[libcxx-commits] [libcxx] [libc++][flat_set] Applied `[[nodiscard]]` (PR #169739)
Hristo Hristov via libcxx-commits
libcxx-commits at lists.llvm.org
Wed Nov 26 14:46:50 PST 2025
https://github.com/H-G-Hristov created https://github.com/llvm/llvm-project/pull/169739
[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
>From 1b6a2508425f0245c4d7d68d4dda3a6eeb7fd4b0 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Thu, 27 Nov 2025 00:46:24 +0200
Subject: [PATCH] [libc++][flat_set] Applied `[[nodiscard]]`
[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
---
libcxx/include/__flat_set/flat_set.h | 88 +++++++++++--------
.../diagnostics/flat_map.nodiscard.verify.cpp | 2 +-
.../diagnostics/flat_set.nodiscard.verify.cpp | 88 +++++++++++++++++--
3 files changed, 135 insertions(+), 43 deletions(-)
diff --git a/libcxx/include/__flat_set/flat_set.h b/libcxx/include/__flat_set/flat_set.h
index 0c8fdb5a803c8..1be38f10ea9f3 100644
--- a/libcxx/include/__flat_set/flat_set.h
+++ b/libcxx/include/__flat_set/flat_set.h
@@ -339,38 +339,42 @@ class flat_set {
}
// iterators
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator begin() noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator begin() noexcept {
return iterator(std::as_const(__keys_).begin());
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator begin() const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator begin() const noexcept {
return const_iterator(__keys_.begin());
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator end() noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator end() noexcept {
return iterator(std::as_const(__keys_).end());
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator end() const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator end() const noexcept {
return const_iterator(__keys_.end());
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 reverse_iterator rbegin() noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 reverse_iterator rbegin() noexcept {
return reverse_iterator(end());
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator rbegin() const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator rbegin() const noexcept {
return const_reverse_iterator(end());
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 reverse_iterator rend() noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 reverse_iterator rend() noexcept {
return reverse_iterator(begin());
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator rend() const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator rend() const noexcept {
return const_reverse_iterator(begin());
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator cbegin() const noexcept { return begin(); }
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator cend() const noexcept { return end(); }
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator crbegin() const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator cbegin() const noexcept {
+ return begin();
+ }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator cend() const noexcept {
+ return end();
+ }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator crbegin() const noexcept {
return const_reverse_iterator(end());
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator crend() const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator crend() const noexcept {
return const_reverse_iterator(begin());
}
@@ -379,9 +383,13 @@ class flat_set {
return __keys_.empty();
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type size() const noexcept { return __keys_.size(); }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type size() const noexcept {
+ return __keys_.size();
+ }
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type max_size() const noexcept { return __keys_.max_size(); }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type max_size() const noexcept {
+ return __keys_.max_size();
+ }
// [flat.set.modifiers], modifiers
template <class... _Args>
@@ -466,7 +474,7 @@ class flat_set {
insert(sorted_unique, __il.begin(), __il.end());
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 container_type extract() && {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 container_type extract() && {
auto __guard = std::__make_scope_guard([&]() noexcept { clear() /* noexcept */; });
auto __ret = std::move(__keys_);
return __ret;
@@ -528,111 +536,117 @@ class flat_set {
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void clear() noexcept { __keys_.clear(); }
// observers
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 key_compare key_comp() const { return __compare_; }
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 value_compare value_comp() const { return __compare_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 key_compare key_comp() const { return __compare_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 value_compare value_comp() const {
+ return __compare_;
+ }
// set operations
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator find(const key_type& __x) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator find(const key_type& __x) {
return __find_impl(*this, __x);
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator find(const key_type& __x) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator find(const key_type& __x) const {
return __find_impl(*this, __x);
}
template <class _Kp>
requires __is_transparent_v<_Compare>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator find(const _Kp& __x) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator find(const _Kp& __x) {
return __find_impl(*this, __x);
}
template <class _Kp>
requires __is_transparent_v<_Compare>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator find(const _Kp& __x) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator find(const _Kp& __x) const {
return __find_impl(*this, __x);
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type count(const key_type& __x) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type count(const key_type& __x) const {
return contains(__x) ? 1 : 0;
}
template <class _Kp>
requires __is_transparent_v<_Compare>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type count(const _Kp& __x) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type count(const _Kp& __x) const {
return contains(__x) ? 1 : 0;
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 bool contains(const key_type& __x) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 bool contains(const key_type& __x) const {
return find(__x) != end();
}
template <class _Kp>
requires __is_transparent_v<_Compare>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 bool contains(const _Kp& __x) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 bool contains(const _Kp& __x) const {
return find(__x) != end();
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator lower_bound(const key_type& __x) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator lower_bound(const key_type& __x) {
const auto& __keys = __keys_;
return iterator(std::lower_bound(__keys.begin(), __keys.end(), __x, __compare_));
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator lower_bound(const key_type& __x) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator
+ lower_bound(const key_type& __x) const {
return const_iterator(std::lower_bound(__keys_.begin(), __keys_.end(), __x, __compare_));
}
template <class _Kp>
requires __is_transparent_v<_Compare>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator lower_bound(const _Kp& __x) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator lower_bound(const _Kp& __x) {
const auto& __keys = __keys_;
return iterator(std::lower_bound(__keys.begin(), __keys.end(), __x, __compare_));
}
template <class _Kp>
requires __is_transparent_v<_Compare>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator lower_bound(const _Kp& __x) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator lower_bound(const _Kp& __x) const {
return const_iterator(std::lower_bound(__keys_.begin(), __keys_.end(), __x, __compare_));
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator upper_bound(const key_type& __x) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator upper_bound(const key_type& __x) {
const auto& __keys = __keys_;
return iterator(std::upper_bound(__keys.begin(), __keys.end(), __x, __compare_));
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator upper_bound(const key_type& __x) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator
+ upper_bound(const key_type& __x) const {
return const_iterator(std::upper_bound(__keys_.begin(), __keys_.end(), __x, __compare_));
}
template <class _Kp>
requires __is_transparent_v<_Compare>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator upper_bound(const _Kp& __x) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator upper_bound(const _Kp& __x) {
const auto& __keys = __keys_;
return iterator(std::upper_bound(__keys.begin(), __keys.end(), __x, __compare_));
}
template <class _Kp>
requires __is_transparent_v<_Compare>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator upper_bound(const _Kp& __x) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator upper_bound(const _Kp& __x) const {
return const_iterator(std::upper_bound(__keys_.begin(), __keys_.end(), __x, __compare_));
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<iterator, iterator> equal_range(const key_type& __x) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<iterator, iterator>
+ equal_range(const key_type& __x) {
return __equal_range_impl(*this, __x);
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<const_iterator, const_iterator>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<const_iterator, const_iterator>
equal_range(const key_type& __x) const {
return __equal_range_impl(*this, __x);
}
template <class _Kp>
requires __is_transparent_v<_Compare>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<iterator, iterator> equal_range(const _Kp& __x) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<iterator, iterator>
+ equal_range(const _Kp& __x) {
return __equal_range_impl(*this, __x);
}
template <class _Kp>
requires __is_transparent_v<_Compare>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<const_iterator, const_iterator>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<const_iterator, const_iterator>
equal_range(const _Kp& __x) const {
return __equal_range_impl(*this, __x);
}
diff --git a/libcxx/test/libcxx/diagnostics/flat_map.nodiscard.verify.cpp b/libcxx/test/libcxx/diagnostics/flat_map.nodiscard.verify.cpp
index d569616b99bf4..7d75083157aef 100644
--- a/libcxx/test/libcxx/diagnostics/flat_map.nodiscard.verify.cpp
+++ b/libcxx/test/libcxx/diagnostics/flat_map.nodiscard.verify.cpp
@@ -10,7 +10,7 @@
// <flat_map>
-// [[nodiscard]] bool empty() const noexcept;
+// Check that functions are marked [[nodiscard]]
#include <flat_map>
#include <utility>
diff --git a/libcxx/test/libcxx/diagnostics/flat_set.nodiscard.verify.cpp b/libcxx/test/libcxx/diagnostics/flat_set.nodiscard.verify.cpp
index 161fe533eabac..89d60456a20c1 100644
--- a/libcxx/test/libcxx/diagnostics/flat_set.nodiscard.verify.cpp
+++ b/libcxx/test/libcxx/diagnostics/flat_set.nodiscard.verify.cpp
@@ -6,15 +6,93 @@
//
//===----------------------------------------------------------------------===//
-// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// REQUIRES: std-at-least-c++23
// <flat_set>
-// [[nodiscard]] bool empty() const noexcept;
+// Check that functions are marked [[nodiscard]]
#include <flat_set>
+#include <utility>
-void f() {
- std::flat_set<int> c;
- c.empty(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+template <typename T>
+struct TransparentKey {
+ T t;
+
+ constexpr explicit operator T() const { return t; }
+};
+
+struct TransparentCompare {
+ using is_transparent = void; // This makes the comparator transparent
+
+ template <typename T>
+ constexpr bool operator()(const T& t, const TransparentKey<T>& transparent) const {
+ return t < transparent.t;
+ }
+
+ template <typename T>
+ constexpr bool operator()(const TransparentKey<T>& transparent, const T& t) const {
+ return transparent.t < t;
+ }
+
+ template <typename T>
+ constexpr bool operator()(const T& t1, const T& t2) const {
+ return t1 < t2;
+ }
+};
+
+void test() {
+ std::flat_set<int, TransparentCompare> fs;
+ const std::flat_set<int, TransparentCompare> cfs;
+
+ fs.begin(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.begin(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.end(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.end(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.rbegin(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.rbegin(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.rend(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.rend(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.cbegin(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.cend(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.crbegin(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.crend(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ fs.empty(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.size(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.max_size(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ std::move(fs).extract(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ fs.key_comp(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.value_comp(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ int key = 0;
+ TransparentKey<int> tkey;
+
+ fs.find(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.find(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.find(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.find(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ fs.count(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.count(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ fs.contains(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.contains(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ fs.lower_bound(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.lower_bound(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.lower_bound(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.lower_bound(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ fs.upper_bound(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.upper_bound(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.upper_bound(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.upper_bound(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ fs.equal_range(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.equal_range(key); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ fs.equal_range(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cfs.equal_range(tkey); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
}
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