[hist] More constructors for RHistEngine and RHist

They simplify creation of multi-dimensional histograms in user code.

Author: hahnjo

hist/histv7/inc/ROOT/RHist.hxx

@@ -75,14 +75,35 @@ public:
    /// \param[in] axes the axis objects, must have size > 0
    explicit RHist(std::vector<RAxisVariant> axes) : fEngine(std::move(axes)), fStats(fEngine.GetNDimensions()) {}
 
-   /// Construct a one-dimensional histogram engine with a regular axis.
+   /// Construct a histogram.
+   ///
+   /// Note that there is no perfect forwarding of the axis objects. If that is needed, use the
+   /// \ref RHist(std::vector<RAxisVariant> axes) "overload accepting a std::vector".
+   ///
+   /// \param[in] axes the axis objects, must have size > 0
+   explicit RHist(std::initializer_list<RAxisVariant> axes) : RHist(std::vector(axes)) {}
+
+   /// Construct a histogram.
+   ///
+   /// Note that there is no perfect forwarding of the axis objects. If that is needed, use the
+   /// \ref RHist(std::vector<RAxisVariant> axes) "overload accepting a std::vector".
+   ///
+   /// \param[in] axis1 the first axis object
+   /// \param[in] axes the remaining axis objects
+   template <typename... Axes>
+   explicit RHist(const RAxisVariant &axis1, const Axes &...axes) : RHist(std::vector<RAxisVariant>{axis1, axes...})
+   {
+   }
+
+   /// Construct a one-dimensional histogram with a regular axis.
    ///
    /// \param[in] nNormalBins the number of normal bins, must be > 0
    /// \param[in] interval the axis interval (lower end inclusive, upper end exclusive)
    /// \par See also
    /// the \ref RRegularAxis::RRegularAxis(std::uint64_t nNormalBins, std::pair<double, double> interval, bool
    /// enableFlowBins) "constructor of RRegularAxis"
-   RHist(std::uint64_t nNormalBins, std::pair<double, double> interval) : RHist({RRegularAxis(nNormalBins, interval)})
+   RHist(std::uint64_t nNormalBins, std::pair<double, double> interval)
+      : RHist(std::vector<RAxisVariant>{RRegularAxis(nNormalBins, interval)})
    {
    }
 

hist/histv7/inc/ROOT/RHistEngine.hxx

@@ -84,6 +84,27 @@ public:
       fBinContents.resize(fAxes.ComputeTotalNBins());
    }
 
+   /// Construct a histogram engine.
+   ///
+   /// Note that there is no perfect forwarding of the axis objects. If that is needed, use the
+   /// \ref RHistEngine(std::vector<RAxisVariant> axes) "overload accepting a std::vector".
+   ///
+   /// \param[in] axes the axis objects, must have size > 0
+   explicit RHistEngine(std::initializer_list<RAxisVariant> axes) : RHistEngine(std::vector(axes)) {}
+
+   /// Construct a histogram engine.
+   ///
+   /// Note that there is no perfect forwarding of the axis objects. If that is needed, use the
+   /// \ref RHistEngine(std::vector<RAxisVariant> axes) "overload accepting a std::vector".
+   ///
+   /// \param[in] axis1 the first axis object
+   /// \param[in] axes the remaining axis objects
+   template <typename... Axes>
+   explicit RHistEngine(const RAxisVariant &axis1, const Axes &...axes)
+      : RHistEngine(std::vector<RAxisVariant>{axis1, axes...})
+   {
+   }
+
    /// Construct a one-dimensional histogram engine with a regular axis.
    ///
    /// \param[in] nNormalBins the number of normal bins, must be > 0
@@ -92,7 +113,7 @@ public:
    /// the \ref RRegularAxis::RRegularAxis(std::uint64_t nNormalBins, std::pair<double, double> interval, bool
    /// enableFlowBins) "constructor of RRegularAxis"
    RHistEngine(std::uint64_t nNormalBins, std::pair<double, double> interval)
-      : RHistEngine({RRegularAxis(nNormalBins, interval)})
+      : RHistEngine(std::vector<RAxisVariant>{RRegularAxis(nNormalBins, interval)})
    {
    }
 

hist/histv7/test/hist_engine.cxx

@@ -59,6 +59,14 @@ TEST(RHistEngine, Constructor)
    EXPECT_EQ(engine.GetNDimensions(), 1);
    engine = RHistEngine<int>({variableBinAxis, categoricalAxis});
    EXPECT_EQ(engine.GetNDimensions(), 2);
+
+   // Templated constructors
+   engine = RHistEngine<int>(variableBinAxis);
+   EXPECT_EQ(engine.GetNDimensions(), 1);
+   engine = RHistEngine<int>(variableBinAxis, categoricalAxis);
+   EXPECT_EQ(engine.GetNDimensions(), 2);
+   engine = RHistEngine<int>(variableBinAxis, categoricalAxis, regularAxis);
+   EXPECT_EQ(engine.GetNDimensions(), 3);
 }
 
 TEST(RHistEngine, GetBinContentInvalidNumberOfArguments)

hist/histv7/test/hist_hist.cxx

@@ -45,6 +45,14 @@ TEST(RHist, Constructor)
    EXPECT_EQ(hist.GetNDimensions(), 1);
    hist = RHist<int>({regularAxis, regularAxis});
    EXPECT_EQ(hist.GetNDimensions(), 2);
+
+   // Templated constructors
+   hist = RHist<int>(regularAxis);
+   EXPECT_EQ(hist.GetNDimensions(), 1);
+   hist = RHist<int>(regularAxis, regularAxis);
+   EXPECT_EQ(hist.GetNDimensions(), 2);
+   hist = RHist<int>(regularAxis, regularAxis, regularAxis);
+   EXPECT_EQ(hist.GetNDimensions(), 3);
 }
 
 TEST(RHist, Add)