Change Tensor::unroll() to take iterator pairs.

include/deal.II/base/tensor.h

@@ -383,6 +383,17 @@ class Tensor<0, dim, Number>
   constexpr DEAL_II_CUDA_HOST_DEV real_type
   norm_square() const;
 
+  /**
+   * Fill a range with all tensor elements. Since this type of Tensor only has
+   * one entry this just copies the value of this tensor into <tt>*begin</tt>.
+   *
+   * The template type Number must be convertible to the type of
+   * <tt>*begin</tt>.
+   */
+  template <class Iterator>
+  void
+  unroll(const Iterator begin, const Iterator end) const;
+
   /**
    * Read or write the data of this object to or from a stream for the purpose
    * of serialization using the [BOOST serialization
@@ -792,11 +803,28 @@ class Tensor
    * This function unrolls all tensor entries into a single, linearly numbered
    * vector. As usual in C++, the rightmost index of the tensor marches
    * fastest.
+   *
+   * @deprecated Use the more general function that takes a pair of iterators
+   * instead.
    */
   template <typename OtherNumber>
-  void
+  DEAL_II_DEPRECATED_EARLY void
   unroll(Vector<OtherNumber> &result) const;
 
+  /**
+   * Fill a range with all tensor elements.
+   *
+   * This function unrolls all tensor entries into a single, linearly numbered
+   * sequence. The order of the elements is the one given by
+   * component_to_unrolled_index().
+   *
+   * The template type Number must be convertible to the type of
+   * <tt>*begin</tt>.
+   */
+  template <class Iterator>
+  void
+  unroll(const Iterator begin, const Iterator end) const;
+
   /**
    * Return an unrolled index in the range $[0,\text{dim}^{\text{rank}}-1]$
    * for the element of the tensor indexed by the argument to the function.
@@ -1038,6 +1066,7 @@ constexpr inline DEAL_II_ALWAYS_INLINE
 }
 
 
+
 template <int dim, typename Number>
 template <typename OtherNumber>
 constexpr inline DEAL_II_ALWAYS_INLINE
@@ -1210,6 +1239,7 @@ constexpr DEAL_II_CUDA_HOST_DEV inline DEAL_II_ALWAYS_INLINE
 }
 
 
+
 template <int dim, typename Number>
 template <typename Iterator>
 Iterator
@@ -1218,12 +1248,15 @@ Tensor<0, dim, Number>::unroll_recursion(const Iterator current,
 {
   Assert(dim != 0,
          ExcMessage("Cannot unroll an object of type Tensor<0,0,Number>"));
-  Assert(current != end, ExcMessage("Cannot put value in end iterator"));
+  Assert(std::distance(current, end) == 1,
+         ExcMessage("The provided iterator range must contain exactly one "
+                    "element."));
   *current = value;
-  return current + 1;
+  return std::next(current);
 }
 
 
+
 template <int dim, typename Number>
 constexpr inline void
 Tensor<0, dim, Number>::clear()
@@ -1234,6 +1267,18 @@ Tensor<0, dim, Number>::clear()
 }
 
 
+
+template <int dim, typename Number>
+template <class Iterator>
+inline void
+Tensor<0, dim, Number>::unroll(const Iterator begin, const Iterator end) const
+{
+  AssertDimension(std::distance(begin, end), n_independent_components);
+  unroll_recursion(begin, end);
+}
+
+
+
 template <int dim, typename Number>
 template <class Archive>
 inline void
@@ -1703,18 +1748,29 @@ constexpr inline DEAL_II_ALWAYS_INLINE DEAL_II_CUDA_HOST_DEV
 }
 
 
+
 template <int rank_, int dim, typename Number>
 template <typename OtherNumber>
 inline void
 Tensor<rank_, dim, Number>::unroll(Vector<OtherNumber> &result) const
 {
-  AssertDimension(result.size(),
-                  (Utilities::fixed_power<rank_, unsigned int>(dim)));
+  unroll(result.begin(), result.end());
+}
+
+
 
-  unroll_recursion(result.begin(), result.end());
+template <int rank_, int dim, typename Number>
+template <class Iterator>
+inline void
+Tensor<rank_, dim, Number>::unroll(const Iterator begin,
+                                   const Iterator end) const
+{
+  AssertDimension(std::distance(begin, end), n_independent_components);
+  unroll_recursion(begin, end);
 }
 
 
+
 template <int rank_, int dim, typename Number>
 template <typename Iterator>
 Iterator