constinit, here we come

Following up on b13421011d9377676e1adc282634991d5064a866 "better data
structures for some static const vars":

* Make the o3tl::sorted_vector ctor taking an initializer_list constexpr.
<https://wg21.link/P0202R3> "Add Constexpr Modifiers to Functions in
<algorithm> and <utility> Headers", which will be in C++2a and is already
implemented by recent libc++ and libstdc++ according to
<https://en.cppreference.com/w/cpp/compiler_support
#C.2B.2B2a_library_features>, makes std::sort constexpr but explicitly keeps
std::stable_sort non-constexpr.  ("Algorithms stable_partition, inplace_merge
and stable_sort allocate memory, construct variables using placement new, use
unique_ptr and do other things not acceptable in constexpr expressions.
Making those algorithms constexpr seems to be a hard task that would require
a lot of intrinsics.  Those algorithms are not marked with constexpr in this
wording.")  But keep o3tl::sorted_vector::Resort (which was introduced in
c59355e936446fe55960209e543b072acb6b2170 "fdo#58793: re-implement
SwpHintsArray::Resort()") using std::stable_sort instead of std::sort, in
case that is relevant for its pre-exisiting uses.

* <https://wg21.link/P1004R2> "Making std::vector constexpr", which was voted
into C++2a according to <https://wg21.link/n4829> "Editors' Report --
Programming Languages -- C++", will make the relevant parts of std::vector
constexpr.  But none of libc++, libstdc++, and MSVC implement that as of now.

* Introduce HAVE_CPP_CONSTINIT_SORTED_VECTOR to hide the constinit behind for
now for the one case from b13421011d9377676e1adc282634991d5064a866 "better
data structures for some static const vars" that can clearly be constinit
once constexpr std::vector is supported by compilers.  The other three cases
(s_aContainerDocumentCommands in
chart2/source/controller/main/CommandDispatchContainer.cxx,
aMetricCompatibleMap in vcl/source/font/PhysicalFontCollection.cxx, and
aBlacklist in writerfilter/source/dmapper/PropertyMap.cxx) would each need a
constexpr OUString first.  (Technically, the constinit would not even be
needed, but it nicely documents our intent that this will actually be
initialized at compile-time once compilers support that.)

Change-Id: Ibeb138f120528be3a7a09b3912143bf795fbab29
Reviewed-on: https://gerrit.libreoffice.org/79556
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/include/o3tl/sorted_vector.hxx b/include/o3tl/sorted_vector.hxx
index 527f34e568c7..102287d52a21 100644
--- a/include/o3tl/sorted_vector.hxx
+++ b/include/o3tl/sorted_vector.hxx
@@ -45,10 +45,10 @@ public:
     typedef typename std::vector<Value>::difference_type difference_type;
     typedef typename std::vector<Value>::size_type size_type;
 
-    sorted_vector( std::initializer_list<Value> init )
+    constexpr sorted_vector( std::initializer_list<Value> init )
         : m_vector(init)
     {
-        Resort();
+        std::sort(m_vector.begin(), m_vector.end(), Compare());
     }
     sorted_vector() = default;
     sorted_vector(sorted_vector const&) = default;