C使用基于范围的指针循环和向量时的错误[重复]

Question

在处理指针向量时，我偶然发现了一个奇怪的bug：

mwe.cpp代码：

#include <iostream>
#include <vector>

class A {
  public:
    int x = 42; // making this unsigned causes output to be only positive
    int y;      // removing this makes bug disappear
};

class B {
  public:
    B(std::vector<A *> pt_vec) {
      std::cout << pt_vec[0]->x << std::endl;
    };
};

int main() {
  std::vector<A *> pt_vec;

  A a;                 //  these 3 lines can
  std::vector<A> vec;  //  be replaced with
  vec.push_back(a);    //  std::vector<A> vec {A()};
                       //  the bug will still appear

  for (A el: vec) {         // doing this without for (or
    pt_vec.push_back(&el);  // with iterators/good ol' for)
  }                         // makes bug disappear

  B b = B(pt_vec);

  //exit(1);    // uncommenting this makes bug disappear
}

如何复制：

$ gcc --version
gcc (Gentoo 9.2.0-r2 p3) 9.2.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ -O0 -Wall -Wpedantic mwe.cpp -o mwe
mwe.cpp: In function ‘int main()’:
mwe.cpp:27:5: warning: variable ‘b’ set but not used [-Wunused-but-set-variable]
   27 |   B b = B(pt_vec);
      |     ^

（ARCH上的GCC 9.2.0也复制了这个Bug，但Apple clang 11.0.0给出了预期的输出）

预期产出：

$ ./mwe
42

实际产量：

$ ./mwe 
1533476528
$ ./mwe
-1607700816
$ ./mwe
<insert random huge number here>

我是否无意中在某处引入了未定义行为？如果没有，为什么这没有按预期工作？

Answer 1

for（A el： vec）意味着您在迭代时正在创建vec中每个元素的副本。

您要将用于（A