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[libc++][unordered_multimap] Applied [[nodiscard]] #171659

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@H-G-Hristov H-G-Hristov commented Dec 10, 2025 •
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[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

@H-G-Hristov H-G-Hristov requested a review from a team as a code owner December 10, 2025 17:41
@llvmbot llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Dec 10, 2025
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llvmbot commented Dec 10, 2025

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Author: Hristo Hristov (H-G-Hristov)

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[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

Full diff: https://github.com/llvm/llvm-project/pull/171659.diff

2 Files Affected:

  • (modified) libcxx/include/unordered_map (+37-37)
  • (added) libcxx/test/libcxx/diagnostics/unordered_multimap.nodiscard.verify.cpp (+103)
diff --git a/libcxx/include/unordered_map b/libcxx/include/unordered_map
index 2afc8805cb4c7..dc1dc6451de0b 100644
--- a/libcxx/include/unordered_map
+++ b/libcxx/include/unordered_map
@@ -1798,20 +1798,20 @@ public:
   _LIBCPP_HIDE_FROM_ABI unordered_multimap& operator=(initializer_list<value_type> __il);
 #  endif // _LIBCPP_CXX03_LANG
 
-  _LIBCPP_HIDE_FROM_ABI allocator_type get_allocator() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI allocator_type get_allocator() const _NOEXCEPT {
     return allocator_type(__table_.__node_alloc());
   }
 
   [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool empty() const _NOEXCEPT { return __table_.size() == 0; }
-  _LIBCPP_HIDE_FROM_ABI size_type size() const _NOEXCEPT { return __table_.size(); }
-  _LIBCPP_HIDE_FROM_ABI size_type max_size() const _NOEXCEPT { return __table_.max_size(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI size_type size() const _NOEXCEPT { return __table_.size(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI size_type max_size() const _NOEXCEPT { return __table_.max_size(); }
 
-  _LIBCPP_HIDE_FROM_ABI iterator begin() _NOEXCEPT { return __table_.begin(); }
-  _LIBCPP_HIDE_FROM_ABI iterator end() _NOEXCEPT { return __table_.end(); }
-  _LIBCPP_HIDE_FROM_ABI const_iterator begin() const _NOEXCEPT { return __table_.begin(); }
-  _LIBCPP_HIDE_FROM_ABI const_iterator end() const _NOEXCEPT { return __table_.end(); }
-  _LIBCPP_HIDE_FROM_ABI const_iterator cbegin() const _NOEXCEPT { return __table_.begin(); }
-  _LIBCPP_HIDE_FROM_ABI const_iterator cend() const _NOEXCEPT { return __table_.end(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI iterator begin() _NOEXCEPT { return __table_.begin(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI iterator end() _NOEXCEPT { return __table_.end(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const_iterator begin() const _NOEXCEPT { return __table_.begin(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const_iterator end() const _NOEXCEPT { return __table_.end(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const_iterator cbegin() const _NOEXCEPT { return __table_.begin(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const_iterator cend() const _NOEXCEPT { return __table_.end(); }
 
   _LIBCPP_HIDE_FROM_ABI iterator insert(const value_type& __x) { return __table_.__emplace_multi(__x); }
 
@@ -1879,10 +1879,10 @@ public:
                                         "node_type with incompatible allocator passed to unordered_multimap::insert()");
     return __table_.template __node_handle_insert_multi<node_type>(__hint.__i_, std::move(__nh));
   }
-  _LIBCPP_HIDE_FROM_ABI node_type extract(key_type const& __key) {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI node_type extract(key_type const& __key) {
     return __table_.template __node_handle_extract<node_type>(__key);
   }
-  _LIBCPP_HIDE_FROM_ABI node_type extract(const_iterator __it) {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI node_type extract(const_iterator __it) {
     return __table_.template __node_handle_extract<node_type>(__it.__i_);
   }
 
@@ -1916,71 +1916,71 @@ public:
     __table_.swap(__u.__table_);
   }
 
-  _LIBCPP_HIDE_FROM_ABI hasher hash_function() const { return __table_.hash_function().hash_function(); }
-  _LIBCPP_HIDE_FROM_ABI key_equal key_eq() const { return __table_.key_eq().key_eq(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI hasher hash_function() const { return __table_.hash_function().hash_function(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI key_equal key_eq() const { return __table_.key_eq().key_eq(); }
 
-  _LIBCPP_HIDE_FROM_ABI iterator find(const key_type& __k) { return __table_.find(__k); }
-  _LIBCPP_HIDE_FROM_ABI const_iterator find(const key_type& __k) const { return __table_.find(__k); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI iterator find(const key_type& __k) { return __table_.find(__k); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const_iterator find(const key_type& __k) const { return __table_.find(__k); }
 #  if _LIBCPP_STD_VER >= 20
   template <class _K2, enable_if_t<__is_transparent_v<hasher, _K2> && __is_transparent_v<key_equal, _K2>>* = nullptr>
-  _LIBCPP_HIDE_FROM_ABI iterator find(const _K2& __k) {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI iterator find(const _K2& __k) {
     return __table_.find(__k);
   }
   template <class _K2, enable_if_t<__is_transparent_v<hasher, _K2> && __is_transparent_v<key_equal, _K2>>* = nullptr>
-  _LIBCPP_HIDE_FROM_ABI const_iterator find(const _K2& __k) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const_iterator find(const _K2& __k) const {
     return __table_.find(__k);
   }
 #  endif // _LIBCPP_STD_VER >= 20
 
-  _LIBCPP_HIDE_FROM_ABI size_type count(const key_type& __k) const { return __table_.__count_multi(__k); }
+  _[[__nodiscard__]] LIBCPP_HIDE_FROM_ABI size_type count(const key_type& __k) const { return __table_.__count_multi(__k); }
 #  if _LIBCPP_STD_VER >= 20
   template <class _K2, enable_if_t<__is_transparent_v<hasher, _K2> && __is_transparent_v<key_equal, _K2>>* = nullptr>
-  _LIBCPP_HIDE_FROM_ABI size_type count(const _K2& __k) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI size_type count(const _K2& __k) const {
     return __table_.__count_multi(__k);
   }
 #  endif // _LIBCPP_STD_VER >= 20
 
 #  if _LIBCPP_STD_VER >= 20
-  _LIBCPP_HIDE_FROM_ABI bool contains(const key_type& __k) const { return find(__k) != end(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool contains(const key_type& __k) const { return find(__k) != end(); }
 
   template <class _K2, enable_if_t<__is_transparent_v<hasher, _K2> && __is_transparent_v<key_equal, _K2>>* = nullptr>
-  _LIBCPP_HIDE_FROM_ABI bool contains(const _K2& __k) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool contains(const _K2& __k) const {
     return find(__k) != end();
   }
 #  endif // _LIBCPP_STD_VER >= 20
 
-  _LIBCPP_HIDE_FROM_ABI pair<iterator, iterator> equal_range(const key_type& __k) {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI pair<iterator, iterator> equal_range(const key_type& __k) {
     return __table_.__equal_range_multi(__k);
   }
-  _LIBCPP_HIDE_FROM_ABI pair<const_iterator, const_iterator> equal_range(const key_type& __k) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI pair<const_iterator, const_iterator> equal_range(const key_type& __k) const {
     return __table_.__equal_range_multi(__k);
   }
 #  if _LIBCPP_STD_VER >= 20
   template <class _K2, enable_if_t<__is_transparent_v<hasher, _K2> && __is_transparent_v<key_equal, _K2>>* = nullptr>
-  _LIBCPP_HIDE_FROM_ABI pair<iterator, iterator> equal_range(const _K2& __k) {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI pair<iterator, iterator> equal_range(const _K2& __k) {
     return __table_.__equal_range_multi(__k);
   }
   template <class _K2, enable_if_t<__is_transparent_v<hasher, _K2> && __is_transparent_v<key_equal, _K2>>* = nullptr>
-  _LIBCPP_HIDE_FROM_ABI pair<const_iterator, const_iterator> equal_range(const _K2& __k) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI pair<const_iterator, const_iterator> equal_range(const _K2& __k) const {
     return __table_.__equal_range_multi(__k);
   }
 #  endif // _LIBCPP_STD_VER >= 20
 
-  _LIBCPP_HIDE_FROM_ABI size_type bucket_count() const _NOEXCEPT { return __table_.bucket_count(); }
-  _LIBCPP_HIDE_FROM_ABI size_type max_bucket_count() const _NOEXCEPT { return __table_.max_bucket_count(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI size_type bucket_count() const _NOEXCEPT { return __table_.bucket_count(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI size_type max_bucket_count() const _NOEXCEPT { return __table_.max_bucket_count(); }
 
-  _LIBCPP_HIDE_FROM_ABI size_type bucket_size(size_type __n) const { return __table_.bucket_size(__n); }
-  _LIBCPP_HIDE_FROM_ABI size_type bucket(const key_type& __k) const { return __table_.bucket(__k); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI size_type bucket_size(size_type __n) const { return __table_.bucket_size(__n); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI size_type bucket(const key_type& __k) const { return __table_.bucket(__k); }
 
-  _LIBCPP_HIDE_FROM_ABI local_iterator begin(size_type __n) { return __table_.begin(__n); }
-  _LIBCPP_HIDE_FROM_ABI local_iterator end(size_type __n) { return __table_.end(__n); }
-  _LIBCPP_HIDE_FROM_ABI const_local_iterator begin(size_type __n) const { return __table_.cbegin(__n); }
-  _LIBCPP_HIDE_FROM_ABI const_local_iterator end(size_type __n) const { return __table_.cend(__n); }
-  _LIBCPP_HIDE_FROM_ABI const_local_iterator cbegin(size_type __n) const { return __table_.cbegin(__n); }
-  _LIBCPP_HIDE_FROM_ABI const_local_iterator cend(size_type __n) const { return __table_.cend(__n); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI local_iterator begin(size_type __n) { return __table_.begin(__n); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI local_iterator end(size_type __n) { return __table_.end(__n); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const_local_iterator begin(size_type __n) const { return __table_.cbegin(__n); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const_local_iterator end(size_type __n) const { return __table_.cend(__n); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const_local_iterator cbegin(size_type __n) const { return __table_.cbegin(__n); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const_local_iterator cend(size_type __n) const { return __table_.cend(__n); }
 
-  _LIBCPP_HIDE_FROM_ABI float load_factor() const _NOEXCEPT { return __table_.load_factor(); }
-  _LIBCPP_HIDE_FROM_ABI float max_load_factor() const _NOEXCEPT { return __table_.max_load_factor(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI float load_factor() const _NOEXCEPT { return __table_.load_factor(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI float max_load_factor() const _NOEXCEPT { return __table_.max_load_factor(); }
   _LIBCPP_HIDE_FROM_ABI void max_load_factor(float __mlf) { __table_.max_load_factor(__mlf); }
   _LIBCPP_HIDE_FROM_ABI void rehash(size_type __n) { __table_.__rehash_multi(__n); }
   _LIBCPP_HIDE_FROM_ABI void reserve(size_type __n) { __table_.__reserve_multi(__n); }
diff --git a/libcxx/test/libcxx/diagnostics/unordered_multimap.nodiscard.verify.cpp b/libcxx/test/libcxx/diagnostics/unordered_multimap.nodiscard.verify.cpp
new file mode 100644
index 0000000000000..c4786a50d38f4
--- /dev/null
+++ b/libcxx/test/libcxx/diagnostics/unordered_multimap.nodiscard.verify.cpp
@@ -0,0 +1,103 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++11
+
+// Check that functions are marked [[nodiscard]]
+
+#include <unoredered_map>
+#include <utility>
+
+#include "test_macros.h"
+
+struct TransparentKey {};
+
+struct StoredKey {
+  friend bool operator==(StoredKey const&, StoredKey const&) { return true; }
+  friend bool operator==(StoredKey const&, TransparentKey const&) { return true; }
+};
+
+struct TransparentKeyHash {
+  using is_transparent = void;
+
+  constexpr std::size_t operator()(TransparentKey const&) const { return 0; }
+  constexpr std::size_t operator()(StoredKey const&) const { return 0; }
+};
+
+void test() {
+  std::unordered_map<int, int> m;
+  const std::unordered_map<int, int> cm;
+
+  m.get_allocator(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  m.empty();    // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  m.size();     // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  m.max_size(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  m.begin();   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  m.end();     // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.begin();  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.end();    // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.cbegin(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.cend();   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  int key = 0;
+
+#if TEST_STD_VER >= 17
+  m.extract(0);         // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  m.extract(m.begin()); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
+
+  m.hash_function(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  m.key_eq();        // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  m.find(key);  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.find(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#if TEST_STD_VER >= 20
+  std::unordered_map<StoredKey, int, TransparentKeyHash, std::equal_to<>> tm;
+  const std::unordered_map<StoredKey, int, TransparentKeyHash, std::equal_to<>> ctm;
+
+  TransparentKey tkey;
+
+  tm.find(tkey);  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  ctm.find(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
+
+#if TEST_STD_VER >= 20
+  m.count(key);  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.count(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  tm.contains(tkey);  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  ctm.contains(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
+
+  m.equal_range(key);  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.equal_range(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#if TEST_STD_VER >= 20
+  tm.equal_range(tkey);  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  ctm.equal_range(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
+
+  m.bucket_count();     // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  m.max_bucket_count(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  int size = 0;
+
+  m.bucket_size(size); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  m.bucket(key);       // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  m.begin(size);   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  m.end(size);     // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.begin(size);  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.end(size);    // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.cbegin(size); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cm.cend(size);   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  m.load_factor();     // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  m.max_load_factor(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}

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github-actions bot commented Dec 10, 2025 •
edited
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✅ With the latest revision this PR passed the C/C++ code formatter.

@H-G-Hristov H-G-Hristov force-pushed the hgh/libcxx/nodiscard-to-unordered_multimap branch 2 times, most recently from 517c909 to accb8d5 Compare December 10, 2025 18:09
@H-G-Hristov
[llibc++][unordered_multimap] Applied [[nodiscard]]
7b230dc 
`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue.

- https://libcxx.llvm.org/CodingGuidelines.htm
- https://wg21.link/unord.multimap
@H-G-Hristov H-G-Hristov force-pushed the hgh/libcxx/nodiscard-to-unordered_multimap branch from accb8d5 to 7b230dc Compare December 10, 2025 18:29
@H-G-Hristov H-G-Hristov changed the title [llibc++][unordered_multimap] Applied [[nodiscard]] [libc++][unordered_multimap] Applied [[nodiscard]] Dec 11, 2025
@H-G-Hristov H-G-Hristov mentioned this pull request Dec 13, 2025
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