STL列表迭代器不会更新我的对象 [英] STL list iterator won't update my object

问题描述

我使用list iterator将Pets的所有年龄设置为1，但更改不会在for循环之外持续：

I am using a list iterator to set all the ages of the Pets to 1 but the change won't persist outside the for loop:

#include <iostream>
#include <stdio.h>
#include <list>

using namespace std;

class Pet{
  public:
  int age;
};

class Person{
  public:
  list<Pet> pets;
};


int main(int argc, char **argv) {
  Person bob;
  Pet p1;
  p1.age = 0;
  bob.pets.push_back(p1);

  cout << "Start with: "<<p1.age << endl;

  std::list<Pet>::iterator itPet;
  for (itPet = bob.pets.begin(); itPet != bob.pets.end(); ++itPet) {
    Pet p = (*itPet);
    p.age = 1;
    cout << "Right after setting to 1: "<<p.age << endl;
  }

  cout << "After the for loop: "<<p1.age << endl;
  return 0;
}

输出：

Start with: 0
Right after setting to 1: 1
After the for loop: 0

为什么p1没有更新？如果不是p1又更新了什么？

Why is p1 not updated? And what has been updated if not p1?

谢谢！

推荐答案

您只需修改副本：声明

Pet p = (*itPet);

将 * itPet 的值复制到 p 然后会更新。您可以使用以下代码验证迭代器使用的对象：

copies the value of *itPet to p which then gets updated. You can verify that the object used by the iterator by using this code:

p.age = 1;
cout << "Right after setting to 1: p.age="<<p.age << " itPet->age=" << itPet->age << '\n';

您想给我们一个参考：

Pet& p = *itPet;

用于验证列表中的对象是否已更改的方法不起作用但是：标准C ++库容器会复制插入的对象，并且不会保留对原始对象的引用。也就是说， p1 不会被更改，但列表中的元素会被更改：

The approach you are using to verify whether the objects in the list are changed doesn't work, either, though: the standard C++ library containers make a copy of the objects inserted and don't keep a reference to the original object. That is, p1 won't be changed but the element in the list is changed:

for (std::list<Pet>::const_iterator it(bob.pets.begin()), end(bob.pets.end());
     it != end; ++it) {
    std::cout << "list after change: " << it->age << '\n';
}

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