custom_iterator

userland/cpp/custom_iterator.cpp

@@ -0,0 +1,50 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-iterators
+
+#include <cassert>
+#include <map>
+#include <iostream>
+
+class MyMap {
+  public:
+    std::map<int,int> map;
+    class iterator {
+        using It = decltype(map)::iterator;
+        It it;
+      public:
+        using difference_type = It::difference_type;
+        using value_type = It::value_type;
+        using pointer = It::pointer;
+        using reference = It::reference;
+        using iterator_category = std::bidirectional_iterator_tag;
+
+        iterator(It begin) : it(begin) {}
+        iterator& operator++() {it++; return *this;}
+        iterator operator++(int) {auto retval = *this; ++(*this); return retval;}
+        iterator& operator--() {it--; return *this;}
+        iterator operator--(int) {auto retval = *this; --(*this); return retval;}
+        bool operator==(iterator other) const { return it == other.it; }
+        bool operator!=(iterator other) const { return !(*this == other); }
+        value_type operator*() {
+            auto pair = *it;
+            return std::make_pair(2*pair.first, 3*pair.second);
+        }
+    };
+    iterator begin() { return iterator(map.begin()); }
+    iterator end() { return iterator(map.end()); }
+};
+
+int main() {
+    MyMap map;
+    map.map.emplace(0, 10);
+    map.map.emplace(1, 11);
+    map.map.emplace(2, 12);
+
+    auto it = map.begin();
+    assert((*it++ == std::pair<const int, int>(0, 30)));
+    assert((*it++ == std::pair<const int, int>(2, 33)));
+    assert((*it++ == std::pair<const int, int>(4, 36)));
+
+    for (const auto& v : map) {
+        std::cout << v.first << " " << v.second << std::endl;
+    }
+}