Remove uses of std::iterator (deprecated in C++17)

Detailed reasoning for the deprecation is provided by the paper proposing deprecation (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0174r2.html) and a related LWG issue (https://cplusplus.github.io/LWG/issue2438).

This was the only issue preventing a clean compile with VS 15.7.2 with c++17/c++latest set as the target language

The issue could be resolved in two ways. Providing a custom replacement to std::iterator (a very simple structure) or by providing the 5 required typedefs. The only functional difference from my reading is that the typedefs are not immediately available to the implementer with the inheritance. I find the inline typedefs to be clearer hence the selection in this commit

include/xlnt/workbook/worksheet_iterator.hpp

@@ -37,18 +37,22 @@ class worksheet;
 // because one needs to point at a const workbook and the other needs
 // to point at a non-const workbook stored as a member variable, respectively.
 
-/// <summary>
-/// Alias the parent class of this iterator to increase clarity.
-/// </summary>
-using ws_iter_type = std::iterator<std::bidirectional_iterator_tag,
-    worksheet, std::ptrdiff_t, worksheet *, worksheet>;
 
 /// <summary>
 /// An iterator which is used to iterate over the worksheets in a workbook.
 /// </summary>
-class XLNT_API worksheet_iterator : public ws_iter_type
+class XLNT_API worksheet_iterator
 {
 public:
+    /// <summary>
+    /// iterator tags required for use with standard algorithms and adapters
+    /// </summary>
+    using iterator_category = std::bidirectional_iterator_tag;
+    using value_type = worksheet;
+    using difference_type = std::ptrdiff_t;
+    using pointer = worksheet *;
+    using reference = worksheet; // intentionally value
+
     /// <summary>
     /// Constructs a worksheet iterator from a workbook and sheet index.
     /// </summary>
@@ -106,18 +110,21 @@ private:
 };
 
 
-/// <summary>
-/// Alias the parent class of this iterator to increase clarity.
-/// </summary>
-using c_ws_iter_type = std::iterator<std::bidirectional_iterator_tag,
-    const worksheet, std::ptrdiff_t, const worksheet *, const worksheet>;
-
 /// <summary>
 /// An iterator which is used to iterate over the worksheets in a const workbook.
 /// </summary>
-class XLNT_API const_worksheet_iterator : public c_ws_iter_type
+class XLNT_API const_worksheet_iterator
 {
 public:
+    /// <summary>
+    /// iterator tags required for use with standard algorithms and adapters
+    /// </summary>
+    using iterator_category = std::bidirectional_iterator_tag;
+    using value_type = const worksheet;
+    using difference_type = std::ptrdiff_t;
+    using pointer = const worksheet *;
+    using reference = const worksheet; // intentionally value
+
     /// <summary>
     /// Constructs a worksheet iterator from a workbook and sheet index.
     /// </summary>

include/xlnt/worksheet/cell_iterator.hpp

@@ -40,18 +40,21 @@ class cell;
 class cell_reference;
 class range_reference;
 
-/// <summary>
-/// Alias the parent class of this iterator to increase clarity.
-/// </summary>
-using c_iter_type = std::iterator<std::bidirectional_iterator_tag,
-    cell, std::ptrdiff_t, cell *, cell>;
-
 /// <summary>
 /// A cell iterator iterates over a 1D range by row or by column.
 /// </summary>
-class XLNT_API cell_iterator : public c_iter_type
+class XLNT_API cell_iterator
 {
 public:
+    /// <summary>
+    /// iterator tags required for use with standard algorithms and adapters
+    /// </summary>
+    using iterator_category = std::bidirectional_iterator_tag;
+    using value_type = cell;
+    using difference_type = std::ptrdiff_t;
+    using pointer = cell *;
+    using reference = cell; // intentionally value
+
     /// <summary>
     /// Constructs a cell_iterator from a worksheet, range, and iteration settings.
     /// </summary>
@@ -144,18 +147,21 @@ private:
     bool wrap_;
 };
 
-/// <summary>
-/// Alias the parent class of this iterator to increase clarity.
-/// </summary>
-using cc_iter_type = std::iterator<std::bidirectional_iterator_tag,
-    const cell, std::ptrdiff_t, const cell *, const cell>;
-
 /// <summary>
 /// A cell iterator iterates over a 1D range by row or by column.
 /// </summary>
-class XLNT_API const_cell_iterator : public cc_iter_type
+class XLNT_API const_cell_iterator
 {
 public:
+    /// <summary>
+    /// iterator tags required for use with standard algorithms and adapters
+    /// </summary>
+    using iterator_category = std::bidirectional_iterator_tag;
+    using value_type = const cell;
+    using difference_type = std::ptrdiff_t;
+    using pointer = const cell *;
+    using reference = const cell; // intentionally value
+
     /// <summary>
     /// Constructs a cell_iterator from a worksheet, range, and iteration settings.
     /// </summary>

include/xlnt/worksheet/range_iterator.hpp

@@ -35,19 +35,22 @@ namespace xlnt {
 
 class cell_vector;
 
-/// <summary>
-/// Alias the parent class of this iterator to increase clarity.
-/// </summary>
-using r_iter_type = std::iterator<std::bidirectional_iterator_tag,
-    cell_vector, std::ptrdiff_t, cell_vector *, cell_vector>;
-
 /// <summary>
 /// An iterator used by worksheet and range for traversing
 /// a 2D grid of cells by row/column then across that row/column.
 /// </summary>
-class XLNT_API range_iterator : public r_iter_type
+class XLNT_API range_iterator
 {
 public:
+    /// <summary>
+    /// iterator tags required for use with standard algorithms and adapters
+    /// </summary>
+    using iterator_category = std::bidirectional_iterator_tag;
+    using value_type = cell_vector;
+    using difference_type = std::ptrdiff_t;
+    using pointer = cell_vector *;
+    using reference = cell_vector; // intentionally value
+
     /// <summary>
     /// Constructs a range iterator on a worksheet, cell pointing to the current
     /// row or column, range bounds, an order, and whether or not to skip null column/rows.
@@ -127,19 +130,22 @@ private:
     bool skip_null_;
 };
 
-/// <summary>
-/// Alias the parent class of this iterator to increase clarity.
-/// </summary>
-using cr_iter_type = std::iterator<std::bidirectional_iterator_tag,
-    const cell_vector, std::ptrdiff_t, const cell_vector *, const cell_vector>;
-
 /// <summary>
 /// A const version of range_iterator which does not allow modification
 /// to the dereferenced cell_vector.
 /// </summary>
-class XLNT_API const_range_iterator : public cr_iter_type
+class XLNT_API const_range_iterator
 {
 public:
+    /// <summary>
+    /// this iterator meets the interface requirements of bidirection_iterator
+    /// </summary>
+    using iterator_category = std::bidirectional_iterator_tag;
+    using value_type = const cell_vector;
+    using difference_type = std::ptrdiff_t;
+    using pointer = const cell_vector *;
+    using reference = const cell_vector; // intentionally value
+
     /// <summary>
     /// Constructs a range iterator on a worksheet, cell pointing to the current
     /// row or column, range bounds, an order, and whether or not to skip null column/rows.