[libcxx-commits] [libcxx] [libc++][test] Refactor increasing_allocator (PR #115671)

[Peng Liu via libcxx-commits]

================
@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef TEST_SUPPORT_INCREASING_ALLOCATOR_H
+#define TEST_SUPPORT_INCREASING_ALLOCATOR_H
+
+#include <cstddef>
+#include <memory>
+
+#include "test_macros.h"
+
+// The increasing_allocator is a custom allocator that enforces an increasing minimum allocation size,
+// ensuring that it allocates an increasing amount of memory, possibly exceeding the requested amount.
+// This unique behavior is particularly useful for testing the shrink_to_fit functionality in std::vector,
+// vector<bool>, and std::basic_string, ensuring that shrink_to_fit does not increase the capacity of
+// the allocated memory.
+
+template <typename T>
+struct increasing_allocator {
+  using value_type         = T;
+  std::size_t min_elements = 1000;
+  increasing_allocator()   = default;
+
+  template <typename U>
+  TEST_CONSTEXPR_CXX20 increasing_allocator(const increasing_allocator<U>& other) TEST_NOEXCEPT
+      : min_elements(other.min_elements) {}
+
+#if TEST_STD_VER >= 23
+  TEST_CONSTEXPR_CXX23 std::allocation_result<T*> allocate_at_least(std::size_t n) {
+    if (n < min_elements)
+      n = min_elements;
+    min_elements += 1000;
+    return std::allocator<T>{}.allocate_at_least(n);
+  }
+#endif // TEST_STD_VER >= 23
+
+  TEST_CONSTEXPR_CXX20 T* allocate(std::size_t n) {
+#if TEST_STD_VER >= 23
+    return allocate_at_least(n).ptr;
+#else
----------------
winner245 wrote:

Maybe another way to avoid the size mismatch problem is to use operator new and delete, which eliminates the need to manually specify the deallocation size:

```cpp
template <typename T>
struct increasing_allocator {
  using value_type         = T;
  std::size_t min_elements = 1000;
  increasing_allocator()   = default;
  
#if TEST_STD_VER >= 23
  std::allocation_result<T*> allocate_at_least(std::size_t n) {
    if (n < min_elements)
      n = min_elements;
    min_elements += 1000;
    return {static_cast<T*>(::operator new(n * sizeof(T))), n};
  }
#endif // TEST_STD_VER >= 23

  T* allocate(std::size_t n) {
    return static_cast<T*>(::operator new(n * sizeof(T)));
  }

  void deallocate(T* p, std::size_t) TEST_NOEXCEPT { ::operator delete(p); }
};
```

https://github.com/llvm/llvm-project/pull/115671