[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!

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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philnik777 approved these changes Nov 19, 2025

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

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

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