意外C++队列行为，弹出后丢失对象

Question

queue<vector<int>>

的行为不符合预期。我似乎无法访问pop()后对vector的引用.

#include <vector>
#include <cstdio>
#include <queue>
#include <iostream>
using namespace std;
struct TreeNode {
string val;
TreeNode(string x) : val(x) {}
};
int main() {
queue<int> q = {};

q.push(1);
q.push(2);
int& test = q.front();
q.pop();
// queue<int>; front() and then pop() behaves as expected
cout << "test1 " << test  << " expect: 1" << endl;
TreeNode n1("node a");
TreeNode n2("node b");
queue<TreeNode> q2 = {};

q2.push(n1);
q2.push(n2);
TreeNode& test2 = q2.front();
q2.pop();
// queue<TreeNode>; front() and then pop() behaves as expected    
cout << "test2 " << test2.val  << " expect: node b" << endl;

vector<int> v1 = {0,1,2};
vector<int> v2 = {2,3,4};
queue<vector<int>> q3 = {};
q3.push(v1);
q3.push(v2);
vector<int>& test3 = q3.front();
// front() alone returns what I expected
cout << "test3 size " << test3.size() << " expect: size 3" << endl;
vector<int>& test4 = q3.front();
q3.pop();
// however front() and then pop() does not behave as expected    
cout << "test4 size " << test3.size() << " expect: size 4" << endl;
return 0;
}

输出：

test1 1 expect: 1
test2 node a expect: node b
test3 size 3 expect: size 3
test4 size 0 expect: size 4
Process finished with exit code 0

问题：

上面的例子有没有代码异味？我应该总是期望在pop()后丢失引用吗？我应该永远不要在pop()后使用引用吗？

还是vector特例？

EDIT: knowing that dangling reference is always bad practice. I made some changes to the code and now have some follow up questions.

后续问题：

queue<int> q = {};
q.push(1);
q.push(2);
// making a copy here
// follow up question 1: is this now correct?
int test = q.front();
q.pop();

vector<int> v1 = {0,1,2};
vector<int> v2 = {2,3,4};
queue<vector<int>> q3 = {};
q3.push(v1);
q3.push(v2);
// I am trying to make a copy here but the compiler complains:
// Parameter type mismatch: expression must be rvalue
// follow up question 2: why can't I make a copy of the vector but I can make a copy of the int in the previous example?
vector<int> test3 = q3.front();
q3.pop()

Answer 1

您正在存储对对象的引用，然后销毁该对象。将其从队列中pop后，它就不再在队列中。要么创建对象的副本，要么在完成之前不要pop它。

同样，永远不要尝试访问不再存在的对象。结果将是不可预测的。