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

[Peng Liu via libcxx-commits]

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@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+// 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"
+
+#if TEST_STD_VER >= 23
+
+// increasing_allocator is a custom allocator that maintains an incrementing minimum allocation size,
+// requiring that each call to allocate_at_least allocate at least this minimum size, which may exceed
+// the requested amount. This unique design makes increasing_allocator 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>
+  constexpr increasing_allocator(const increasing_allocator<U>& other) noexcept : min_elements(other.min_elements) {}
+
+  constexpr 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);
+  }
+  constexpr T* allocate(std::size_t n) { return std::allocator<T>{}.allocate(n); }
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winner245 wrote:

This makes sense to me. I just double checked the `shrink_to_fit` implementation of `vector`. Prior to C++23, it used `allocate`, and since C++23 it uses  `allocate_at_least` . So by requiring both `allocate` and `allocate_at_least` to allocate the increasing amount of memory, we can make `increasing_allocator` more useful for testing both implementations of `shrink_to_fit`. 

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