[libc++][variant] Applied [[nodiscard]]
#172058
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H-G-Hristov
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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 - https://wg21.link/variant
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@llvm/pr-subscribers-libcxx Author: Hristo Hristov (H-G-Hristov) Changes
Full diff: https://github.com/llvm/llvm-project/pull/172058.diff 2 Files Affected:
diff --git a/libcxx/include/variant b/libcxx/include/variant
index df587ccf23843..b93009b3353cb 100644
--- a/libcxx/include/variant
+++ b/libcxx/include/variant
@@ -288,7 +288,7 @@ _LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
class _LIBCPP_EXPORTED_FROM_ABI bad_variant_access : public exception {
public:
- const char* what() const _NOEXCEPT override;
+ [[__nodiscard__]] const char* what() const _NOEXCEPT override;
};
_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
@@ -1282,11 +1282,11 @@ public:
return __impl_.template __emplace<_Ip>(__il, std::forward<_Args>(__args)...);
}
- _LIBCPP_HIDE_FROM_ABI constexpr bool valueless_by_exception() const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool valueless_by_exception() const noexcept {
return __impl_.valueless_by_exception();
}
- _LIBCPP_HIDE_FROM_ABI constexpr size_t index() const noexcept { return __impl_.index(); }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr size_t index() const noexcept { return __impl_.index(); }
template < bool _Dummy = true,
enable_if_t< __all<(__dependent_type<is_move_constructible<_Types>, _Dummy>::value &&
@@ -1329,7 +1329,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool __holds_alternative(const variant<_Types...
}
template <class _Tp, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr bool holds_alternative(const variant<_Types...>& __v) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool holds_alternative(const variant<_Types...>& __v) noexcept {
return std::__holds_alternative<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}
@@ -1343,21 +1343,23 @@ _LIBCPP_HIDE_FROM_ABI constexpr auto&& __generic_get(_Vp&& __v) {
}
template <size_t _Ip, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr variant_alternative_t<_Ip, variant<_Types...>>& get(variant<_Types...>& __v) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr variant_alternative_t<_Ip, variant<_Types...>>&
+get(variant<_Types...>& __v) {
static_assert(_Ip < sizeof...(_Types));
static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
return std::__generic_get<_Ip>(__v);
}
template <size_t _Ip, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr variant_alternative_t<_Ip, variant<_Types...>>&& get(variant<_Types...>&& __v) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr variant_alternative_t<_Ip, variant<_Types...>>&&
+get(variant<_Types...>&& __v) {
static_assert(_Ip < sizeof...(_Types));
static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
return std::__generic_get<_Ip>(std::move(__v));
}
template <size_t _Ip, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr const variant_alternative_t<_Ip, variant<_Types...>>&
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const variant_alternative_t<_Ip, variant<_Types...>>&
get(const variant<_Types...>& __v) {
static_assert(_Ip < sizeof...(_Types));
static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
@@ -1365,7 +1367,7 @@ get(const variant<_Types...>& __v) {
}
template <size_t _Ip, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr const variant_alternative_t<_Ip, variant<_Types...>>&&
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const variant_alternative_t<_Ip, variant<_Types...>>&&
get(const variant<_Types...>&& __v) {
static_assert(_Ip < sizeof...(_Types));
static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
@@ -1373,25 +1375,25 @@ get(const variant<_Types...>&& __v) {
}
template <class _Tp, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp& get(variant<_Types...>& __v) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp& get(variant<_Types...>& __v) {
static_assert(!is_void_v<_Tp>);
return std::get<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}
template <class _Tp, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp&& get(variant<_Types...>&& __v) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& get(variant<_Types...>&& __v) {
static_assert(!is_void_v<_Tp>);
return std::get<__find_exactly_one_t<_Tp, _Types...>::value>(std::move(__v));
}
template <class _Tp, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr const _Tp& get(const variant<_Types...>& __v) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& get(const variant<_Types...>& __v) {
static_assert(!is_void_v<_Tp>);
return std::get<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}
template <class _Tp, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& get(const variant<_Types...>&& __v) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& get(const variant<_Types...>&& __v) {
static_assert(!is_void_v<_Tp>);
return std::get<__find_exactly_one_t<_Tp, _Types...>::value>(std::move(__v));
}
@@ -1403,7 +1405,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr auto* __generic_get_if(_Vp* __v) noexcept {
}
template <size_t _Ip, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<variant_alternative_t<_Ip, variant<_Types...>>>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<variant_alternative_t<_Ip, variant<_Types...>>>
get_if(variant<_Types...>* __v) noexcept {
static_assert(_Ip < sizeof...(_Types));
static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
@@ -1411,7 +1413,7 @@ get_if(variant<_Types...>* __v) noexcept {
}
template <size_t _Ip, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<const variant_alternative_t<_Ip, variant<_Types...>>>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<const variant_alternative_t<_Ip, variant<_Types...>>>
get_if(const variant<_Types...>* __v) noexcept {
static_assert(_Ip < sizeof...(_Types));
static_assert(!is_void_v<variant_alternative_t<_Ip, variant<_Types...>>>);
@@ -1419,13 +1421,13 @@ get_if(const variant<_Types...>* __v) noexcept {
}
template <class _Tp, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<_Tp> get_if(variant<_Types...>* __v) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<_Tp> get_if(variant<_Types...>* __v) noexcept {
static_assert(!is_void_v<_Tp>);
return std::get_if<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}
template <class _Tp, class... _Types>
-_LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<const _Tp> get_if(const variant<_Types...>* __v) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr add_pointer_t<const _Tp> get_if(const variant<_Types...>* __v) noexcept {
static_assert(!is_void_v<_Tp>);
return std::get_if<__find_exactly_one_t<_Tp, _Types...>::value>(__v);
}
@@ -1606,7 +1608,7 @@ struct hash< __enable_hash_helper<variant<_Types...>, remove_const_t<_Types>...>
using result_type _LIBCPP_DEPRECATED_IN_CXX17 = size_t;
# endif
- _LIBCPP_HIDE_FROM_ABI size_t operator()(const variant<_Types...>& __v) const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI size_t operator()(const variant<_Types...>& __v) const {
using __variant_detail::__visitation::__variant;
size_t __res =
__v.valueless_by_exception()
diff --git a/libcxx/test/libcxx/utilities/variant/nodiscard.verify.cpp b/libcxx/test/libcxx/utilities/variant/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..b74a66f446311
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/variant/nodiscard.verify.cpp
@@ -0,0 +1,74 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++17
+
+// Check that functions are marked [[nodiscard]]
+
+#include <variant>
+
+#include <test_macros.h>
+
+void test() {
+ {
+ std::bad_variant_access ex;
+
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ ex.what();
+ }
+
+ {
+ struct First {};
+ struct Second {};
+
+ std::variant<First, Second> v;
+ const std::variant<First, Second> cv;
+
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cv.valueless_by_exception();
+
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cv.index();
+
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::holds_alternative<First>(v);
+
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<0>(v);
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<0>(cv);
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<0>(std::move(v));
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<0>(std::move(cv));
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<First>(v);
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<First>(cv);
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<First>(std::move(v));
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get<First>(std::move(cv));
+
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get_if<0>(&v);
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get_if<0>(&cv);
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get_if<First>(&v);
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::get_if<First>(&cv);
+ }
+
+ {
+ std::hash< std::variant<int, float>> hash;
+
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ hash(std::variant<int, float>{});
+ }
+}
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[[nodiscard]]should be applied to functions where discarding the return value is most likely a correctness issue.