[ntuple] add fpclass check for double --> float conversion

Author: jblomer

tree/ntuple/src/RColumnElement.hxx

@@ -162,6 +162,18 @@ inline void EnsureValidRange(SourceT val [[maybe_unused]])
    }
 }
 
+/// Currently ensures that the floating point class doesn't change in double --> float conversion
+template <typename DestT, typename SourceT>
+inline void EnsureValidConversion(DestT dst [[maybe_unused]], SourceT src [[maybe_unused]])
+{
+   if constexpr (std::is_same_v<DestT, float> && std::is_same_v<SourceT, double>) {
+      if (std::fpclassify(src) != std::fpclassify(dst)) {
+         throw ROOT::RException(R__FAIL(std::string("floating point class mismatch: ") + std::to_string(src) +
+                                        " on disk to " + std::to_string(dst) + " in memory"));
+      }
+   }
+}
+
 /// \brief Pack `count` elements into narrower (or wider) type
 ///
 /// Used to convert in-memory elements to smaller column types of comatible types
@@ -192,6 +204,7 @@ inline void CastUnpack(void *destination, const void *source, std::size_t count)
       ByteSwapIfNecessary(val);
       EnsureValidRange<DestT, SourceT>(val);
       dst[i] = val;
+      EnsureValidConversion<DestT, SourceT>(dst[i], val);
    }
 }
 
@@ -230,6 +243,7 @@ inline void CastSplitUnpack(void *destination, const void *source, std::size_t c
       ByteSwapIfNecessary(val);
       EnsureValidRange<DestT, SourceT>(val);
       dst[i] = val;
+      EnsureValidConversion<DestT, SourceT>(dst[i], val);
    }
 }
 

tree/ntuple/test/ntuple_cast.cxx

@@ -266,6 +266,30 @@ TEST(RNTuple, TypeCastReal)
    EXPECT_FLOAT_EQ(*ptrDouble32, -11.043);
 }
 
+TEST(RNTuple, TypeCastRealFpClass)
+{
+   FileRaii fileGuard("test_ntuple_type_cast_real_fpclass.root");
+
+   {
+      auto model = RNTupleModel::Create();
+      auto ptrDouble = model->MakeField<double>("double");
+
+      auto writer = RNTupleWriter::Recreate(std::move(model), "ntpl", fileGuard.GetPath());
+      *ptrDouble = std::numeric_limits<double>::max();
+      writer->Fill();
+   }
+
+   auto castModel = RNTupleModel::Create();
+   auto ptrDouble = castModel->MakeField<float>("double");
+   auto reader = RNTupleReader::Open(std::move(castModel), "ntpl", fileGuard.GetPath());
+   try {
+      reader->LoadEntry(0);
+      FAIL() << "floating point mismatch should throw";
+   } catch (const ROOT::RException &err) {
+      EXPECT_THAT(err.what(), testing::HasSubstr("floating point class mismatch"));
+   }
+}
+
 TEST(RNTuple, TypeCastChar)
 {
    FileRaii fileGuard("test_ntuple_type_cast_char.root");