libcxx/test/std/utilities/utility/pairs/pairs.pair/ctor.brace-init.P1951.pass.cpp

   1 //===----------------------------------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 // REQUIRES: c++11 || c++14 || c++17 || c++20
  10
  11 // This test makes sure that we don't apply P1951 before C++23, since that is
  12 // a breaking change. The examples in this test are taken from Richard Smith's
  13 // comments on https://llvm.org/D109066.
  14
  15 #include <cassert>
  16 #include <utility>
  17 #include <vector>
  18
  19 struct A {
  20     int *p_;
  21     A(int *p) : p_(p) { *p_ += 1; }
  22     A(const A& a) : p_(a.p_) { *p_ += 1; }
  23     ~A() { *p_ -= 1; }
  24 };
  25
  26 int main(int, char**) {
  27     // Example 1:
  28     // Without P1951, we call the `pair(int, const A&)` constructor (the converting constructor is not usable because
  29     // we can't deduce from an initializer list), which creates the A temporary as part of the call to f. With P1951,
  30     // we call the `pair(U&&, V&&)` constructor, which creates a A temporary inside the pair constructor, and that
  31     // temporary doesn't live long enough any more.
  32     {
  33         int i = 0;
  34         auto f = [&](std::pair<std::vector<int>, const A&>) { assert(i >= 1); };
  35         f({{42, 43}, &i});
  36     }
  37
  38     // Example 2:
  39     // Here, n doesn't need to be captured if we call the `pair(const int&, const long&)` constructor, because
  40     // the lvalue-to-rvalue conversion happens in the lambda. But if we call the `pair(U&&, V&&)` constructor
  41     // (deducing V = int), then n does need to be captured.
  42     {
  43         const int n = 5;
  44         (void) []{ std::pair<int, long>({1}, n); };
  45     }
  46
  47     return 0;
  48 }