custom_iterator

README.adoc

@@ -20213,6 +20213,13 @@ Programs under link:userland/cpp/[] are examples of https://en.wikipedia.org/wik
 *** link:userland/cpp/multimap.cpp[]: `std::multimap`
 ** <<algorithms>> contains a benchmark comparison of different c++ containers
 
+[[cpp-standards]]
+==== C++ standards
+
+Like for C, you have to pay for the standards... insane. So we just use the closest free drafts instead.
+
+https://stackoverflow.com/questions/81656/where-do-i-find-the-current-c-or-c-standard-documents
+
 [[cpp-initialization-types]]
 ==== C++ initialization types
 
@@ -20470,13 +20477,6 @@ https://stackoverflow.com/questions/51031060/are-c17-parallel-algorithms-impleme
 
 link:userland/cpp/parallel_sort.cpp[]
 
-[[cpp-standards]]
-==== C++ standards
-
-Like for C, you have to pay for the standards... insane. So we just use the closest free drafts instead.
-
-https://stackoverflow.com/questions/81656/where-do-i-find-the-current-c-or-c-standard-documents
-
 [[cpp17]]
 ===== C++17 N4659 standards draft
 
@@ -20517,6 +20517,20 @@ Replaces decltype with type of an expression at compile time.
 
 More powerful than `auto` as you can use it in more places.
 
+[[cpp-concepts]]
+==== C++ concepts
+
+[[cpp-iterators]]
+===== C++ iterators
+
+https://stackoverflow.com/questions/37031805/preparation-for-stditerator-being-deprecated/38103394
+
+link:userland/cpp/custom_iterator.cpp[]: there is no way to easily define a nice custom iterator, you just have to wrap existing iterators and add a gazillion wrapper methods:
+
+* https://stackoverflow.com/questions/8054273/how-to-implement-an-stl-style-iterator-and-avoid-common-pitfalls
+* https://stackoverflow.com/questions/3582608/how-to-correctly-implement-custom-iterators-and-const-iterators
+* https://stackoverflow.com/questions/6471019/can-should-i-inherit-from-an-stl-iterator
+
 === POSIX
 
 Programs under link:userland/posix/[] are examples of POSIX C programming.

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;
+    }
+}