default ctor for workbook iterator

include/xlnt/workbook/worksheet_iterator.hpp

@@ -53,6 +53,11 @@ public:
     using pointer = worksheet *;
     using reference = worksheet; // intentionally value
 
+    /// <summary>
+    /// Default Constructs a worksheet iterator
+    /// </summary>
+    worksheet_iterator() = default;
+
     /// <summary>
     /// Constructs a worksheet iterator from a workbook and sheet index.
     /// </summary>
@@ -135,12 +140,12 @@ private:
     /// <summary>
     /// The target workbook of this iterator.
     /// </summary>
-    workbook *wb_;
+    workbook *wb_ = nullptr;
 
     /// <summary>
     /// The index of the worksheet in wb_ this iterator is currently pointing to.
     /// </summary>
-    std::size_t index_;
+    std::size_t index_ = 0;
 };
 
 
@@ -159,6 +164,11 @@ public:
     using pointer = const worksheet *;
     using reference = const worksheet; // intentionally value
 
+    /// <summary>
+    /// Default Constructs a worksheet iterator
+    /// </summary>
+    const_worksheet_iterator() = default;
+
     /// <summary>
     /// Constructs a worksheet iterator from a workbook and sheet index.
     /// </summary>
@@ -234,12 +244,12 @@ private:
     /// <summary>
     /// The target workbook of this iterator.
     /// </summary>
-    const workbook *wb_;
+    const workbook *wb_ = nullptr;
 
     /// <summary>
     /// The index of the worksheet in wb_ this iterator is currently pointing to.
     /// </summary>
-    std::size_t index_;
+    std::size_t index_ = 0;
 };
 
 } // namespace xlnt