[hist] Support weighted filling with user-defined types

Weaken SupportsWeightedFilling to forbid integral types.

Author: hahnjo

hist/histv7/inc/ROOT/RHist.hxx

@@ -32,11 +32,11 @@ hist.Fill(8.5);
 // hist.GetBinContent(ROOT::Experimental::RBinIndex(3)) will return 1
 \endcode
 
-The class is templated on the bin content type. For counting, as in the example above, it may be an integer type such as
-`int` or `long`. Narrower types such as `unsigned char` or `short` are supported, but may overflow due to their limited
-range and must be used with care. For weighted filling, the bin content type must be a floating-point type such as
-`float` or `double`, or the special type RBinWithError. Note that `float` has a limited significand precision of 24
-bits.
+The class is templated on the bin content type. For counting, as in the example above, it may be an integral type such
+as `int` or `long`. Narrower types such as `unsigned char` or `short` are supported, but may overflow due to their
+limited range and must be used with care. For weighted filling, the bin content type must not be an integral type, but
+a floating-point type such as `float` or `double`, or the special type RBinWithError. Note that `float` has a limited
+significand precision of 24 bits.
 
 An object can have arbitrary dimensionality determined at run-time. The axis configuration is passed as a vector of
 RAxisVariant:
@@ -220,8 +220,7 @@ public:
 
    /// Fill an entry into the histogram with a weight.
    ///
-   /// This overload is only available for floating-point bin content types (see
-   /// \ref RHistEngine::SupportsWeightedFilling).
+   /// This overload is not available for integral bin content types (see \ref RHistEngine::SupportsWeightedFilling).
    ///
    /// \code
    /// ROOT::Experimental::RHist<float> hist({/* two dimensions */});
@@ -259,7 +258,7 @@ public:
    /// ROOT::Experimental::RHist<float> hist({/* two dimensions */});
    /// hist.Fill(8.5, 10.5, ROOT::Experimental::RWeight(0.8));
    /// \endcode
-   /// This is only available for floating-point bin content types (see \ref RHistEngine::SupportsWeightedFilling).
+   /// This is not available for integral bin content types (see \ref RHistEngine::SupportsWeightedFilling).
    ///
    /// If one of the arguments is outside the corresponding axis and flow bins are disabled, the entry will be silently
    /// discarded.

hist/histv7/inc/ROOT/RHistEngine.hxx

@@ -7,7 +7,6 @@
 
 #include "RAxes.hxx"
 #include "RBinIndex.hxx"
-#include "RBinWithError.hxx"
 #include "RHistUtils.hxx"
 #include "RLinearizedIndex.hxx"
 #include "RRegularAxis.hxx"
@@ -37,11 +36,11 @@ hist.Fill(8.5);
 // hist.GetBinContent(ROOT::Experimental::RBinIndex(3)) will return 1
 \endcode
 
-The class is templated on the bin content type. For counting, as in the example above, it may be an integer type such as
-`int` or `long`. Narrower types such as `unsigned char` or `short` are supported, but may overflow due to their limited
-range and must be used with care. For weighted filling, the bin content type must be a floating-point type such as
-`float` or `double`, or the special type RBinWithError. Note that `float` has a limited significand precision of 24
-bits.
+The class is templated on the bin content type. For counting, as in the example above, it may be an integral type such
+as `int` or `long`. Narrower types such as `unsigned char` or `short` are supported, but may overflow due to their
+limited range and must be used with care. For weighted filling, the bin content type must not be an integral type, but
+a floating-point type such as `float` or `double`, or the special type RBinWithError. Note that `float` has a limited
+significand precision of 24 bits.
 
 An object can have arbitrary dimensionality determined at run-time. The axis configuration is passed as a vector of
 RAxisVariant:
@@ -209,8 +208,7 @@ public:
    }
 
    /// Whether this histogram engine type supports weighted filling.
-   static constexpr bool SupportsWeightedFilling =
-      std::is_floating_point_v<BinContentType> || std::is_same_v<BinContentType, RBinWithError>;
+   static constexpr bool SupportsWeightedFilling = !std::is_integral_v<BinContentType>;
 
    /// Fill an entry into the histogram.
    ///
@@ -247,7 +245,7 @@ public:
 
    /// Fill an entry into the histogram with a weight.
    ///
-   /// This overload is only available for floating-point bin content types (see \ref SupportsWeightedFilling).
+   /// This overload is not available for integral bin content types (see \ref SupportsWeightedFilling).
    ///
    /// \code
    /// ROOT::Experimental::RHistEngine<float> hist({/* two dimensions */});
@@ -269,7 +267,7 @@ public:
    template <typename... A>
    void Fill(const std::tuple<A...> &args, RWeight weight)
    {
-      static_assert(SupportsWeightedFilling, "weighted filling is only supported for floating-point bin content types");
+      static_assert(SupportsWeightedFilling, "weighted filling is not supported for integral bin content types");
 
       // We could rely on RAxes::ComputeGlobalIndex to check the number of arguments, but its exception message might
       // be confusing for users.
@@ -295,7 +293,7 @@ public:
    /// ROOT::Experimental::RHistEngine<float> hist({/* two dimensions */});
    /// hist.Fill(8.5, 10.5, ROOT::Experimental::RWeight(0.8));
    /// \endcode
-   /// This is only available for floating-point bin content types (see \ref SupportsWeightedFilling).
+   /// This is not available for integral bin content types (see \ref SupportsWeightedFilling).
    ///
    /// If one of the arguments is outside the corresponding axis and flow bins are disabled, the entry will be silently
    /// discarded.
@@ -312,8 +310,7 @@ public:
    {
       auto t = std::forward_as_tuple(args...);
       if constexpr (std::is_same_v<typename Internal::LastType<A...>::type, RWeight>) {
-         static_assert(SupportsWeightedFilling,
-                       "weighted filling is only supported for floating-point bin content types");
+         static_assert(SupportsWeightedFilling, "weighted filling is not supported for integral bin content types");
          static constexpr std::size_t N = sizeof...(A) - 1;
          if (N != fAxes.GetNDimensions()) {
             throw std::invalid_argument("invalid number of arguments to Fill");

hist/histv7/test/hist_user.cxx

@@ -19,6 +19,12 @@ struct User {
       return old;
    }
 
+   User &operator+=(double w)
+   {
+      fValue += w;
+      return *this;
+   }
+
    User &operator+=(const User &rhs)
    {
       fValue += rhs.fValue;
@@ -98,3 +104,18 @@ TEST(RHistEngineUser, Fill)
    std::array<RBinIndex, 1> indices = {9};
    EXPECT_EQ(engine.GetBinContent(indices).fValue, 1);
 }
+
+TEST(RHistEngineUser, FillWeight)
+{
+   // Weighted filling uses operator+=(double)
+   static constexpr std::size_t Bins = 20;
+   const RRegularAxis axis(Bins, {0, Bins});
+   RHistEngine<User> engine({axis});
+
+   engine.Fill(8.5, RWeight(0.8));
+   engine.Fill(std::make_tuple(9.5), RWeight(0.9));
+
+   EXPECT_EQ(engine.GetBinContent(RBinIndex(8)).fValue, 0.8);
+   std::array<RBinIndex, 1> indices = {9};
+   EXPECT_EQ(engine.GetBinContent(indices).fValue, 0.9);
+}