I first declare a vector of string called test. Then I push_back string Hello and World and let a be a reference of test[0].And then I push_back a into test. However , I printed a before and after push_back respectively and observed that a became nothing after pushed into test. Why a becomes nothing? How does the vector work while pushing back a reference (a) of its element to itself ? Does that mean a is no longer a reference of test[0] ?
Thanks .

Remarks: If I push_back test[0] , a also becomes nothing. But test[0] is still "Hello".

#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main()
{
     vector<string> test;
     test.push_back("Hello");
     test.push_back("World");
     string& a = test[0];
     cout << "1"<< a << "\n";
     test.push_back(a);          //or : test.push_back(test[0]);
     cout << "2"<< a << "\n";
}

Live Demo

output:

1Hello
2

Update:

I got it ,thanks to answers and comments below. I printed the size and capacity of test and observed that they are both 2 . When test.push_back(a)is performed , the vector test allocates new memory and copy its old elements to the new memory . Thus a , the reference of it old elements , become undefined.

Here is the similar code using reserve . I think the reason why a becomes undefined is same as my original question. (Let me know if I'm wrong.)

#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main()
{
     vector<string> test;
     test.push_back("Hello");
     test.push_back("World");
     string& a = test[0];
     cout << "1"<< a << "\n";
     cout << "size:" << test.size() << " capacity:" <<test.capacity() <<"\n";
     test.reserve(3);
     cout << "2"<< a << "\n";
}

output:

1Hello
size:2 capacity:2
2

test.push_back(a);          //or : test.push_back(test[0]);

Adds a copy of a as the last element of test. The fact that a is a reference to an element in test is not relevant at all.

After that call, a could be dangling reference. Using it as you have in the line

 cout << "2"<< a << "\n";

is cause for undefined behavior.

test[0], on the other hand, returns a reference to the first element of test. It could be a reference to a different object than what a references.

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