Iterators are now working far more reliably and have more extensive tests that randomize inputs and fuzz them to check they keep working.

scratchpad/matrixittest.cpp

@@ -0,0 +1,77 @@
+#include <algorithm>
+#include <iostream>
+
+struct ItStep {
+  int cell;
+  size_t x;
+  size_t y;
+  bool row;
+};
+
+class MatrixIterator1 {
+  public:
+    class iterator
+    {
+    public:
+      Matrix &mat;
+      size_t x = 0;
+      size_t y = 0;
+
+      using iterator_category = std::input_iterator_tag;
+      using value_type = ItStep;
+      using difference_type = ItStep;
+      using pointer = ItStep *;
+      using reference = ItStep;
+
+      explicit iterator(Matrix &_mat)
+        : mat(_mat) {}
+
+      iterator& operator++() {
+        x++;
+        if(x < mat[0].size()) {
+          return *this;
+        } else {
+          x = 0;
+          y++;
+          return *this;
+        }
+      }
+
+      iterator operator++(int) {
+        iterator retval = *this;
+        ++(*this);
+        return retval;
+      }
+
+      bool operator==(iterator other) const {
+        return x == other.x && y == other.y;
+      }
+
+      reference operator*() const {
+        return {
+          .cell = mat[y][x],
+          .x = x,
+          .y = y,
+          .row = x >= mat[0].size() - 1
+        };
+      }
+  };
+
+  Matrix &mat;
+
+  MatrixIterator1(Matrix &mat_) :
+    mat(mat_)
+  {
+  }
+
+  iterator begin() {
+    return iterator(mat);
+  }
+
+  iterator end() {
+    iterator it(mat);
+    it.y = mat.size();
+    it.x = 0;
+    return it;
+  }
+};