[libc++] Implement P1072R10 (std::basic_string::resize_and_overwrite)

Reviewed By: Quuxplusone, #libc, Mordante

Spies: mzeren-vmw, ckennelly, arichardson, ldionne, Mordante, libcxx-commits, Quuxplusone

Differential Revision: https://reviews.llvm.org/D113013

libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.pass.cpp

@@ -0,0 +1,105 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++20
+
+// <string>
+
+// template<class Operation>
+// void resize_and_overwrite(size_type n, Operation op)
+
+#include <cassert>
+#include <string>
+
+#include "make_string.h"
+#include "test_macros.h"
+
+template <class S>
+constexpr void test_appending(size_t k, size_t N, size_t new_capacity) {
+  assert(N > k);
+  assert(new_capacity >= N);
+  auto s = S(k, 'a');
+  s.resize_and_overwrite(new_capacity, [&](auto* p, auto n) {
+    assert(n == new_capacity);
+    LIBCPP_ASSERT(s.size() == new_capacity);
+    LIBCPP_ASSERT(s.begin().base() == p);
+    assert(std::all_of(p, p + k, [](const auto ch) { return ch == 'a'; }));
+    std::fill(p + k, p + n, 'b');
+    p[n] = 'c'; // will be overwritten
+    return N;
+  });
+  const S expected = S(k, 'a') + S(N - k, 'b');
+  assert(s == expected);
+  assert(s.c_str()[N] == '\0');
+}
+
+template <class S>
+constexpr void test_truncating(size_t o, size_t N) {
+  assert(N < o);
+  auto s = S(o, 'a');
+  s.resize_and_overwrite(N, [&](auto* p, auto n) {
+    assert(n == N);
+    LIBCPP_ASSERT(s.size() == n);
+    LIBCPP_ASSERT(s.begin().base() == p);
+    assert(std::all_of(p, p + n, [](auto ch) { return ch == 'a'; }));
+    p[n - 1] = 'b';
+    p[n] = 'c'; // will be overwritten
+    return n;
+  });
+  const S expected = S(N - 1, 'a') + S(1, 'b');
+  assert(s == expected);
+  assert(s.c_str()[N] == '\0');
+}
+
+template <class CharT>
+constexpr bool test() {
+  using S = std::basic_string<CharT>;
+  test_appending<S>(10, 15, 15);
+  test_appending<S>(10, 15, 20);
+  test_appending<S>(10, 40, 40);
+  test_appending<S>(10, 40, 50);
+  test_appending<S>(30, 35, 35);
+  test_appending<S>(30, 35, 45);
+  test_appending<S>(10, 15, 30);
+  test_truncating<S>(15, 10);
+  test_truncating<S>(40, 35);
+  test_truncating<S>(40, 10);
+
+  return true;
+}
+
+void test_value_categories() {
+  std::string s;
+  s.resize_and_overwrite(10, [](char*&, size_t&) { return 0; });
+  struct RefQualified {
+    int operator()(char*, size_t) && { return 0; }
+  };
+  s.resize_and_overwrite(10, RefQualified{});
+}
+
+int main(int, char**) {
+  test<char>();
+  test<char8_t>();
+  test<char16_t>();
+  test<char32_t>();
+
+#if defined(__cpp_lib_constexpr_string) && __cpp_lib_constexpr_string >= 201907L
+  static_assert(test<char>());
+  static_assert(test<char8_t>());
+  static_assert(test<char16_t>());
+  static_assert(test<char32_t>());
+#endif
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#if defined(__cpp_lib_constexpr_string) && __cpp_lib_constexpr_string >= 201907L
+  static_assert(test<wchar_t>());
+#endif
+#endif
+  return 0;
+}