[libc++] Applied [[nodiscard]] to some general utilities
#169322
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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 The following has been annotated in this patch: - [x] `bind_back`, `bind_front`, `bind` - [x] `function`, `mem_fn` - [x] `reference_wrapper`
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@llvm/pr-subscribers-libcxx Author: Hristo Hristov (H-G-Hristov) Changes
The following has been annotated in this patch:
Full diff: https://github.com/llvm/llvm-project/pull/169322.diff 7 Files Affected:
diff --git a/libcxx/include/__functional/bind.h b/libcxx/include/__functional/bind.h
index def9e4c4ec7a9..328dc3bf3dabc 100644
--- a/libcxx/include/__functional/bind.h
+++ b/libcxx/include/__functional/bind.h
@@ -278,14 +278,14 @@ template <class _Rp, class _Fp, class... _BoundArgs>
struct is_bind_expression<__bind_r<_Rp, _Fp, _BoundArgs...> > : public true_type {};
template <class _Fp, class... _BoundArgs>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bind<_Fp, _BoundArgs...>
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bind<_Fp, _BoundArgs...>
bind(_Fp&& __f, _BoundArgs&&... __bound_args) {
typedef __bind<_Fp, _BoundArgs...> type;
return type(std::forward<_Fp>(__f), std::forward<_BoundArgs>(__bound_args)...);
}
template <class _Rp, class _Fp, class... _BoundArgs>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bind_r<_Rp, _Fp, _BoundArgs...>
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bind_r<_Rp, _Fp, _BoundArgs...>
bind(_Fp&& __f, _BoundArgs&&... __bound_args) {
typedef __bind_r<_Rp, _Fp, _BoundArgs...> type;
return type(std::forward<_Fp>(__f), std::forward<_BoundArgs>(__bound_args)...);
diff --git a/libcxx/include/__functional/bind_back.h b/libcxx/include/__functional/bind_back.h
index e44768d2283c0..41177144d81fe 100644
--- a/libcxx/include/__functional/bind_back.h
+++ b/libcxx/include/__functional/bind_back.h
@@ -64,7 +64,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr auto __bind_back(_Fn&& __f, _Args&&... __args) n
# if _LIBCPP_STD_VER >= 23
template <class _Fn, class... _Args>
-_LIBCPP_HIDE_FROM_ABI constexpr auto bind_back(_Fn&& __f, _Args&&... __args) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto bind_back(_Fn&& __f, _Args&&... __args) {
static_assert(is_constructible_v<decay_t<_Fn>, _Fn>, "bind_back requires decay_t<F> to be constructible from F");
static_assert(is_move_constructible_v<decay_t<_Fn>>, "bind_back requires decay_t<F> to be move constructible");
static_assert((is_constructible_v<decay_t<_Args>, _Args> && ...),
diff --git a/libcxx/include/__functional/bind_front.h b/libcxx/include/__functional/bind_front.h
index 87ef3affe80b6..427accf96339d 100644
--- a/libcxx/include/__functional/bind_front.h
+++ b/libcxx/include/__functional/bind_front.h
@@ -43,7 +43,7 @@ struct __bind_front_t : __perfect_forward<__bind_front_op, _Fn, _BoundArgs...> {
template <class _Fn, class... _Args>
requires is_constructible_v<decay_t<_Fn>, _Fn> && is_move_constructible_v<decay_t<_Fn>> &&
(is_constructible_v<decay_t<_Args>, _Args> && ...) && (is_move_constructible_v<decay_t<_Args>> && ...)
-_LIBCPP_HIDE_FROM_ABI constexpr auto bind_front(_Fn&& __f, _Args&&... __args) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto bind_front(_Fn&& __f, _Args&&... __args) {
return __bind_front_t<decay_t<_Fn>, decay_t<_Args>...>(std::forward<_Fn>(__f), std::forward<_Args>(__args)...);
}
diff --git a/libcxx/include/__functional/function.h b/libcxx/include/__functional/function.h
index c768fd90d01b4..121417f90ff01 100644
--- a/libcxx/include/__functional/function.h
+++ b/libcxx/include/__functional/function.h
@@ -672,11 +672,11 @@ class function<_Rp(_ArgTypes...)>
# if _LIBCPP_HAS_RTTI
// function target access:
- _LIBCPP_HIDE_FROM_ABI const std::type_info& target_type() const _NOEXCEPT;
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const std::type_info& target_type() const _NOEXCEPT;
template <typename _Tp>
- _LIBCPP_HIDE_FROM_ABI _Tp* target() _NOEXCEPT;
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _Tp* target() _NOEXCEPT;
template <typename _Tp>
- _LIBCPP_HIDE_FROM_ABI const _Tp* target() const _NOEXCEPT;
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI const _Tp* target() const _NOEXCEPT;
# endif // _LIBCPP_HAS_RTTI
};
diff --git a/libcxx/include/__functional/mem_fn.h b/libcxx/include/__functional/mem_fn.h
index 690393988c5a5..1c9340c4f4183 100644
--- a/libcxx/include/__functional/mem_fn.h
+++ b/libcxx/include/__functional/mem_fn.h
@@ -43,7 +43,8 @@ class __mem_fn : public __weak_result_type<_Tp> {
};
template <class _Rp, class _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __mem_fn<_Rp _Tp::*> mem_fn(_Rp _Tp::*__pm) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __mem_fn<_Rp _Tp::*>
+mem_fn(_Rp _Tp::* __pm) _NOEXCEPT {
return __mem_fn<_Rp _Tp::*>(__pm);
}
diff --git a/libcxx/include/__functional/reference_wrapper.h b/libcxx/include/__functional/reference_wrapper.h
index 148703b21d84a..1bb33cb033dcb 100644
--- a/libcxx/include/__functional/reference_wrapper.h
+++ b/libcxx/include/__functional/reference_wrapper.h
@@ -58,11 +58,11 @@ class reference_wrapper : public __weak_result_type<_Tp> {
// access
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 operator type&() const _NOEXCEPT { return *__f_; }
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 type& get() const _NOEXCEPT { return *__f_; }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 type& get() const _NOEXCEPT { return *__f_; }
// invoke
template <class... _ArgTypes>
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __invoke_result_t<type&, _ArgTypes...>
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __invoke_result_t<type&, _ArgTypes...>
operator()(_ArgTypes&&... __args) const
#if _LIBCPP_STD_VER >= 17
// Since is_nothrow_invocable requires C++17 LWG3764 is not backported
@@ -128,23 +128,25 @@ reference_wrapper(_Tp&) -> reference_wrapper<_Tp>;
#endif
template <class _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference_wrapper<_Tp> ref(_Tp& __t) _NOEXCEPT {
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX20 reference_wrapper<_Tp> ref(_Tp& __t) _NOEXCEPT {
return reference_wrapper<_Tp>(__t);
}
template <class _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference_wrapper<_Tp>
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference_wrapper<_Tp>
ref(reference_wrapper<_Tp> __t) _NOEXCEPT {
return __t;
}
template <class _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference_wrapper<const _Tp> cref(const _Tp& __t) _NOEXCEPT {
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference_wrapper<const _Tp>
+cref(const _Tp& __t) _NOEXCEPT {
return reference_wrapper<const _Tp>(__t);
}
template <class _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference_wrapper<const _Tp>
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference_wrapper<const _Tp>
cref(reference_wrapper<_Tp> __t) _NOEXCEPT {
return __t;
}
diff --git a/libcxx/test/libcxx/diagnostics/functional.nodiscard.verify.cpp b/libcxx/test/libcxx/diagnostics/functional.nodiscard.verify.cpp
index 4307976e9e442..d07ea7d2e0f5f 100644
--- a/libcxx/test/libcxx/diagnostics/functional.nodiscard.verify.cpp
+++ b/libcxx/test/libcxx/diagnostics/functional.nodiscard.verify.cpp
@@ -6,7 +6,7 @@
//
//===----------------------------------------------------------------------===//
-// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: c++03
// check that <functional> functions are marked [[nodiscard]]
@@ -16,5 +16,47 @@
void test() {
int i = 0;
- std::identity()(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ // Function wrappers
+
+#if !defined(TEST_HAS_NO_RTTI)
+ std::function<void(int)> f;
+ const std::function<void(int)> cf;
+
+ f.target_type(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ f.target<void(int)>(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cf.target<void(int)>(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
+ struct ZMT {
+ void member_function() {};
+ };
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::mem_fn(&ZMT::member_function);
+
+ // Identity
+
+ std::identity{}(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ // Partial function application
+
+#if TEST_STD_VER >= 23
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::bind_back([](int a) { return a; }, 94);
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::bind_front([](int a) { return a; }, 94);
+#endif
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::bind([](int a) { return a; }, 94);
+ // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::bind<float>([](int a) { return a; }, 94);
+
+ // Reference wrappers
+ std::reference_wrapper<int> rw{i};
+ rw.get(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::function<int(int)> vf;
+ auto rwf = std::ref(vf);
+ rwf(1); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ std::ref(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::cref(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
}
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philnik777
requested changes
Nov 24, 2025
philnik777
approved these changes
Nov 25, 2025
Contributor
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Is it intended to add |
Contributor
Author
Yes. I am implementing the changes in chunks trying to balance between what's more often used while keeping the PRs neither too small nor too big. |
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[[nodiscard]]should be applied to functions where discarding the return value is most likely a correctness issue.The following functions/classes have been annotated in this patch:
bind_back,bind_front,bindfunction,mem_fnreference_wrapper