Add has_value to cell_iterator

When iterating over a range that doesn't ignore null rows/columns,
the operator*() throws when dereferencing an iterator to a null cell.
Add has_value() to cell_iterator to be able to make a check without
relying on exceptions.
And add a test case.

include/xlnt/worksheet/cell_iterator.hpp

@@ -136,6 +136,13 @@ public:
     /// </summary>
     cell_iterator operator++(int);
 
+    /// <summary>
+    /// When iterating over a range that doesn't ignore null cells, operator*()
+    /// will throw when trying to access the cells that are null. This method
+    /// checks the existence of a cell.
+    /// </summary>
+    bool has_value() const;
+
 private:
     /// <summary>
     /// If true, cells that don't exist in the worksheet will be skipped during iteration.
@@ -263,6 +270,13 @@ public:
     /// </summary>
     const_cell_iterator operator++(int);
 
+    /// <summary>
+    /// When iterating over a range that doesn't ignore null cells, operator*()
+    /// will throw when trying to access the cells that are null. This method
+    /// checks the existence of a cell.
+    /// </summary>
+    bool has_value() const;
+
 private:
     /// <summary>
     /// If true, cells that don't exist in the worksheet will be skipped during iteration.

source/worksheet/cell_iterator.cpp

@@ -278,4 +278,12 @@ const const_cell_iterator::reference const_cell_iterator::operator*() const
 {
     return ws_.cell(cursor_);
 }
+
+bool cell_iterator::has_value() const{
+    return ws_.has_cell(cursor_);
+}
+
+bool const_cell_iterator::has_value() const{
+    return ws_.has_cell(cursor_);
+}
 } // namespace xlnt

tests/worksheet/worksheet_test_suite.cpp

@@ -78,6 +78,7 @@ public:
         register_test(test_lowest_row_or_props);
         register_test(test_highest_row);
         register_test(test_highest_row_or_props);
+        register_test(test_iterator_has_value);
         register_test(test_const_iterators);
         register_test(test_const_reverse_iterators);
         register_test(test_column_major_iterators);
@@ -584,6 +585,31 @@ public:
         xlnt_assert_equals(ws.highest_row_or_props(), 11);
     }
 
+    void test_iterator_has_value()
+    {
+        xlnt::workbook wb;
+        auto ws = wb.active_sheet();
+
+        // make a worksheet with a blank row and column
+        ws.cell("A1").value("A1");
+        ws.cell("A3").value("A3");
+        ws.cell("C1").value("C1");
+
+        xlnt_assert_equals(ws.columns(false)[0].begin().has_value(), true);
+        xlnt_assert_equals(ws.rows(false)[0].begin().has_value(), true);
+
+        xlnt_assert_equals(ws.columns(false)[1].begin().has_value(), false);
+        xlnt_assert_equals(ws.rows(false)[1].begin().has_value(), false);
+
+        // also test const interators.
+        xlnt_assert_equals(ws.columns(false)[0].cbegin().has_value(), true);
+        xlnt_assert_equals(ws.rows(false)[0].cbegin().has_value(), true);
+
+        xlnt_assert_equals(ws.columns(false)[1].cbegin().has_value(), false);
+        xlnt_assert_equals(ws.rows(false)[1].cbegin().has_value(), false);
+
+    }
+
     void test_const_iterators()
     {
         xlnt::workbook wb;