replace boost iterator with C++ iterator and remove dependency

Author: lonvia

CMakeLists.txt

@@ -60,9 +60,6 @@ endif()
 
 message(STATUS "Building in C++${CMAKE_CXX_STANDARD} mode")
 
-find_package(Boost 1.66 REQUIRED COMPONENTS)
-include_directories(SYSTEM ${Boost_INCLUDE_DIRS})
-
 # Modules without any Python code and just one source file.
 foreach(PYMOD geom index io area)
     pybind11_add_module(${PYMOD} lib/${PYMOD}.cc)

README.md

@@ -28,8 +28,7 @@ For other versions, a source wheel is provided. Make sure to install all
 external dependencies first. On Debian/Ubuntu-like systems, the following
 command installs all required packages:
 
-    sudo apt-get install build-essential cmake libboost-dev \
-                         libexpat1-dev zlib1g-dev libbz2-dev
+    sudo apt-get install build-essential cmake libexpat1-dev zlib1g-dev libbz2-dev
 
 
 ### Installing from source
@@ -45,16 +44,14 @@ pyosmium has the following dependencies:
  * [expat](https://libexpat.github.io/)
  * [libz](https://www.zlib.net/)
  * [libbz2](https://www.sourceware.org/bzip2/)
- * [Boost](https://www.boost.org/) variant and iterator >= 1.70
  * [Python Requests](https://docs.python-requests.org/)
  * [scikit-build-core](https://scikit-build-core.readthedocs.io)
  * a C++17-compatible compiler (Clang 13+, GCC 10+ are supported)
 
 
 ### Compiling from Source
 
-Make sure to install the development packages for expat, libz, libbz2
-and boost.
+Make sure to install the development packages for expat, libz and libbz2.
 
 The appropriate versions for Libosmium and Protozero will be downloaded into
 the `contrib` directory when building the source package:

README.rst

@@ -17,11 +17,10 @@ pyosmium can be installed with pip:
 
 The Pypi source package already comes bundled with a matching version of
 libosmium, protozero and pybind11. Pyosmium additionally depends on
-expat, libz, libbz2 and Boost variant and iterator. You need to install
+expat, libz and libbz2. You need to install
 development packages for these libraries. On Debian/Ubuntu do::
 
-    sudo apt-get install build-essential cmake libboost-dev \
-                       libexpat1-dev zlib1g-dev libbz2-dev
+    sudo apt-get install build-essential cmake libexpat1-dev zlib1g-dev libbz2-dev
 
 
 Python >= 3.8 is supported. Pypy is known not to work.

docs/index.md

@@ -28,7 +28,6 @@ the following additional dependencies need to be available:
  * [expat](https://libexpat.github.io/)
  * [libz](https://www.zlib.net/)
  * [libbz2](https://www.sourceware.org/bzip2/)
- * [Boost](https://www.boost.org/) variant and iterator >= 1.41
  * [Python Requests](https://docs.python-requests.org/en/master/)
  * a recent C++ compiler (Clang 3.4+, GCC 4.8+)
 
@@ -41,7 +40,7 @@ of the build process:
 On Debian/Ubuntu-like systems, the following command installs all required
 packages:
 
-    sudo apt-get install python3-dev build-essential cmake libboost-dev \
+    sudo apt-get install python3-dev build-essential cmake \
                          libexpat1-dev zlib1g-dev libbz2-dev
 
 Compatible versions of libosmium and protozero are shipped with the source

lib/merge_input_reader.cc

@@ -2,14 +2,13 @@
  *
  * This file is part of pyosmium. (https://osmcode.org/pyosmium/)
  *
- * Copyright (C) 2025 Sarah Hoffmann <lonvia@denofr.de> and others.
+ * Copyright (C) 2026 Sarah Hoffmann <lonvia@denofr.de> and others.
  * For a full list of authors see the git log.
  */
 #include <pybind11/pybind11.h>
 
 #include <vector>
-
-#include <boost/iterator/function_output_iterator.hpp>
+#include <iterator>
 
 #include <osmium/osm/object_comparisons.hpp>
 #include <osmium/io/any_input.hpp>
@@ -30,20 +29,21 @@ namespace {
     /**
      *  Copy the first OSM object with a given Id to the output. Keep
      *  track of the Id of each object to do this.
-     *
-     *  We are using this functor class instead of a simple lambda, because the
-     *  lambda doesn't build on MSVC.
      */
     class copy_first_with_id {
         osmium::io::Writer* writer;
         osmium::object_id_type id = 0;
 
     public:
+        using iterator_category = std::output_iterator_tag;
+        using value_type = const osmium::OSMObject;
+        using reference = const osmium::OSMObject&;
+
         explicit copy_first_with_id(osmium::io::Writer& w) :
             writer(&w) {
         }
 
-        void operator()(const osmium::OSMObject& obj) {
+        void push_back(const osmium::OSMObject& obj) {
             if (obj.id() != id) {
                 if (obj.visible()) {
                     (*writer)(obj);
@@ -112,9 +112,8 @@ class MergeInputReader
             std::reverse(objects.ptr_begin(), objects.ptr_end());
             objects.sort(osmium::object_order_type_id_reverse_version());
 
-            auto output_it = boost::make_function_output_iterator(
-                    copy_first_with_id(*writer.get())
-                    );
+            copy_first_with_id copy_first{*writer.get()};
+            auto output_it = std::back_inserter(copy_first);
 
             std::set_union(objects.begin(),
                     objects.end(),