[libcxx-commits] [libcxx] [libc++] Added `[[nodiscard]]` to `fstream.native_handle`, `span.at` and `in/out_ptr` (PR #167097)
Hristo Hristov via libcxx-commits
libcxx-commits at lists.llvm.org
Sat Nov 8 09:03:12 PST 2025
https://github.com/H-G-Hristov updated https://github.com/llvm/llvm-project/pull/167097
>From ea08a3c757dadff14d847c9a8b7b2df54eb0f717 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sat, 8 Nov 2025 07:55:22 +0200
Subject: [PATCH] [libc++] Added `[[nodiscard]]` to `fstream.native_handle`,
`span.at` and `in/out_ptr`
...according to Coding Guidelines: `[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue.
At the time of impelentation the ``nodiscard` policy has not been established yet. I did it as a batch as the changes are trivial and minimal.
---
libcxx/include/__memory/inout_ptr.h | 2 +-
libcxx/include/__memory/out_ptr.h | 2 +-
libcxx/include/fstream | 14 +++++---
libcxx/include/span | 4 +--
.../views/views.span/nodiscard.verify.cpp | 30 ++++++++++++++++
.../fstreams/nodiscard.verify.cpp | 34 +++++++++++++++++++
.../smartptr/adapt/nodiscard.verify.cpp | 23 +++++++++++++
7 files changed, 101 insertions(+), 8 deletions(-)
create mode 100644 libcxx/test/libcxx/containers/views/views.span/nodiscard.verify.cpp
create mode 100644 libcxx/test/libcxx/input.output/file.streams/fstreams/nodiscard.verify.cpp
create mode 100644 libcxx/test/libcxx/utilities/smartptr/adapt/nodiscard.verify.cpp
diff --git a/libcxx/include/__memory/inout_ptr.h b/libcxx/include/__memory/inout_ptr.h
index ef345fe469bca..0fa685afb2ee6 100644
--- a/libcxx/include/__memory/inout_ptr.h
+++ b/libcxx/include/__memory/inout_ptr.h
@@ -96,7 +96,7 @@ class inout_ptr_t {
};
template <class _Pointer = void, class _Smart, class... _Args>
-_LIBCPP_HIDE_FROM_ABI auto inout_ptr(_Smart& __s, _Args&&... __args) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI auto inout_ptr(_Smart& __s, _Args&&... __args) {
using _Ptr = conditional_t<is_void_v<_Pointer>, __pointer_of_t<_Smart>, _Pointer>;
return std::inout_ptr_t<_Smart, _Ptr, _Args&&...>(__s, std::forward<_Args>(__args)...);
}
diff --git a/libcxx/include/__memory/out_ptr.h b/libcxx/include/__memory/out_ptr.h
index e498e3307b9d6..23a77f6a0f7e9 100644
--- a/libcxx/include/__memory/out_ptr.h
+++ b/libcxx/include/__memory/out_ptr.h
@@ -88,7 +88,7 @@ class out_ptr_t {
};
template <class _Pointer = void, class _Smart, class... _Args>
-_LIBCPP_HIDE_FROM_ABI auto out_ptr(_Smart& __s, _Args&&... __args) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI auto out_ptr(_Smart& __s, _Args&&... __args) {
using _Ptr = conditional_t<is_void_v<_Pointer>, __pointer_of_t<_Smart>, _Pointer>;
return std::out_ptr_t<_Smart, _Ptr, _Args&&...>(__s, std::forward<_Args>(__args)...);
}
diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index 1f88d134fe061..2371e460ddf9e 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -269,7 +269,7 @@ public:
_LIBCPP_HIDE_FROM_ABI basic_filebuf* __open(int __fd, ios_base::openmode __mode);
basic_filebuf* close();
# if _LIBCPP_STD_VER >= 26
- _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept {
_LIBCPP_ASSERT_UNCATEGORIZED(this->is_open(), "File must be opened");
# if defined(_LIBCPP_WIN32API)
return std::__filebuf_windows_native_handle(__file_);
@@ -1165,7 +1165,9 @@ public:
_LIBCPP_HIDE_FROM_ABI basic_filebuf<char_type, traits_type>* rdbuf() const;
# if _LIBCPP_STD_VER >= 26
- _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept { return rdbuf()->native_handle(); }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept {
+ return rdbuf()->native_handle();
+ }
# endif
_LIBCPP_HIDE_FROM_ABI bool is_open() const;
void open(const char* __s, ios_base::openmode __mode = ios_base::in);
@@ -1321,7 +1323,9 @@ public:
_LIBCPP_HIDE_FROM_ABI basic_filebuf<char_type, traits_type>* rdbuf() const;
# if _LIBCPP_STD_VER >= 26
- _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept { return rdbuf()->native_handle(); }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept {
+ return rdbuf()->native_handle();
+ }
# endif
_LIBCPP_HIDE_FROM_ABI bool is_open() const;
void open(const char* __s, ios_base::openmode __mode = ios_base::out);
@@ -1483,7 +1487,9 @@ public:
_LIBCPP_HIDE_FROM_ABI basic_filebuf<char_type, traits_type>* rdbuf() const;
# if _LIBCPP_STD_VER >= 26
- _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept { return rdbuf()->native_handle(); }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept {
+ return rdbuf()->native_handle();
+ }
# endif
_LIBCPP_HIDE_FROM_ABI bool is_open() const;
_LIBCPP_HIDE_FROM_ABI void open(const char* __s, ios_base::openmode __mode = ios_base::in | ios_base::out);
diff --git a/libcxx/include/span b/libcxx/include/span
index 3d4f9e4ba7831..68fa3d5987310 100644
--- a/libcxx/include/span
+++ b/libcxx/include/span
@@ -362,7 +362,7 @@ public:
}
# if _LIBCPP_STD_VER >= 26
- _LIBCPP_HIDE_FROM_ABI constexpr reference at(size_type __index) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr reference at(size_type __index) const {
if (__index >= size())
std::__throw_out_of_range("span");
return __data_[__index];
@@ -527,7 +527,7 @@ public:
}
# if _LIBCPP_STD_VER >= 26
- _LIBCPP_HIDE_FROM_ABI constexpr reference at(size_type __index) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr reference at(size_type __index) const {
if (__index >= size())
std::__throw_out_of_range("span");
return __data_[__index];
diff --git a/libcxx/test/libcxx/containers/views/views.span/nodiscard.verify.cpp b/libcxx/test/libcxx/containers/views/views.span/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..6deec90aa0f00
--- /dev/null
+++ b/libcxx/test/libcxx/containers/views/views.span/nodiscard.verify.cpp
@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++26
+
+// <span>
+
+// Check that functions are marked [[nodiscard]]
+
+#include <array>
+#include <span>
+#include <vector>
+
+void test() {
+ { // Test with static extent
+ std::array arr{94, 92};
+ std::span sp{arr};
+ sp.at(0); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+ { // Test with dynamic extent
+ std::vector vec{94, 92};
+ std::span sp{vec};
+ sp.at(0); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+}
diff --git a/libcxx/test/libcxx/input.output/file.streams/fstreams/nodiscard.verify.cpp b/libcxx/test/libcxx/input.output/file.streams/fstreams/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..4aeef4141693c
--- /dev/null
+++ b/libcxx/test/libcxx/input.output/file.streams/fstreams/nodiscard.verify.cpp
@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++26
+
+// <fstream>
+
+// Check that functions are marked [[nodiscard]]
+
+#include <fstream>
+
+void test() {
+ {
+ std::filebuf fb;
+ fb.native_handle(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+ {
+ std::fstream fs;
+ fs.native_handle(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+ {
+ std::ifstream fs;
+ fs.native_handle(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+ {
+ std::ofstream fs;
+ fs.native_handle(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+}
diff --git a/libcxx/test/libcxx/utilities/smartptr/adapt/nodiscard.verify.cpp b/libcxx/test/libcxx/utilities/smartptr/adapt/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..c026594f58d71
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/smartptr/adapt/nodiscard.verify.cpp
@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++23
+
+// <memory>
+
+// Check that functions are marked [[nodiscard]]
+
+#include <memory>
+
+void test() {
+ std::unique_ptr<int> uPtr;
+ // [inout.ptr]
+ std::inout_ptr(uPtr); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ // [out.ptr]
+ std::out_ptr(uPtr); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}
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