Partially implement C++20's P1020/P1973.

[ldionne]

Summary

This commit adds std::make_unique_for_overwrite.

[ckennelly]

Fixing lint errors

[ckennelly]

• Fix synposis
• Restrict tests to C++20

[ldionne]

Thanks a lot for the patch! Do you think you could update the patch with the rest of the paper too? I'm fine reviewing it all in one patch (actually I find it easier to validate that the whole paper is implemented that way).

Also, I don't see any feature test macro in the paper -- can you confirm that's the case, and that we're not forgetting to add one?

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.create/make_unique_for_overwrite.array2.compile.fail.cpp

10–11 ↗

(On Diff #300522)

You don't need these includes (here and elsewhere).

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.create/make_unique_for_overwrite.array3.compile.fail.cpp

14 ↗

(On Diff #300522)

Can you make these tests be .verify.cpp instead, and check for the error message?

[ckennelly]

In D90111#2353912, @ldionne wrote:

Thanks a lot for the patch! Do you think you could update the patch with the rest of the paper too? I'm fine reviewing it all in one patch (actually I find it easier to validate that the whole paper is implemented that way).

I looked at it, but I realized I needed to implement P0674R1 as a prereq for the make_shared_for_overwrite array bits, which looked more involved than trying to implement all 3 papers in one go

Also, I don't see any feature test macro in the paper -- can you confirm that's the case, and that we're not forgetting to add one?

There is a feature test macro, but because I did not implement the make_shared_for_overwrite parts

[ckennelly]

Apply feedback on review

• Remove unused includes
• Use .verify.cpp to check expected error

[mvels]

Sorry, didn't see this earlier as this drowned in my email filters :/

[ldionne]

Arrays in std::shared_ptr is https://reviews.llvm.org/D62641. I've been meaning to review it forever but never get the time to get into it fully. If you'd like, please consider taking a first pass at it.

[EricWF]

I'm personally fine taking small bits of a paper.
I would rather it than have a full implementation get stuck in review.

This patch LGTM apart from the negative compilation tests. I suspect those can be SFINAE tests instead.

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.create/make_unique_for_overwrite.array2.compile.verify.cpp

13

Can we not write positive compilation tests for this? Using things like is_invocable?

Negative compilation tests should be the last resort.

[EricWF]

After talking to @ldionne, the negative compilation tests aren't inappropriate here. I'm happy to see the stay.

[ckennelly]

Restrict verify tests to C++20

We do not expose std::make_unique_for_overwrite on pre-C++20 builds, so we will
not get the expected error message.

[ckennelly]

The test failures look like unrelated tests with flakes.

@ldionne: Do you want to get patch for P0674R1 reviewed first and I can turn this into a complete implementation of P1020/P1973 or should we land the partial implementation?

[ckennelly]

Ping

[ckennelly]

Ping

[ckennelly]

Ping

[ckennelly]

Ping

[ckennelly]

Ping

[ldionne]

In D90111#2485054, @ckennelly wrote:

Ping

We've been working hard to fix some underlying issues in std::shared_ptr first. We'll do that, and then get to this patch. It's on my radar, don't worry, it's just that the mess we needed to fix before we get to this ended up being larger than expected.

[ckennelly]

Any updates on the shared_ptr work?

[ldionne]

In D90111#2590844, @ckennelly wrote:

Any updates on the shared_ptr work?

It's coming together. The review is basically done but Zoe's trying to figure out a few CI failures related to the change. If you wanted, you could probably cherry-pick https://reviews.llvm.org/D62641 locally and base your work on top of that.

[ldionne]

@mumbleskates @ckennelly Late is better than never. I think this should be unblocked if we want to implement the full paper now.

[ldionne]

Supersedded by D140913, closing.

libcxx/include/memory

 {
     bad_weak_ptr() noexcept;
 };
 
 template<class T, class... Args> unique_ptr<T> make_unique(Args&&... args);     // C++14
 template<class T>                unique_ptr<T> make_unique(size_t n);           // C++14
 template<class T, class... Args> unspecified   make_unique(Args&&...) = delete; // C++14, T == U[N]
 
+template<class T> unique_ptr<T> make_unique_for_overwrite();     // C++20
+template<class T> unique_ptr<T> make_unique_for_overwrite(size_t n);           // C++20
+template<class T, class... Args> unspecified   make_unique_for_overwrite(Args&&...) = delete; // C++20, T == U[N]
+
 template<class E, class T, class Y, class D>
     basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, unique_ptr<Y, D> const& p);
 
 template<class T>
 class shared_ptr
 {
 public:
     typedef T element_type;
make_unique(size_t __n)
     typedef typename remove_extent<_Tp>::type _Up;
     return unique_ptr<_Tp>(new _Up[__n]());
 }
 
 template<class _Tp, class... _Args>
     typename __unique_if<_Tp>::__unique_array_known_bound
     make_unique(_Args&&...) = delete;
 
+#if _LIBCPP_STD_VER > 17
+
+template<class _Tp>
+inline _LIBCPP_INLINE_VISIBILITY
+typename __unique_if<_Tp>::__unique_single
+make_unique_for_overwrite()
+{
+    return unique_ptr<_Tp>(new _Tp);
+}
+
+template<class _Tp>
+inline _LIBCPP_INLINE_VISIBILITY
+typename __unique_if<_Tp>::__unique_array_unknown_bound
+make_unique_for_overwrite(size_t __n)
+{
+    typedef typename remove_extent<_Tp>::type _Up;
+    return unique_ptr<_Tp>(new _Up[__n]);
+}
+
+template<class _Tp, class... _Args>
+    typename __unique_if<_Tp>::__unique_array_known_bound
+    make_unique_for_overwrite(_Args&&...) = delete;
+
+#endif  // _LIBCPP_STD_VER > 17
+
 #endif  // _LIBCPP_STD_VER > 11
 
 template <class _Tp, class _Dp>
 #ifdef _LIBCPP_CXX03_LANG
 struct _LIBCPP_TEMPLATE_VIS hash<unique_ptr<_Tp, _Dp> >
 #else
 struct _LIBCPP_TEMPLATE_VIS hash<__enable_hash_helper<
     unique_ptr<_Tp, _Dp>, typename unique_ptr<_Tp, _Dp>::pointer> >

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.create/make_unique_for_overwrite.array.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+#include <memory>
+#include <string>
+#include <cassert>
+
+#include "test_macros.h"
+
+struct InitializationVerifier {
+  static constexpr int kValue = 0x24;
+
+  static void* operator new(size_t n) = delete;
+
+  static void* operator new[](size_t n) {
+    void* ret = ::operator new[](n);
+    memset(ret, kValue, n);
+    return ret;
+  }
+
+  int a;
+  int b;
+};
+
+//    The only way to create an unique_ptr<T[]> is to default construct them.
+
+class foo {
+public:
+  foo() : val_(3) {}
+  int get() const { return val_; }
+
+private:
+  int val_;
+};
+
+int main(int, char**) {
+  {
+    auto p1 = std::make_unique_for_overwrite<InitializationVerifier[]>(5);
+    int pattern;
+    memset(&pattern, InitializationVerifier::kValue, sizeof(pattern));
+    for (int i = 0; i < 5; ++i) {
+      assert(p1[i].a == pattern);
+      assert(p1[i].b == pattern);
+    }
+  }
+
+  {
+    auto p2 = std::make_unique_for_overwrite<std::string[]>(5);
+    for (int i = 0; i < 5; ++i)
+      assert(p2[i].size() == 0);
+  }
+
+  {
+    auto p3 = std::make_unique_for_overwrite<foo[]>(7);
+    for (int i = 0; i < 7; ++i)
+      assert(p3[i].get() == 3);
+  }
+
+  return 0;
+}

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.create/make_unique_for_overwrite.array1.compile.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+#include <memory>
+#include <string>
+
+int main(int, char**) {
+  auto up1 = std::make_unique_for_overwrite<std::string[]>(
+      "error"); // expected-error@-1 {{no matching function for call to 'make_unique_for_overwrite'}}
+
+  return 0;
+}

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.create/make_unique_for_overwrite.array2.compile.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+#include <memory>
+
+int main(int, char**) {
+  auto up2 = std::make_unique_for_overwrite<int[]>(
+      10, 20, 30,
+      40); // expected-error@-2 {{no matching function for call to 'make_unique_for_overwrite'}}
+
+  return 0;
+}

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.create/make_unique_for_overwrite.array3.compile.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+#include <memory>
+
+int main(int, char**) {
+  std::make_unique_for_overwrite<
+      int[5]>(); // expected-error@-1 {{call to deleted function 'make_unique_for_overwrite'}}
+
+  return 0;
+}

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.create/make_unique_for_overwrite.array4.compile.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+#include <memory>
+
+int main(int, char**) {
+  std::make_unique_for_overwrite<int[5]>(
+      11, 22, 33, 44,
+      55); // expected-error@-2 {{call to deleted function 'make_unique_for_overwrite'}}
+
+  return 0;
+}

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.create/make_unique_for_overwrite.single.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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, c++17
+#include <memory>
+#include <string>
+#include <cassert>
+
+#include "test_macros.h"
+
+struct InitializationVerifier {
+  static constexpr int kValue = 0x42;
+
+  static void* operator new(size_t n) {
+    void* ret = ::operator new(n);
+    memset(ret, kValue, n);
+    return ret;
+  }
+
+  static void* operator new[](size_t n) = delete;
+
+  int a;
+  int b;
+};
+
+int main(int, char**) {
+  {
+    std::unique_ptr<InitializationVerifier> p1 =
+        std::make_unique_for_overwrite<InitializationVerifier>();
+
+    int pattern;
+    memset(&pattern, InitializationVerifier::kValue, sizeof(pattern));
+    assert(p1->a == pattern);
+    assert(p1->b == pattern);
+  }
+
+  {
+    std::unique_ptr<std::string> p2 =
+        std::make_unique_for_overwrite<std::string>();
+    assert(*p2 == "");
+  }
+
+  return 0;
+}

libcxx/www/cxx2a_status.html

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   <tr><td><a href="https://wg21.link/P1006R1">P1006R1</a></td><td>LWG</td><td>Constexpr in std::pointer_traits</td><td>San Diego</td><td>Complete</td><td>8.0</td></tr>
   <tr><td><a href="https://wg21.link/P1007R3">P1007R3</a></td><td>LWG</td><td><tt>std::assume_aligned</tt></td><td>San Diego</td><td><i> </i></td><td></td></tr>
-  <tr><td><a href="https://wg21.link/P1020R1">P1020R1</a></td><td>LWG</td><td>Smart pointer creation with default initialization</td><td>San Diego</td><td><i> </i></td><td></td></tr>
+  <tr><td><a href="https://wg21.link/P1020R1">P1020R1</a></td><td>LWG</td><td>Smart pointer creation with default initialization</td><td>San Diego</td><td><i>In Progress</i></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1032R1">P1032R1</a></td><td>LWG</td><td>Misc constexpr bits</td><td>San Diego</td><td><i> </i></td><td></td></tr>
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+  <tr><td><a href="https://wg21.link/P1973">P1973</a></td><td>LWG</td><td>Rename "_default_init" Functions, Rev1</td><td>Prague</td><td><i>In Progress</i></td><td></td></tr>
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