Add test of THistImpl using vector<atomic<INT>>.

hist/hist/v7/test/atomics.cxx

+#include "gtest/gtest.h"
+
+#include "ROOT/THist.hxx"
+
+#include <atomic>
+#include <vector>
+
+/** Basic tests for histograms of integral precision using vector<atomic> storage.
+   NOTE that floating point precision is not supported - it would need a hand-written
+   fetch_add implementation or the arrival of P0020 in C++. */
+
+using namespace ROOT::Experimental;
+
+// Storage using vector<atomic<>>
+template <class PRECISION>
+using atomicvec_t = std::vector<std::atomic<PRECISION>>;
+
+// A THistData using vector<atomic<>> as storage.
+template <int DIM, class PRECISION>
+using atomic_histdata_t = Detail::THistData<DIM, PRECISION, atomicvec_t, THistStatContent>;
+
+// THistImpl uses op+= to add to bin content; provide the relevant overload.
+template <class INTEGRAL, class = decltype(std::declval<std::atomic<INTEGRAL>>().fetch_add(INTEGRAL()))>
+std::atomic<INTEGRAL> &operator+=(std::atomic<INTEGRAL> &lhs, INTEGRAL rhs)
+{
+   lhs.fetch_add(rhs);
+   return lhs;
+}
+
+// Test creation of THistImpl with atomic precision.
+TEST(HistAtomicPrecisionTest, Create)
+{
+   Detail::THistImpl<atomic_histdata_t<1, int>, TAxisEquidistant> h1I(TAxisEquidistant{100, 0., 1});
+   Detail::THistImpl<atomic_histdata_t<2, char>, TAxisEquidistant, TAxisEquidistant> h2C(TAxisEquidistant{100, 0., 1},
+                                                                                         TAxisEquidistant{10, -1., 1});
+   Detail::THistImpl<atomic_histdata_t<1, long long>, TAxisIrregular> h1LLIrr(TAxisIrregular{{0., 0.1, 0.5, 1}});
+}
+
+// Test filling of THistImpl with atomic precision.
+TEST(HistAtomicPrecisionTest, Fill1Int)
+{
+   Detail::THistImpl<atomic_histdata_t<1, int>, TAxisEquidistant> h1I(TAxisEquidistant{100, 0., 1});
+   h1I.Fill({0.2222}, 12);
+   EXPECT_EQ(12, h1I.GetBinContent({0.2222}));
+}
+
+TEST(HistAtomicPrecisionTest, Fill2LongLong)
+{
+   Detail::THistImpl<atomic_histdata_t<2, long long>, TAxisEquidistant, TAxisEquidistant> hist(
+      TAxisEquidistant{100, 0., 1}, TAxisEquidistant{10, -1., 1});
+   hist.Fill({0.1111, -0.2222}, 42ll);
+   EXPECT_EQ(42ll, hist.GetBinContent({0.1111, -0.2222}));
+}
+
+TEST(HistAtomicPrecisionTest, Fill1CharIrr)
+{
+   Detail::THistImpl<atomic_histdata_t<1, char>, TAxisIrregular> hist(TAxisIrregular{{0., 0.1, 0.5, 1}});
+   hist.Fill({0.1111}, 17);
+   EXPECT_EQ(17, hist.GetBinContent({0.1111}));
+}