[libcxx-commits] [libcxx] [String] Allow fancy pointer as pointer type of basic_string allocator (PR #191023)

[Nikita Belenkiy via libcxx-commits]

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@@ -444,6 +445,235 @@ TEST_CONSTEXPR_CXX20 thread_unsafe_shared_ptr<T> make_thread_unsafe_shared(Args.
 }
 } // namespace detail
 
+template <class T>
+class fancy_pointer {
+public:
+  // For the std::pointer_traits interface.
+  using pointer         = T*;
+  using element_type    = T;
+  using difference_type = std::ptrdiff_t;
+
+  template <class U>
+  using rebind = fancy_pointer<U>;
+
+  // For the std::iterator_traits interface.
+  using value_type        = typename std::remove_cv<T>::type;
+  using reference         = typename std::add_lvalue_reference<T>::type;
+  using iterator_category = std::random_access_iterator_tag;
+
+  TEST_CONSTEXPR_CXX14 fancy_pointer() TEST_NOEXCEPT : ptr_(nullptr) {}
+  TEST_CONSTEXPR_CXX14 fancy_pointer(T* p) TEST_NOEXCEPT : ptr_(p) {}
+
+  template <typename T2>
+  TEST_CONSTEXPR_CXX14 explicit fancy_pointer(const fancy_pointer<T2>& other) TEST_NOEXCEPT
+      : ptr_(static_cast<T*>(other.get())) {}
+
+  TEST_CONSTEXPR_CXX14 operator T*() const TEST_NOEXCEPT { return ptr_; }
+  TEST_CONSTEXPR_CXX14 fancy_pointer(fancy_pointer&&) TEST_NOEXCEPT                 = default;
+  TEST_CONSTEXPR_CXX14 fancy_pointer(const fancy_pointer&) TEST_NOEXCEPT            = default;
+  TEST_CONSTEXPR_CXX14 fancy_pointer& operator=(fancy_pointer&&) TEST_NOEXCEPT      = default;
+  TEST_CONSTEXPR_CXX14 fancy_pointer& operator=(const fancy_pointer&) TEST_NOEXCEPT = default;
+  TEST_CONSTEXPR_CXX20 ~fancy_pointer() TEST_NOEXCEPT                               = default;
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kitsnet wrote:

Should we also make sure that basic_string works correctly with non-trivial destructors?

Anyway, even non-trivial but still equivalent to defaulted are not enough for proper testing of fancy pointers like offset_ptr. We will need to add tests showing that the special member functions of fancy pointers that change their value representation on copying are actually called. Which means we need either real offset_ptr (and then we may be limited to the choice of platforms where they work) or an object with a copy counter (and then we need to rely on the knowledge of the implementation to estimate the correct values of the counter).

https://github.com/llvm/llvm-project/pull/191023