worksheet_iterator - operator* const overloads

Users may still want to dereference a const iterator (note: not a const_iterator).
Also use the "reference" typedef to ensure there is only 1 source of information

include/xlnt/workbook/worksheet_iterator.hpp

@@ -88,7 +88,14 @@ public:
     /// If the iterator points to one-past-the-end of the workbook, an invalid_parameter
     /// exception will be thrown.
     /// </summary>
-    worksheet operator*();
+    reference operator*();
+
+    /// <summary>
+    /// Dereferences the iterator to return the worksheet it is pointing to.
+    /// If the iterator points to one-past-the-end of the workbook, an invalid_parameter
+    /// exception will be thrown.
+    /// </summary>
+    const reference operator*() const;
 
     /// <summary>
     /// Returns true if this iterator points to the same worksheet as comparand.
@@ -175,7 +182,7 @@ public:
     /// If the iterator points to one-past-the-end of the workbook, an invalid_parameter
     /// exception will be thrown.
     /// </summary>
-    const worksheet operator*();
+    const reference operator*() const;
 
     /// <summary>
     /// Returns true if this iterator points to the same worksheet as comparand.

source/workbook/worksheet_iterator.cpp

@@ -32,7 +32,12 @@ worksheet_iterator::worksheet_iterator(workbook &wb, std::size_t index)
 {
 }
 
-worksheet worksheet_iterator::operator*()
+worksheet_iterator::reference worksheet_iterator::operator*()
+{
+    return (*wb_)[index_];
+}
+
+const worksheet_iterator::reference worksheet_iterator::operator*() const
 {
     return (*wb_)[index_];
 }
@@ -65,7 +70,7 @@ const_worksheet_iterator::const_worksheet_iterator(const workbook &wb, std::size
 {
 }
 
-const worksheet const_worksheet_iterator::operator*()
+const const_worksheet_iterator::reference const_worksheet_iterator::operator*() const
 {
     return wb_->sheet_by_index(index_);
 }