Port nb::bind_map from pybind11 (#114)

This commit adds a port of nanobind's `nb::bind_map<T>` feature to create
bindings of STL map types (`map`, `unordered_map`). 

The implementation contains the following simplifications:

1. The C++17 constexpr feature was used to considerably reduce the
   size of the implementation.

2. The key/value/item views are simple wrappers without the need for
   polymorphism or STL unique pointers. They are created once per map type.

The commit also includes a port of the associated pybind11 test suite parts.

Co-authored by: Nicholas Junge <[email protected]>
Co-authored-by: Wenzel Jakob <[email protected]>

include/nanobind/nb_class.h

@@ -239,6 +239,9 @@ template <op_id id, op_type ot, typename L = undefined_t, typename R = undefined
 template <typename T, typename SFINAE = int>
 struct is_copy_constructible : std::is_copy_constructible<T> { };
 
+template <typename T>
+constexpr bool is_copy_constructible_v = is_copy_constructible<T>::value;
+
 NAMESPACE_END(detail)
 
 template <typename T, typename... Ts>
@@ -275,7 +278,7 @@ class class_ : public object {
         if constexpr (!std::is_same_v<Alias, T>)
             d.flags |= (uint32_t) detail::type_flags::is_trampoline;
 
-        if constexpr (detail::is_copy_constructible<T>::value) {
+        if constexpr (detail::is_copy_constructible_v<T>) {
             d.flags |= (uint32_t) detail::type_flags::is_copy_constructible;
 
             if constexpr (!std::is_trivially_copy_constructible_v<T>) {

include/nanobind/stl/bind_map.h

@@ -0,0 +1,120 @@
+/*
+    nanobind/stl/bind_map.h: Automatic creation of bindings for map-style containers
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#pragma once
+
+#include <nanobind/nanobind.h>
+#include <nanobind/make_iterator.h>
+#include <nanobind/stl/detail/traits.h>
+
+NAMESPACE_BEGIN(NB_NAMESPACE)
+
+template <typename Map, typename... Args>
+class_<Map> bind_map(handle scope, const char *name, Args &&...args) {
+    using Key = typename Map::key_type;
+    using Value = typename Map::mapped_type;
+
+    auto cl = class_<Map>(scope, name, std::forward<Args>(args)...)
+        .def(init<>())
+
+        .def("__len__", &Map::size)
+
+        .def("__bool__",
+             [](const Map &m) { return !m.empty(); },
+             "Check whether the map is nonempty")
+
+        .def("__contains__",
+             [](const Map &m, const Key &k) { return m.find(k) != m.end(); })
+
+        .def("__contains__", // fallback for incompatible types
+             [](const Map &, handle) { return false; })
+
+        .def("__iter__",
+             [](Map &m) {
+                 return make_key_iterator(type<Map>(), "KeyIterator",
+                                          m.begin(), m.end());
+             },
+             keep_alive<0, 1>())
+
+        .def("__getitem__",
+             [](Map &m, const Key &k) -> Value & {
+                 auto it = m.find(k);
+                 if (it == m.end())
+                     throw key_error();
+                 return it->second;
+             },
+             rv_policy::reference_internal
+        )
+
+        .def("__delitem__",
+            [](Map &m, const Key &k) {
+                auto it = m.find(k);
+                if (it == m.end())
+                    throw key_error();
+                m.erase(it);
+            }
+        );
+
+    // Assignment operator for copy-assignable/copy-constructible types
+    if constexpr (detail::is_copy_assignable_v<Value> ||
+                  detail::is_copy_constructible_v<Value>) {
+        cl.def("__setitem__", [](Map &m, const Key &k, const Value &v) {
+            if constexpr (detail::is_copy_assignable_v<Value>) {
+                m[k] = v;
+            } else {
+                auto r = m.emplace(k, v);
+                if (!r.second) {
+                    // Value is not copy-assignable. Erase and retry
+                    m.erase(r.first);
+                    m.emplace(k, v);
+                }
+            }
+        });
+    }
+
+    // Item, value, and key views
+    struct KeyView   { Map &map; };
+    struct ValueView { Map &map; };
+    struct ItemView  { Map &map; };
+
+    class_<ItemView>(cl, "ItemView")
+        .def("__len__", [](ItemView &v) { return v.map.size(); })
+        .def("__iter__",
+             [](ItemView &v) {
+                 return make_iterator(type<Map>(), "ItemIterator",
+                                      v.map.begin(), v.map.end());
+             },
+             keep_alive<0, 1>());
+
+    class_<KeyView>(cl, "KeyView")
+        .def("__contains__", [](KeyView &v, const Key &k) { return v.map.find(k) != v.map.end(); })
+        .def("__contains__", [](KeyView &, handle) { return false; })
+        .def("__len__", [](KeyView &v) { return v.map.size(); })
+        .def("__iter__",
+             [](KeyView &v) {
+                 return make_key_iterator(type<Map>(), "KeyIterator",
+                                          v.map.begin(), v.map.end());
+             },
+             keep_alive<0, 1>());
+
+    class_<ValueView>(cl, "ValueView")
+        .def("__len__", [](ValueView &v) { return v.map.size(); })
+        .def("__iter__",
+             [](ValueView &v) {
+                 return make_value_iterator(type<Map>(), "ValueIterator",
+                                            v.map.begin(), v.map.end());
+             },
+             keep_alive<0, 1>());
+
+    cl.def("keys", [](Map &m)   { return new KeyView{m}; }, keep_alive<0, 1>());
+    cl.def("values", [](Map &m) { return new ValueView{m}; }, keep_alive<0, 1>());
+    cl.def("items", [](Map &m)  { return new ItemView{m}; }, keep_alive<0, 1>());
+
+    return cl;
+}
+
+NAMESPACE_END(NB_NAMESPACE)

include/nanobind/stl/detail/traits.h

@@ -31,13 +31,36 @@ struct is_copy_constructible<
         is_copy_constructible<typename T::value_type>::value;
 };
 
+// std::pair is copy-constructible <=> both constituents are copy-constructible
 template <typename T1, typename T2>
 struct is_copy_constructible<std::pair<T1, T2>> {
     static constexpr bool value =
-        is_copy_constructible<T1>::value ||
+        is_copy_constructible<T1>::value &&
         is_copy_constructible<T2>::value;
 };
 
+// Analogous template for checking copy-assignability
+template <typename T, typename SFINAE = int>
+struct is_copy_assignable : std::is_copy_assignable<T> { };
+
+template <typename T>
+struct is_copy_assignable<T,
+                          enable_if_t<std::is_copy_assignable_v<T> &&
+                                      std::is_same_v<typename T::value_type &,
+                                                     typename T::reference>>> {
+    static constexpr bool value = is_copy_assignable<typename T::value_type>::value;
+};
+
+template <typename T1, typename T2>
+struct is_copy_assignable<std::pair<T1, T2>> {
+    static constexpr bool value =
+            is_copy_assignable<T1>::value &&
+            is_copy_assignable<T2>::value;
+};
+
+template <typename T>
+constexpr bool is_copy_assignable_v = is_copy_assignable<T>::value;
+
 NAMESPACE_END(detail)
 NAMESPACE_END(NB_NAMESPACE)
 

tests/CMakeLists.txt

@@ -13,6 +13,7 @@ nanobind_add_module(test_functions_ext test_functions.cpp ${NB_EXTRA_ARGS})
 nanobind_add_module(test_classes_ext test_classes.cpp ${NB_EXTRA_ARGS})
 nanobind_add_module(test_holders_ext test_holders.cpp ${NB_EXTRA_ARGS})
 nanobind_add_module(test_stl_ext test_stl.cpp ${NB_EXTRA_ARGS})
+nanobind_add_module(test_bind_map_ext test_stl_bind_map.cpp ${NB_EXTRA_ARGS})
 nanobind_add_module(test_enum_ext test_enum.cpp ${NB_EXTRA_ARGS})
 nanobind_add_module(test_tensor_ext test_tensor.cpp ${NB_EXTRA_ARGS})
 nanobind_add_module(test_intrusive_ext test_intrusive.cpp object.cpp object.h ${NB_EXTRA_ARGS})
@@ -40,6 +41,7 @@ set(TEST_FILES
   test_classes.py
   test_holders.py
   test_stl.py
+  test_stl_bind_map.py
   test_enum.py
   test_tensor.py
   test_intrusive.py

tests/test_stl_bind_map.cpp

@@ -0,0 +1,76 @@
+#include <map>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include <nanobind/stl/bind_map.h>
+#include <nanobind/stl/string.h>
+#include <nanobind/stl/vector.h>
+
+namespace nb = nanobind;
+
+// testing for insertion of non-copyable class
+class E_nc {
+public:
+    explicit E_nc(int i) : value{i} {}
+    E_nc(const E_nc &) = delete;
+    E_nc &operator=(const E_nc &) = delete;
+    E_nc(E_nc &&) = default;
+    E_nc &operator=(E_nc &&) = default;
+
+    int value;
+};
+
+template <class Map>
+Map *times_ten(int n) {
+    auto *m = new Map();
+    for (int i = 1; i <= n; i++) {
+        m->emplace(int(i), E_nc(10 * i));
+    }
+    return m;
+}
+
+template <class NestMap>
+NestMap *times_hundred(int n) {
+    auto *m = new NestMap();
+    for (int i = 1; i <= n; i++) {
+        for (int j = 1; j <= n; j++) {
+            (*m)[i].emplace(int(j * 10), E_nc(100 * j));
+        }
+    }
+    return m;
+}
+
+NB_MODULE(test_bind_map_ext, m) {
+    // test_map_string_double
+    nb::bind_map<std::map<std::string, double>>(m, "MapStringDouble");
+    nb::bind_map<std::unordered_map<std::string, double>>(m, "UnorderedMapStringDouble");
+    // test_map_string_double_const
+    nb::bind_map<std::map<std::string, double const>>(m, "MapStringDoubleConst");
+    nb::bind_map<std::unordered_map<std::string, double const>>(m,
+                                                                "UnorderedMapStringDoubleConst");
+
+    nb::class_<E_nc>(m, "ENC").def(nb::init<int>()).def_readwrite("value", &E_nc::value);
+
+    nb::bind_map<std::map<int, E_nc>>(m, "MapENC");
+    m.def("get_mnc", &times_ten<std::map<int, E_nc>>);
+    nb::bind_map<std::unordered_map<int, E_nc>>(m, "UmapENC");
+    m.def("get_umnc", &times_ten<std::unordered_map<int, E_nc>>);
+    // Issue #1885: binding nested std::map<X, Container<E>> with E non-copyable
+    nb::bind_map<std::map<int, std::vector<E_nc>>>(m, "MapVecENC");
+    m.def("get_nvnc", [](int n) {
+        auto *m = new std::map<int, std::vector<E_nc>>();
+        for (int i = 1; i <= n; i++) {
+            for (int j = 1; j <= n; j++) {
+                (*m)[i].emplace_back(j);
+            }
+        }
+        return m;
+    });
+
+    nb::bind_map<std::map<int, std::map<int, E_nc>>>(m, "MapMapENC");
+    m.def("get_nmnc", &times_hundred<std::map<int, std::map<int, E_nc>>>);
+    nb::bind_map<std::unordered_map<int, std::unordered_map<int, E_nc>>>(m, "UmapUmapENC");
+    m.def("get_numnc", &times_hundred<std::unordered_map<int, std::unordered_map<int, E_nc>>>);
+
+}