From Axel:

- Added missing class header docs.


git-svn-id: http://root.cern.ch/svn/root/trunk@21401 27541ba8-7e3a-0410-8455-c3a389f83636

hist/inc/THnSparse.h

@@ -227,6 +227,38 @@ class THnSparse: public TNamed {
    ClassDef(THnSparse, 1); // Interfaces of sparse n-dimensional histogram
 };
 
+
+
+//______________________________________________________________________________
+//
+// Templated implementation of the abstract base THnSparse.
+// All functionality and the interfaces to be used are in THnSparse!
+//
+// THnSparse does not know how to store any bin content itself. Instead, this
+// is delegated to the derived, templated class: the template parameter decides
+// what the format for the bin content is. In fact it even defines the array
+// itself; possible implementations probably derive from TArray.
+//
+// Typedefs exist for template parematers with ROOT's generic types:
+//
+//   Templated name        Typedef       Bin content type
+//   THnSparseT<TArrayC>   THnSparseC    Char_r
+//   THnSparseT<TArrayS>   THnSparseS    Short_t
+//   THnSparseT<TArrayI>   THnSparseI    Int_t
+//   THnSparseT<TArrayL>   THnSparseL    Long_t
+//   THnSparseT<TArrayF>   THnSparseF    Float_t
+//   THnSparseT<TArrayD>   THnSparseD    Double_t
+//
+// We recommend to use THnSparseC wherever possible, and to map its value space
+// of 256 possible values to e.g. float values outside the class. This saves an
+// enourmous amount of memory. Only if more than 256 values need to be
+// distinguished should e.g. THnSparseS or even THnSparseF be chosen.
+//
+// Implementation detail: the derived, templated class is kept extremely small
+// on purpose. That way the (templated thus inlined) uses of this class will
+// only create a small amount of machine code, in contrast to e.g. STL.
+//______________________________________________________________________________
+
 template <class CONT>
 class THnSparseT: public THnSparse {
  public: