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[libc++] Add nodiscard attribute to std::make_unique/std::make_shared #153115

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[libc++] Add nodiscard attribute to std::make_unique/std::make_shared #153115

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935d246
added nodiscard attribute to std::make_unique()
lukasx999 Aug 12, 2025
30e0103
added nodiscard attribute to std::make_shared()
lukasx999 Aug 12, 2025
cf7c589
Merge branch 'main' into dev
lukasx999 Aug 12, 2025
2a26cb8
added nodiscard attribute to actual make_unique implementation
lukasx999 Aug 12, 2025
e13ed16
added nodiscard attribute to make_unique for C++17
lukasx999 Aug 12, 2025
b229542
added nodiscard for make_shared
lukasx999 Aug 12, 2025
1ae47c2
Merge remote-tracking branch 'origin/dev' into dev
lukasx999 Aug 12, 2025
cb4d239
removed _LIBCPP_NODISCARD_SINCE_CXX17 in favor of [[__nodiscard__]]
lukasx999 Aug 12, 2025
710f97c
removed nodiscard from memory synopsis
lukasx999 Aug 12, 2025
d793dae
removed _LIBCPP_NODISCARD_SINCE_CXX17 from make_shared
lukasx999 Aug 12, 2025
d6ac2b2
added tests for make_unique/make_shared nodiscard attribute
lukasx999 Aug 12, 2025
23356ef
using int instead of S for make_unique/make_shared nodiscard test
lukasx999 Aug 13, 2025
2e14fa1
removed version constraint for make_unique/make_shared nodiscard tests
lukasx999 Aug 13, 2025
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16 changes: 8 additions & 8 deletions libcxx/include/__memory/shared_ptr.h
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Original file line number Diff line number Diff line change
Expand Up @@ -740,7 +740,7 @@ _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)...);
}

Expand All @@ -754,7 +754,7 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _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>>());
}

Expand Down Expand Up @@ -963,17 +963,17 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _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>());
}

Expand All @@ -996,17 +996,17 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _Alloc
}

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);
}

Expand Down
8 changes: 4 additions & 4 deletions libcxx/include/__memory/unique_ptr.h
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Original file line number Diff line number Diff line change
Expand Up @@ -755,12 +755,12 @@ 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);
}
Expand All @@ -773,12 +773,12 @@ 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);
}

Expand Down
39 changes: 39 additions & 0 deletions libcxx/test/libcxx/memory/nodiscard.verify.cpp
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@@ -0,0 +1,39 @@
//===----------------------------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//

// UNSUPPORTED: c++03, c++11, c++14

// check that functions are marked [[nodiscard]] as an extension in C++17

// [[nodiscard]] std::make_unique(Args&&...);
// [[nodiscard]] std::make_shared(Args&&...);
//
// [[nodiscard]] std::make_unique(size_t);
// [[nodiscard]] std::make_shared(size_t);
//
// [[nodiscard]] std::make_unique_for_overwrite();
// [[nodiscard]] std::make_shared_for_overwrite();

#include <memory>

void f() {

struct S {
S() = default;
};

std::make_unique<S>(S{}); // expected-warning {{ignoring return value of function}}
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It looks like you're missing a few test cases. I count 12 new annotations above, but this tests only 6 functions.

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where do you see 12 annotations? I only see 6.

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I also counted 12.

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I'm sorry but you're not being very helpful, you do mind pointing me to a resource that explains how "annotations" in this test system work?

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These .verify.cpp tests use Clang-verify: https://clang.llvm.org/docs/InternalsManual.html#verifying-diagnostics

I think what @philnik777 is referring to is that you added [[__nodiscard__]] to 12 functions or function overloads, yet you are only testing that the diagnostic is firing on 6 functions. This means that you are missing coverage for 6 functions or function overloads to which the attribute was added, but you have nothing testing it.

std::make_shared<S>(S{}); // expected-warning {{ignoring return value of function}}

std::make_unique<S[]>(5); // expected-warning {{ignoring return value of function}}
std::make_shared<S[]>(5); // expected-warning {{ignoring return value of function}}

std::make_unique_for_overwrite<S>(); // expected-warning {{ignoring return value of function}}
std::make_shared_for_overwrite<S>(); // expected-warning {{ignoring return value of function}}

}