[libc++][memory] Applied `[[nodiscard]]` to smart pointers #168483

H-G-Hristov · 2025-11-18T05:04:20Z

Applied [[nodiscard]] where relevant to smart pointers and related functions.

- std::unique_ptr
- std::shared_ptr
- std::weak_ptr

See guidelines:

https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue. For example a locking constructor in unique_lock.

llvmbot · 2025-11-18T05:04:51Z

@llvm/pr-subscribers-libcxx

Author: Hristo Hristov (H-G-Hristov)

Changes

Applied [[nodiscard]] where relevant to smart pointers and related functions.

- std::unique_ptr
- std::shared_ptr
- std::weak_ptr

See guidelines:

https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue. For example a locking constructor in unique_lock.

Patch is 28.10 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/168483.diff

3 Files Affected:

(modified) libcxx/include/__memory/shared_ptr.h (+58-42)
(modified) libcxx/include/__memory/unique_ptr.h (+15-10)
(modified) libcxx/test/libcxx/utilities/smartptr/nodiscard.verify.cpp (+130-3)

diff --git a/libcxx/include/__memory/shared_ptr.h b/libcxx/include/__memory/shared_ptr.h
index ee07efe081e51..76ae50db20b6d 100644
--- a/libcxx/include/__memory/shared_ptr.h
+++ b/libcxx/include/__memory/shared_ptr.h
@@ -568,16 +568,20 @@ class _LIBCPP_SHARED_PTR_TRIVIAL_ABI shared_ptr {
     shared_ptr(__p, __d, __a).swap(*this);
   }
 
-  _LIBCPP_HIDE_FROM_ABI element_type* get() const _NOEXCEPT { return __ptr_; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI element_type* get() const _NOEXCEPT { return __ptr_; }
 
-  _LIBCPP_HIDE_FROM_ABI __add_lvalue_reference_t<element_type> operator*() const _NOEXCEPT { return *__ptr_; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI __add_lvalue_reference_t<element_type> operator*() const _NOEXCEPT {
+    return *__ptr_;
+  }
 
   _LIBCPP_HIDE_FROM_ABI element_type* operator->() const _NOEXCEPT {
     static_assert(!is_array<_Tp>::value, "std::shared_ptr<T>::operator-> is only valid when T is not an array type.");
     return __ptr_;
   }
 
-  _LIBCPP_HIDE_FROM_ABI long use_count() const _NOEXCEPT { return __cntrl_ ? __cntrl_->use_count() : 0; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI long use_count() const _NOEXCEPT {
+    return __cntrl_ ? __cntrl_->use_count() : 0;
+  }
 
 #if _LIBCPP_STD_VER < 20 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_SHARED_PTR_UNIQUE)
   _LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_HIDE_FROM_ABI bool unique() const _NOEXCEPT { return use_count() == 1; }
@@ -586,19 +590,19 @@ class _LIBCPP_SHARED_PTR_TRIVIAL_ABI shared_ptr {
   _LIBCPP_HIDE_FROM_ABI explicit operator bool() const _NOEXCEPT { return get() != nullptr; }
 
   template <class _Up>
-  _LIBCPP_HIDE_FROM_ABI bool owner_before(shared_ptr<_Up> const& __p) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool owner_before(shared_ptr<_Up> const& __p) const _NOEXCEPT {
     return __cntrl_ < __p.__cntrl_;
   }
 
   template <class _Up>
-  _LIBCPP_HIDE_FROM_ABI bool owner_before(weak_ptr<_Up> const& __p) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool owner_before(weak_ptr<_Up> const& __p) const _NOEXCEPT {
     return __cntrl_ < __p.__cntrl_;
   }
 
   _LIBCPP_HIDE_FROM_ABI bool __owner_equivalent(const shared_ptr& __p) const { return __cntrl_ == __p.__cntrl_; }
 
 #if _LIBCPP_STD_VER >= 17
-  _LIBCPP_HIDE_FROM_ABI __add_lvalue_reference_t<element_type> operator[](ptrdiff_t __i) const {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI __add_lvalue_reference_t<element_type> operator[](ptrdiff_t __i) const {
     static_assert(is_array<_Tp>::value, "std::shared_ptr<T>::operator[] is only valid when T is an array type.");
     return __ptr_[__i];
   }
@@ -669,7 +673,7 @@ shared_ptr(unique_ptr<_Tp, _Dp>) -> shared_ptr<_Tp>;
 // std::allocate_shared and std::make_shared
 //
 template <class _Tp, class _Alloc, class... _Args, __enable_if_t<!is_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared(const _Alloc& __a, _Args&&... __args) {
+[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared(const _Alloc& __a, _Args&&... __args) {
   using _ControlBlock          = __shared_ptr_emplace<_Tp, _Alloc>;
   using _ControlBlockAllocator = typename __allocator_traits_rebind<_Alloc, _ControlBlock>::type;
   __allocation_guard<_ControlBlockAllocator> __guard(__a, 1);
@@ -680,21 +684,21 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared(const _Alloc& __a, _Args&&
 }
 
 template <class _Tp, class... _Args, __enable_if_t<!is_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(_Args&&... __args) {
+[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(_Args&&... __args) {
   return std::allocate_shared<_Tp>(allocator<__remove_cv_t<_Tp> >(), std::forward<_Args>(__args)...);
 }
 
 #if _LIBCPP_STD_VER >= 20
 
 template <class _Tp, class _Alloc, __enable_if_t<!is_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _Alloc& __a) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _Alloc& __a) {
   using _ForOverwriteAllocator = __allocator_traits_rebind_t<_Alloc, __for_overwrite_tag>;
   _ForOverwriteAllocator __alloc(__a);
   return std::allocate_shared<_Tp>(__alloc);
 }
 
 template <class _Tp, __enable_if_t<!is_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite() {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite() {
   return std::allocate_shared_for_overwrite<_Tp>(allocator<__remove_cv_t<_Tp>>());
 }
 
@@ -886,67 +890,69 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Array> __allocate_shared_bounded_array(const _
 
 // bounded array variants
 template <class _Tp, class _Alloc, __enable_if_t<is_bounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared(const _Alloc& __a) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared(const _Alloc& __a) {
   return std::__allocate_shared_bounded_array<_Tp>(__a);
 }
 
 template <class _Tp, class _Alloc, __enable_if_t<is_bounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared(const _Alloc& __a, const remove_extent_t<_Tp>& __u) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp>
+allocate_shared(const _Alloc& __a, const remove_extent_t<_Tp>& __u) {
   return std::__allocate_shared_bounded_array<_Tp>(__a, __u);
 }
 
 template <class _Tp, class _Alloc, __enable_if_t<is_bounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _Alloc& __a) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _Alloc& __a) {
   using _ForOverwriteAllocator = __allocator_traits_rebind_t<_Alloc, __for_overwrite_tag>;
   _ForOverwriteAllocator __alloc(__a);
   return std::__allocate_shared_bounded_array<_Tp>(__alloc);
 }
 
 template <class _Tp, __enable_if_t<is_bounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared() {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared() {
   return std::__allocate_shared_bounded_array<_Tp>(allocator<_Tp>());
 }
 
 template <class _Tp, __enable_if_t<is_bounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(const remove_extent_t<_Tp>& __u) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(const remove_extent_t<_Tp>& __u) {
   return std::__allocate_shared_bounded_array<_Tp>(allocator<_Tp>(), __u);
 }
 
 template <class _Tp, __enable_if_t<is_bounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite() {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite() {
   return std::__allocate_shared_bounded_array<_Tp>(allocator<__for_overwrite_tag>());
 }
 
 // unbounded array variants
 template <class _Tp, class _Alloc, __enable_if_t<is_unbounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared(const _Alloc& __a, size_t __n) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared(const _Alloc& __a, size_t __n) {
   return std::__allocate_shared_unbounded_array<_Tp>(__a, __n);
 }
 
 template <class _Tp, class _Alloc, __enable_if_t<is_unbounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared(const _Alloc& __a, size_t __n, const remove_extent_t<_Tp>& __u) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp>
+allocate_shared(const _Alloc& __a, size_t __n, const remove_extent_t<_Tp>& __u) {
   return std::__allocate_shared_unbounded_array<_Tp>(__a, __n, __u);
 }
 
 template <class _Tp, class _Alloc, __enable_if_t<is_unbounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _Alloc& __a, size_t __n) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _Alloc& __a, size_t __n) {
   using _ForOverwriteAllocator = __allocator_traits_rebind_t<_Alloc, __for_overwrite_tag>;
   _ForOverwriteAllocator __alloc(__a);
   return std::__allocate_shared_unbounded_array<_Tp>(__alloc, __n);
 }
 
 template <class _Tp, __enable_if_t<is_unbounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(size_t __n) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(size_t __n) {
   return std::__allocate_shared_unbounded_array<_Tp>(allocator<_Tp>(), __n);
 }
 
 template <class _Tp, __enable_if_t<is_unbounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(size_t __n, const remove_extent_t<_Tp>& __u) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(size_t __n, const remove_extent_t<_Tp>& __u) {
   return std::__allocate_shared_unbounded_array<_Tp>(allocator<_Tp>(), __n, __u);
 }
 
 template <class _Tp, __enable_if_t<is_unbounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite(size_t __n) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite(size_t __n) {
   return std::__allocate_shared_unbounded_array<_Tp>(allocator<__for_overwrite_tag>(), __n);
 }
 
@@ -980,12 +986,14 @@ inline _LIBCPP_HIDE_FROM_ABI bool operator>(const shared_ptr<_Tp>& __x, const sh
 }
 
 template <class _Tp, class _Up>
-inline _LIBCPP_HIDE_FROM_ABI bool operator<=(const shared_ptr<_Tp>& __x, const shared_ptr<_Up>& __y) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI bool
+operator<=(const shared_ptr<_Tp>& __x, const shared_ptr<_Up>& __y) _NOEXCEPT {
   return !(__y < __x);
 }
 
 template <class _Tp, class _Up>
-inline _LIBCPP_HIDE_FROM_ABI bool operator>=(const shared_ptr<_Tp>& __x, const shared_ptr<_Up>& __y) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI bool
+operator>=(const shared_ptr<_Tp>& __x, const shared_ptr<_Up>& __y) _NOEXCEPT {
   return !(__x < __y);
 }
 
@@ -1075,7 +1083,8 @@ inline _LIBCPP_HIDE_FROM_ABI void swap(shared_ptr<_Tp>& __x, shared_ptr<_Tp>& __
 }
 
 template <class _Tp, class _Up>
-inline _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> static_pointer_cast(const shared_ptr<_Up>& __r) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp>
+static_pointer_cast(const shared_ptr<_Up>& __r) _NOEXCEPT {
   return shared_ptr<_Tp>(__r, static_cast< typename shared_ptr<_Tp>::element_type*>(__r.get()));
 }
 
@@ -1083,13 +1092,14 @@ inline _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> static_pointer_cast(const shared_pt
 // We don't backport because it is an evolutionary change.
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp, class _Up>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> static_pointer_cast(shared_ptr<_Up>&& __r) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> static_pointer_cast(shared_ptr<_Up>&& __r) noexcept {
   return shared_ptr<_Tp>(std::move(__r), static_cast<typename shared_ptr<_Tp>::element_type*>(__r.get()));
 }
 #endif
 
 template <class _Tp, class _Up>
-inline _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> dynamic_pointer_cast(const shared_ptr<_Up>& __r) _NOEXCEPT {
+[[__nodiscard__]] inline
+    _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> dynamic_pointer_cast(const shared_ptr<_Up>& __r) _NOEXCEPT {
   typedef typename shared_ptr<_Tp>::element_type _ET;
   _ET* __p = dynamic_cast<_ET*>(__r.get());
   return __p ? shared_ptr<_Tp>(__r, __p) : shared_ptr<_Tp>();
@@ -1099,14 +1109,14 @@ inline _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> dynamic_pointer_cast(const shared_p
 // We don't backport because it is an evolutionary change.
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp, class _Up>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> dynamic_pointer_cast(shared_ptr<_Up>&& __r) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> dynamic_pointer_cast(shared_ptr<_Up>&& __r) noexcept {
   auto* __p = dynamic_cast<typename shared_ptr<_Tp>::element_type*>(__r.get());
   return __p ? shared_ptr<_Tp>(std::move(__r), __p) : shared_ptr<_Tp>();
 }
 #endif
 
 template <class _Tp, class _Up>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> const_pointer_cast(const shared_ptr<_Up>& __r) _NOEXCEPT {
+[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> const_pointer_cast(const shared_ptr<_Up>& __r) _NOEXCEPT {
   typedef typename shared_ptr<_Tp>::element_type _RTp;
   return shared_ptr<_Tp>(__r, const_cast<_RTp*>(__r.get()));
 }
@@ -1115,13 +1125,13 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> const_pointer_cast(const shared_ptr<_Up>&
 // We don't backport because it is an evolutionary change.
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp, class _Up>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> const_pointer_cast(shared_ptr<_Up>&& __r) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> const_pointer_cast(shared_ptr<_Up>&& __r) noexcept {
   return shared_ptr<_Tp>(std::move(__r), const_cast<typename shared_ptr<_Tp>::element_type*>(__r.get()));
 }
 #endif
 
 template <class _Tp, class _Up>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> reinterpret_pointer_cast(const shared_ptr<_Up>& __r) _NOEXCEPT {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> reinterpret_pointer_cast(const shared_ptr<_Up>& __r) _NOEXCEPT {
   return shared_ptr<_Tp>(__r, reinterpret_cast< typename shared_ptr<_Tp>::element_type*>(__r.get()));
 }
 
@@ -1129,7 +1139,7 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> reinterpret_pointer_cast(const shared_ptr<
 // We don't backport because it is an evolutionary change.
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp, class _Up>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> reinterpret_pointer_cast(shared_ptr<_Up>&& __r) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> reinterpret_pointer_cast(shared_ptr<_Up>&& __r) noexcept {
   return shared_ptr<_Tp>(std::move(__r), reinterpret_cast<typename shared_ptr<_Tp>::element_type*>(__r.get()));
 }
 #endif
@@ -1137,7 +1147,7 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> reinterpret_pointer_cast(shared_ptr<_Up>&&
 #if _LIBCPP_HAS_RTTI
 
 template <class _Dp, class _Tp>
-inline _LIBCPP_HIDE_FROM_ABI _Dp* get_deleter(const shared_ptr<_Tp>& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _Dp* get_deleter(const shared_ptr<_Tp>& __p) _NOEXCEPT {
   return __p.template __get_deleter<_Dp>();
 }
 
@@ -1192,15 +1202,19 @@ class _LIBCPP_SHARED_PTR_TRIVIAL_ABI weak_ptr {
   _LIBCPP_HIDE_FROM_ABI void swap(weak_ptr& __r) _NOEXCEPT;
   _LIBCPP_HIDE_FROM_ABI void reset() _NOEXCEPT;
 
-  _LIBCPP_HIDE_FROM_ABI long use_count() const _NOEXCEPT { return __cntrl_ ? __cntrl_->use_count() : 0; }
-  _LIBCPP_HIDE_FROM_ABI bool expired() const _NOEXCEPT { return __cntrl_ == nullptr || __cntrl_->use_count() == 0; }
-  _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> lock() const _NOEXCEPT;
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI long use_count() const _NOEXCEPT {
+    return __cntrl_ ? __cntrl_->use_count() : 0;
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool expired() const _NOEXCEPT {
+    return __cntrl_ == nullptr || __cntrl_->use_count() == 0;
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> lock() const _NOEXCEPT;
   template <class _Up>
-  _LIBCPP_HIDE_FROM_ABI bool owner_before(const shared_ptr<_Up>& __r) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool owner_before(const shared_ptr<_Up>& __r) const _NOEXCEPT {
     return __cntrl_ < __r.__cntrl_;
   }
   template <class _Up>
-  _LIBCPP_HIDE_FROM_ABI bool owner_before(const weak_ptr<_Up>& __r) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI bool owner_before(const weak_ptr<_Up>& __r) const _NOEXCEPT {
     return __cntrl_ < __r.__cntrl_;
   }
 
@@ -1384,13 +1398,15 @@ class enable_shared_from_this {
   _LIBCPP_HIDE_FROM_ABI ~enable_shared_from_this() {}
 
 public:
-  _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> shared_from_this() { return shared_ptr<_Tp>(__weak_this_); }
-  _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp const> shared_from_this() const { return shared_ptr<const _Tp>(__weak_this_); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> shared_from_this() { return shared_ptr<_Tp>(__weak_this_); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp const> shared_from_this() const {
+    return shared_ptr<const _Tp>(__weak_this_);
+  }
 
 #if _LIBCPP_STD_VER >= 17
-  _LIBCPP_HIDE_FROM_ABI weak_ptr<_Tp> weak_from_this() _NOEXCEPT { return __weak_this_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI weak_ptr<_Tp> weak_from_this() _NOEXCEPT { return __weak_this_; }
 
-  _LIBCPP_HIDE_FROM_ABI weak_ptr<const _Tp> weak_from_this() const _NOEXCEPT { return __weak_this_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI weak_ptr<const _Tp> weak_from_this() const _NOEXCEPT { return __weak_this_; }
 #endif // _LIBCPP_STD_VER >= 17
 
   template <class _Up>
diff --git a/libcxx/include/__memory/unique_ptr.h b/libcxx/include/__memory/unique_ptr.h
index 3cf4b97a7f49c..a822c761be203 100644
--- a/libcxx/include/__memory/unique_ptr.h
+++ b/libcxx/include/__memory/unique_ptr.h
@@ -258,14 +258,17 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI unique_ptr {
     return *this;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __add_lvalue_reference_t<_Tp> operator*() const
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __add_lvalue_reference_t<_Tp> operator*() const
       _NOEXCEPT_(_NOEXCEPT_(*std::declval<pointer>())) {
     return *__ptr_;
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 pointer operator->() const _NOEXCEPT { return __ptr_; }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 pointer get() const _NOEXCEPT { return __ptr_; }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 deleter_type& get_deleter() _NOEXCEPT { return __deleter_; }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 const deleter_type& get_deleter() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 pointer get() const _NOEXCEPT { return __ptr_; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 deleter_type& get_deleter() _NOEXCEPT {
+    return __deleter_;
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 const deleter_type&
+  get_deleter() const _NOEXCEPT {
     return __deleter_;
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 explicit operator bool() const _NOEXCEPT {
@@ -672,8 +675,8 @@ operator<=>(const unique_ptr<_T1, _D1>& __x, const unique_ptr<_T2, _D2>& __y) {
 #endif
 
 template <class _T1, class _D1>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 bool
-operator==(const unique_ptr<_T1, _D1>& __x, nullptr_t) _NOEXCEPT {
+inline _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX23 bool operator==(const unique_ptr<_T1, _D1>& __x, nullptr_t) _NOEXCEPT {
   return !__x;
 }
 
@@ -748,12 +751,13 @@ operator<=>(const unique_ptr<_T1, _D1>& __x, nullptr_t) {
 #if _LIBCPP_STD_VER >= 14
 
 template <class _Tp, class... _Args, enable_if_t<!is_array<_Tp>::value, int> = 0>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique(_Args&&... __args) {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique(_Args&&... __args) {
   return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...));
 }
 
 template <class _Tp, enable_if_t<__is_unbounded_array_v<_Tp>, int> = 0>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique(size_t __n) {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique(size_t __n) {
   typedef __remove_extent_t<_Tp> _Up;
   return unique_ptr<_Tp>(__private_constructor_tag(), new _Up[__n](), __n);
 }
@@ -766,12 +770,13 @@ void make_unique(_Args&&...) = delete;
 #if _LIBCPP_STD_VER >= 20
 
 template <class _Tp, enable_if_t<!is_array_v<_Tp>, int> = 0>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique_for_overwrite() {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique_for_overwrite() {
   return unique_ptr<_Tp>(new _Tp);
 }
 
 template <class _Tp, enable_if_t<is_unbounded_array_v<_Tp>, int> = 0>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique_for_overwrite(size_t __n) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp>
+make_unique_for_overwrite(size_t __n) {
   return unique_ptr<_Tp>(__private_constructor_tag(), new __remove_extent_t<_Tp>[__n], __n);
 }
 
diff --git a/libcxx/test/libcxx/utilities/smartptr/nodiscard.verify.cpp b/libcxx/test/libcxx/utilities/smartp...
[truncated]

libcxx/include/__memory/unique_ptr.h

libcxx/include/__memory/shared_ptr.h

libcxx/test/libcxx/utilities/smartptr/nodiscard.verify.cpp

Applied `[[nodiscard]]` where relevant to smart pointers and related functions. See guidelines: - https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant - `[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue. For example a locking constructor in unique_lock.

github-actions · 2025-11-18T17:48:57Z

✅ With the latest revision this PR passed the C/C++ code formatter.

philnik777

LGTM % nit.

...smartptr/util.smartptr.shared/util.smartptr.shared.obs/unique.deprecated_in_cxx17.verify.cpp

H-G-Hristov · 2025-11-19T17:49:23Z

LGTM % nit.

Thank you!

nico · 2025-11-24T19:08:20Z

Thanks for all the [[nodiscard]] patches! They find real bugs.

For make_unique in particular, I'll mention that in Chromium, this has been a false positive in 4 / 4 cases (https://chromium-review.googlesource.com/c/chromium/src/+/7199487). There's a 5th case that's only hit on some platforms that I haven't analyzed yet.

It's a pretty small N, so I don't suggest doing anything in response to this data point, but if other projects are seeing this too, maybe it might make sense to roll back the make_unique nodiscard. I can easily imagine it catching bugs. But maybe creating objects for side effects is common enough that it isn't worth it in total. (Or maybe it is! Just contributing a data point.)

philnik777 · 2025-11-24T21:18:56Z

Thanks for all the [[nodiscard]] patches! They find real bugs.

That good to hear! Do you have any statistics on this? It'd be really interesting to see what kind of bugs this find.

For make_unique in particular, I'll mention that in Chromium, this has been a false positive in 4 / 4 cases (https://chromium-review.googlesource.com/c/chromium/src/+/7199487). There's a 5th case that's only hit on some platforms that I haven't analyzed yet.

It's a pretty small N, so I don't suggest doing anything in response to this data point, but if other projects are seeing this too, maybe it might make sense to roll back the make_unique nodiscard. I can easily imagine it catching bugs. But maybe creating objects for side effects is common enough that it isn't worth it in total. (Or maybe it is! Just contributing a data point.)

3/4 were death tests where I'm guessing using make_unique was just easier? Or are the objects so big that they need to be stored on the heap? Either way, I don't think these are super representative of most code bases. AFAIK death tests are rather niche. Thanks for the info though. If other people hit this as well we may have to roll that part of the patch back, or at least put it behind some flag.

Recent versions of libc++ mark make_unique<>'s result as [[nodiscard]]: llvm/llvm-project#168483 These places all intentionally return make_unique<>'s result, so explicitly annotate them as such. Bug: none Change-Id: I32077a72c7f343d77b70ba22a9f6ba23f4384a58 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7199487 Reviewed-by: Hans Wennborg <[email protected]> Commit-Queue: Nico Weber <[email protected]> Owners-Override: Nico Weber <[email protected]> Cr-Commit-Position: refs/heads/main@{#1549979}

Applied `[[nodiscard]]` where relevant to smart pointers and related functions. - [x] - `std::unique_ptr` - [x] - `std::shared_ptr` - [x] - `std::weak_ptr` See guidelines: - https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant - `[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue. For example a locking constructor in unique_lock. --------- Co-authored-by: Hristo Hristov <[email protected]>

nico · 2025-11-26T17:50:06Z

(Here's a fifth; still haven't looked at the missing thing I mentioned above: https://chromium-review.googlesource.com/c/chromium/src/+/7207549 Also a false positive here; the test uses make_unique<> to force an alloc/free pair.)

Recent versions of libc++ mark make_unique<>'s result as [[nodiscard]]: llvm/llvm-project#168483 This place intentionally returns make_unique<>'s result, so explicitly annotate it as such. Previously: https://chromium-review.googlesource.com/c/chromium/src/+/7199487 This instance here is android-only, which is why it wasn't part of that CL. No behavior change. Bug: none Change-Id: I3293f65934d34cfd63795a6b34008aa4a8d78237 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7207549 Reviewed-by: Hans Wennborg <[email protected]> Commit-Queue: Hans Wennborg <[email protected]> Auto-Submit: Nico Weber <[email protected]> Cr-Commit-Position: refs/heads/main@{#1550606}

nico · 2025-11-26T18:59:01Z

Here's two more that just test that a create/destroy sequence doesn't crash (not as a death test):
https://chromium-review.googlesource.com/c/chromium/src/+/7198792/11/chrome/browser/ash/notifications/idle_app_name_notification_view_unittest.cc
https://chromium-review.googlesource.com/c/chromium/src/+/7198792/11/chromeos/ash/services/secure_channel/ble_characteristics_finder_unittest.cc

For the one I wanted to look into, it also looks like the make_unique<> result was intentionally not used, but the code is a bit convoluted: http://crbug.com/463698813

So for us it's 9/9 false positives.

(But, again, no action needed, just contributing a data point.)

Recent versions of libc++ mark make_unique<>'s result as [[nodiscard]]: llvm/llvm-project#168483 This place intentionally returns make_unique<>'s result, so explicitly annotate it as such. Previously: https://chromium-review.googlesource.com/c/chromium/src/+/7199487 This instance here is android-only, which is why it wasn't part of that CL. No behavior change. Bug: none Change-Id: I3293f65934d34cfd63795a6b34008aa4a8d78237 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7207549 Reviewed-by: Hans Wennborg <[email protected]> Commit-Queue: Hans Wennborg <[email protected]> Auto-Submit: Nico Weber <[email protected]> Cr-Commit-Position: refs/heads/main@{#1550606} NOKEYCHECK=True GitOrigin-RevId: 1383ff0a2548cf347e245d797e1144885da2231f

H-G-Hristov requested a review from a team as a code owner November 18, 2025 05:04

llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Nov 18, 2025

H-G-Hristov force-pushed the hgh/libcxx/nodiscard-to-smartptr branch from a7f9ee6 to b2266e0 Compare November 18, 2025 05:33

frederick-vs-ja reviewed Nov 18, 2025

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H-G-Hristov added 2 commits November 18, 2025 19:44

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H-G-Hristov force-pushed the hgh/libcxx/nodiscard-to-smartptr branch from b2266e0 to ff03257 Compare November 18, 2025 17:47

H-G-Hristov added 6 commits November 18, 2025 19:50

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H-G-Hristov requested a review from philnik777 November 18, 2025 19:14

Merge branch 'main' into hgh/libcxx/nodiscard-to-smartptr

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Update deprecated usage of shared_ptr::unique

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Zingam merged commit 3f151a3 into llvm:main Nov 20, 2025
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H-G-Hristov deleted the hgh/libcxx/nodiscard-to-smartptr branch November 20, 2025 03:11

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