value NA_LOGICAL which is the same a NA_INTEGER.
b) is currently implemented as a C enum with two values. I don't know
of any guarantees on how that is stored except in char or an integer
type -- however it seems common practice to use a 32-bit type (int or
unsigned int would not be distinguishable). (C23 §6.7.3.3) Enums can
have a specified data type, but we do not.
C23 states that bool has 1 value bit and some padding bits (§6.2.6.2) so
it can be stored in char-sized storage (i.e. bytes) or multiples
thereof. And that _Bool is a alternative name for bool.
f) is complier-dependent: for interoperability with C or R, code should
use c_bool from iso_c_binding (Fortran 2003). Fortran compilers store
LOGICAL in compiler-dependent ways, and for a long time we got away with
assuming that was equivalent to int (so LOGICAL values could be passed
to and from with int* on the C/R side). But sometime around GCC 8 they
changed to int_least32_t, which on common platforms is the same as int
but does not need to be.
It seems that in all cases coercion to an integer type coerces false
values to 0 and true values to 1 (and this is guaranteed by C23 at
least). And C23 guarantees that when coercing from an integer type to
bool zero values are coerced to false and non-zero ones to true (bool is
'an unsigned integer type'). However, that does not seem to be true for
C++ as UB sanitizers warn on coercing values other than 0/1.
I believe it to be the intention that c), d) and e) have the same
representation and interwork using the same compiler, but I could not
find that documented and see signs that e) might differ in C17 and C23
modes.
----------------
I need to look again at the C and C++ standards which with my vision I
need to do in very small chunks. Oh for the vision I once had!
--
Brian D. Ripley, rip...@stats.ox.ac.uk
Emeritus Professor of Applied Statistics, University of Oxford
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