[libcxx-commits] [libcxx] [libc++][bitset] Applied `[[nodiscard]]` (PR #170623)
via libcxx-commits
libcxx-commits at lists.llvm.org
Thu Dec 4 00:04:53 PST 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: Hristo Hristov (H-G-Hristov)
<details>
<summary>Changes</summary>
`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html
- https://wg21.link/bitset
---
Full diff: https://github.com/llvm/llvm-project/pull/170623.diff
2 Files Affected:
- (modified) libcxx/include/bitset (+24-15)
- (added) libcxx/test/libcxx/utilities/template.bitset/nodiscard.verify.cpp (+48)
``````````diff
diff --git a/libcxx/include/bitset b/libcxx/include/bitset
index 3453c2fcde71e..82df661ed30ff 100644
--- a/libcxx/include/bitset
+++ b/libcxx/include/bitset
@@ -681,45 +681,54 @@ public:
// element access:
# ifdef _LIBCPP_ABI_BITSET_VECTOR_BOOL_CONST_SUBSCRIPT_RETURN_BOOL
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool operator[](size_t __p) const {
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool operator[](size_t __p) const {
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__p < _Size, "bitset::operator[] index out of bounds");
return __base::__make_ref(__p);
}
# else
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __const_reference operator[](size_t __p) const {
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __const_reference operator[](size_t __p) const {
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__p < _Size, "bitset::operator[] index out of bounds");
return __base::__make_ref(__p);
}
# endif
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 reference operator[](size_t __p) {
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 reference operator[](size_t __p) {
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__p < _Size, "bitset::operator[] index out of bounds");
return __base::__make_ref(__p);
}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unsigned long to_ulong() const { return __base::to_ulong(); }
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unsigned long long to_ullong() const {
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unsigned long to_ulong() const {
+ return __base::to_ulong();
+ }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unsigned long long to_ullong() const {
return __base::to_ullong();
}
template <class _CharT, class _Traits, class _Allocator>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, _Traits, _Allocator>
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, _Traits, _Allocator>
to_string(_CharT __zero = _CharT('0'), _CharT __one = _CharT('1')) const;
template <class _CharT, class _Traits>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, _Traits, allocator<_CharT> >
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI
+ _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, _Traits, allocator<_CharT> >
to_string(_CharT __zero = _CharT('0'), _CharT __one = _CharT('1')) const;
template <class _CharT>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, char_traits<_CharT>, allocator<_CharT> >
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI
+ _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<_CharT, char_traits<_CharT>, allocator<_CharT> >
to_string(_CharT __zero = _CharT('0'), _CharT __one = _CharT('1')) const;
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<char, char_traits<char>, allocator<char> >
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI
+ _LIBCPP_CONSTEXPR_SINCE_CXX23 basic_string<char, char_traits<char>, allocator<char> >
to_string(char __zero = '0', char __one = '1') const;
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 size_t count() const _NOEXCEPT;
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR size_t size() const _NOEXCEPT { return _Size; }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 size_t count() const _NOEXCEPT;
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR size_t size() const _NOEXCEPT { return _Size; }
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool operator==(const bitset& __rhs) const _NOEXCEPT;
# if _LIBCPP_STD_VER <= 17
_LIBCPP_HIDE_FROM_ABI bool operator!=(const bitset& __rhs) const _NOEXCEPT;
# endif
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool test(size_t __pos) const;
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool all() const _NOEXCEPT { return __base::all(); }
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool any() const _NOEXCEPT { return __base::any(); }
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool none() const _NOEXCEPT { return !any(); }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool test(size_t __pos) const;
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool all() const _NOEXCEPT {
+ return __base::all();
+ }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool any() const _NOEXCEPT {
+ return __base::any();
+ }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool none() const _NOEXCEPT { return !any(); }
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bitset operator<<(size_t __pos) const _NOEXCEPT;
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bitset operator>>(size_t __pos) const _NOEXCEPT;
diff --git a/libcxx/test/libcxx/utilities/template.bitset/nodiscard.verify.cpp b/libcxx/test/libcxx/utilities/template.bitset/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..8fc9b5d402d3b
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/template.bitset/nodiscard.verify.cpp
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+// <bitset>
+
+// Check that functions are marked [[nodiscard]]
+
+#include <bitset>
+
+#include "test_macros.h"
+#include "test_allocator.h"
+
+void test() {
+ std::bitset<10> bs;
+ const std::bitset<10> cbs;
+
+ bs[0]; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cbs[0]; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.to_ulong(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.to_ullong(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ struct CharTraits : public std::char_traits<char> {};
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.to_string<char, CharTraits, test_allocator<char>>();
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.to_string<char, CharTraits>();
+#if !defined(TEST_HAS_NO_WIDE_CHARACTERS)
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.to_string<wchar_t>();
+#endif
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.to_string();
+
+ bs.count(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.size(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.test(0); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.all(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.any(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ bs.none(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/170623
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