[libcxx-commits] [libcxx] [libc++][pair] Applied `[[nodiscard]]` (PR #171999)
Hristo Hristov via libcxx-commits
libcxx-commits at lists.llvm.org
Fri Dec 12 05:03:16 PST 2025
https://github.com/H-G-Hristov updated https://github.com/llvm/llvm-project/pull/171999
>From 4ef24c02ee15f122cb9fc6d56c010456224f57b8 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Thu, 4 Dec 2025 07:41:50 +0200
Subject: [PATCH] [libc++][pair] 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
- https://wg21.link/pairs
---
libcxx/include/__utility/pair.h | 32 +++++++-----
.../utility/pairs/nodiscard.verify.cpp | 52 +++++++++++++++++++
2 files changed, 70 insertions(+), 14 deletions(-)
create mode 100644 libcxx/test/libcxx/utilities/utility/pairs/nodiscard.verify.cpp
diff --git a/libcxx/include/__utility/pair.h b/libcxx/include/__utility/pair.h
index d3914f655f2a6..a8232bc9da9fa 100644
--- a/libcxx/include/__utility/pair.h
+++ b/libcxx/include/__utility/pair.h
@@ -539,8 +539,8 @@ swap(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y) noexcept(noexcept(__x
#endif
template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<__unwrap_ref_decay_t<_T1>, __unwrap_ref_decay_t<_T2> >
-make_pair(_T1&& __t1, _T2&& __t2) {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
+pair<__unwrap_ref_decay_t<_T1>, __unwrap_ref_decay_t<_T2> > make_pair(_T1&& __t1, _T2&& __t2) {
return pair<__unwrap_ref_decay_t<_T1>, __unwrap_ref_decay_t<_T2> >(std::forward<_T1>(__t1), std::forward<_T2>(__t2));
}
@@ -612,67 +612,71 @@ struct __get_pair<1> {
};
template <size_t _Ip, class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 typename tuple_element<_Ip, pair<_T1, _T2> >::type&
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
+typename tuple_element<_Ip, pair<_T1, _T2> >::type&
get(pair<_T1, _T2>& __p) _NOEXCEPT {
return __get_pair<_Ip>::get(__p);
}
template <size_t _Ip, class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const typename tuple_element<_Ip, pair<_T1, _T2> >::type&
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX14 const typename tuple_element<_Ip, pair<_T1, _T2> >::type&
get(const pair<_T1, _T2>& __p) _NOEXCEPT {
return __get_pair<_Ip>::get(__p);
}
template <size_t _Ip, class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 typename tuple_element<_Ip, pair<_T1, _T2> >::type&&
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
+typename tuple_element<_Ip, pair<_T1, _T2> >::type&&
get(pair<_T1, _T2>&& __p) _NOEXCEPT {
return __get_pair<_Ip>::get(std::move(__p));
}
template <size_t _Ip, class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const typename tuple_element<_Ip, pair<_T1, _T2> >::type&&
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX14 const typename tuple_element<_Ip, pair<_T1, _T2> >::type&&
get(const pair<_T1, _T2>&& __p) _NOEXCEPT {
return __get_pair<_Ip>::get(std::move(__p));
}
#if _LIBCPP_STD_VER >= 14
template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T1& get(pair<_T1, _T2>& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T1& get(pair<_T1, _T2>& __p) _NOEXCEPT {
return __p.first;
}
template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T1 const& get(pair<_T1, _T2> const& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T1 const& get(pair<_T1, _T2> const& __p) _NOEXCEPT {
return __p.first;
}
template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T1&& get(pair<_T1, _T2>&& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T1&& get(pair<_T1, _T2>&& __p) _NOEXCEPT {
return std::forward<_T1&&>(__p.first);
}
template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T1 const&& get(pair<_T1, _T2> const&& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T1 const&& get(pair<_T1, _T2> const&& __p) _NOEXCEPT {
return std::forward<_T1 const&&>(__p.first);
}
template <class _T2, class _T1>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T2& get(pair<_T1, _T2>& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T2& get(pair<_T1, _T2>& __p) _NOEXCEPT {
return __p.second;
}
template <class _T2, class _T1>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T2 const& get(pair<_T1, _T2> const& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T2 const& get(pair<_T1, _T2> const& __p) _NOEXCEPT {
return __p.second;
}
template <class _T2, class _T1>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T2&& get(pair<_T1, _T2>&& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T2&& get(pair<_T1, _T2>&& __p) _NOEXCEPT {
return std::forward<_T2&&>(__p.second);
}
template <class _T2, class _T1>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T2 const&& get(pair<_T1, _T2> const&& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T2 const&& get(pair<_T1, _T2> const&& __p) _NOEXCEPT {
return std::forward<_T2 const&&>(__p.second);
}
diff --git a/libcxx/test/libcxx/utilities/utility/pairs/nodiscard.verify.cpp b/libcxx/test/libcxx/utilities/utility/pairs/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..46442d4be9691
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/utility/pairs/nodiscard.verify.cpp
@@ -0,0 +1,52 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Check that functions are marked [[nodiscard]]
+
+#include <utility>
+
+#include <test_macros.h>
+
+void test() {
+ struct First {};
+ struct Second {};
+
+ std::pair<First, Second> p;
+ const std::pair<First, Second> cp{};
+
+ std::make_pair(94, 82); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+#if TEST_STD_VER >= 11
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<0>(p);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<0>(cp);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<0>(std::move(p));
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<0>(std::move(cp));
+#endif // TEST_STD_VER >= 11
+#if TEST_STD_VER >= 14
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<First>(p);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<First>(cp);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<First>(std::move(p));
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<First>(std::move(cp));
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<Second>(p);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<Second>(cp);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<Second>(std::move(p));
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<Second>(std::move(cp));
+#endif // TEST_STD_VER >= 14
+}
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