[libc++][mdspan] Applied [[nodiscard]]
#169326
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@llvm/pr-subscribers-libcxx Author: Hristo Hristov (H-G-Hristov) Changes
Full diff: https://github.com/llvm/llvm-project/pull/169326.diff 3 Files Affected:
diff --git a/libcxx/include/__mdspan/extents.h b/libcxx/include/__mdspan/extents.h
index 26219557dbae9..d16bbd2af44f1 100644
--- a/libcxx/include/__mdspan/extents.h
+++ b/libcxx/include/__mdspan/extents.h
@@ -299,11 +299,13 @@ class extents {
public:
// [mdspan.extents.obs], observers of multidimensional index space
- _LIBCPP_HIDE_FROM_ABI static constexpr rank_type rank() noexcept { return __rank_; }
- _LIBCPP_HIDE_FROM_ABI static constexpr rank_type rank_dynamic() noexcept { return __rank_dynamic_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr rank_type rank() noexcept { return __rank_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr rank_type rank_dynamic() noexcept { return __rank_dynamic_; }
- _LIBCPP_HIDE_FROM_ABI constexpr index_type extent(rank_type __r) const noexcept { return __vals_.__value(__r); }
- _LIBCPP_HIDE_FROM_ABI static constexpr size_t static_extent(rank_type __r) noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr index_type extent(rank_type __r) const noexcept {
+ return __vals_.__value(__r);
+ }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr size_t static_extent(rank_type __r) noexcept {
return _Values::__static_value(__r);
}
diff --git a/libcxx/include/__mdspan/mdspan.h b/libcxx/include/__mdspan/mdspan.h
index c0f27678197ce..9f3139a874ff9 100644
--- a/libcxx/include/__mdspan/mdspan.h
+++ b/libcxx/include/__mdspan/mdspan.h
@@ -87,12 +87,14 @@ class mdspan {
using data_handle_type = typename accessor_type::data_handle_type;
using reference = typename accessor_type::reference;
- _LIBCPP_HIDE_FROM_ABI static constexpr rank_type rank() noexcept { return extents_type::rank(); }
- _LIBCPP_HIDE_FROM_ABI static constexpr rank_type rank_dynamic() noexcept { return extents_type::rank_dynamic(); }
- _LIBCPP_HIDE_FROM_ABI static constexpr size_t static_extent(rank_type __r) noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr rank_type rank() noexcept { return extents_type::rank(); }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr rank_type rank_dynamic() noexcept {
+ return extents_type::rank_dynamic();
+ }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr size_t static_extent(rank_type __r) noexcept {
return extents_type::static_extent(__r);
}
- _LIBCPP_HIDE_FROM_ABI constexpr index_type extent(rank_type __r) const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr index_type extent(rank_type __r) const noexcept {
return __map_.extents().extent(__r);
};
@@ -185,7 +187,7 @@ class mdspan {
requires((is_convertible_v<_OtherIndexTypes, index_type> && ...) &&
(is_nothrow_constructible_v<index_type, _OtherIndexTypes> && ...) &&
(sizeof...(_OtherIndexTypes) == rank()))
- _LIBCPP_HIDE_FROM_ABI constexpr reference operator[](_OtherIndexTypes... __indices) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr reference operator[](_OtherIndexTypes... __indices) const {
// Note the standard layouts would also check this, but user provided ones may not, so we
// check the precondition here
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__mdspan_detail::__is_multidimensional_index_in(extents(), __indices...),
@@ -196,7 +198,8 @@ class mdspan {
template <class _OtherIndexType>
requires(is_convertible_v<const _OtherIndexType&, index_type> &&
is_nothrow_constructible_v<index_type, const _OtherIndexType&>)
- _LIBCPP_HIDE_FROM_ABI constexpr reference operator[](const array< _OtherIndexType, rank()>& __indices) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr reference
+ operator[](const array< _OtherIndexType, rank()>& __indices) const {
return __acc_.access(__ptr_, [&]<size_t... _Idxs>(index_sequence<_Idxs...>) {
return __map_(__indices[_Idxs]...);
}(make_index_sequence<rank()>()));
@@ -205,7 +208,7 @@ class mdspan {
template <class _OtherIndexType>
requires(is_convertible_v<const _OtherIndexType&, index_type> &&
is_nothrow_constructible_v<index_type, const _OtherIndexType&>)
- _LIBCPP_HIDE_FROM_ABI constexpr reference operator[](span<_OtherIndexType, rank()> __indices) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr reference operator[](span<_OtherIndexType, rank()> __indices) const {
return __acc_.access(__ptr_, [&]<size_t... _Idxs>(index_sequence<_Idxs...>) {
return __map_(__indices[_Idxs]...);
}(make_index_sequence<rank()>()));
@@ -237,24 +240,28 @@ class mdspan {
swap(__x.__acc_, __y.__acc_);
}
- _LIBCPP_HIDE_FROM_ABI constexpr const extents_type& extents() const noexcept { return __map_.extents(); };
- _LIBCPP_HIDE_FROM_ABI constexpr const data_handle_type& data_handle() const noexcept { return __ptr_; };
- _LIBCPP_HIDE_FROM_ABI constexpr const mapping_type& mapping() const noexcept { return __map_; };
- _LIBCPP_HIDE_FROM_ABI constexpr const accessor_type& accessor() const noexcept { return __acc_; };
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const extents_type& extents() const noexcept {
+ return __map_.extents();
+ };
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const data_handle_type& data_handle() const noexcept { return __ptr_; };
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const mapping_type& mapping() const noexcept { return __map_; };
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const accessor_type& accessor() const noexcept { return __acc_; };
// per LWG-4021 "mdspan::is_always_meow() should be noexcept"
- _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_unique() noexcept { return mapping_type::is_always_unique(); };
- _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_exhaustive() noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_unique() noexcept {
+ return mapping_type::is_always_unique();
+ };
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_exhaustive() noexcept {
return mapping_type::is_always_exhaustive();
};
- _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_strided() noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_strided() noexcept {
return mapping_type::is_always_strided();
};
- _LIBCPP_HIDE_FROM_ABI constexpr bool is_unique() const { return __map_.is_unique(); };
- _LIBCPP_HIDE_FROM_ABI constexpr bool is_exhaustive() const { return __map_.is_exhaustive(); };
- _LIBCPP_HIDE_FROM_ABI constexpr bool is_strided() const { return __map_.is_strided(); };
- _LIBCPP_HIDE_FROM_ABI constexpr index_type stride(rank_type __r) const { return __map_.stride(__r); };
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool is_unique() const { return __map_.is_unique(); };
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool is_exhaustive() const { return __map_.is_exhaustive(); };
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool is_strided() const { return __map_.is_strided(); };
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr index_type stride(rank_type __r) const { return __map_.stride(__r); };
private:
_LIBCPP_NO_UNIQUE_ADDRESS data_handle_type __ptr_{};
diff --git a/libcxx/test/libcxx/containers/views/mdspan/nodiscard.verify.cpp b/libcxx/test/libcxx/containers/views/mdspan/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..5e9a66582ec5c
--- /dev/null
+++ b/libcxx/test/libcxx/containers/views/mdspan/nodiscard.verify.cpp
@@ -0,0 +1,64 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <span>
+
+// Check that functions are marked [[nodiscard]]
+
+#include <array>
+#include <mdspan>
+#include <span>
+
+#include "test_macros.h"
+
+void test() {
+ // mdspan<>
+
+ std::array<int, 4> data;
+ std::mdspan<int, std::extents<std::size_t, 2, 2>> mdsp{data.data(), 2, 2};
+
+ mdsp[0, 1]; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::array arr{0, 1};
+ mdsp[arr]; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::span sp{arr};
+ mdsp[sp]; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ mdsp.rank(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.rank_dynamic(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.static_extent(0); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.extent(0); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ mdsp.extents(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.data_handle(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.mapping(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.accessor(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ mdsp.is_always_unique(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.is_always_exhaustive(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.is_always_strided(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.is_unique(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.is_exhaustive(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.is_strided(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ mdsp.stride(0); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ // Helpers
+
+ std::extents<int, 1, 2> ex;
+ ex.rank(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ ex.rank_dynamic(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ ex.static_extent(0); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ ex.extent(0); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ std::dextents<int, 2> dex;
+ dex.rank(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ dex.rank_dynamic(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ dex.static_extent(0); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ dex.extent(0); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}
|
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✅ With the latest revision this PR passed the C/C++ code formatter. |
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`[[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
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frederick-vs-ja
approved these changes
Nov 27, 2025
tanji-dg
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`[[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
GeneraluseAI
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`[[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
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Thank you! |
philnik777
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Nov 27, 2025
| }(make_index_sequence<rank()>())); | ||
| } | ||
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| _LIBCPP_HIDE_FROM_ABI constexpr size_type size() const noexcept { |
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[[nodiscard]]should be applied to functions where discarding the return value is most likely a correctness issue.