Difference betwen string and int when using std::allocator C++ - c++

Question

I'm trying to learn how std::allocator works and I tried to do a simple task. The task is to delete for example the second element and to shift the elements to the left when the elements has been deleted.
For example, we have this as an input array: 1,2,3 and the output should look like 1,3. And what I get as input is: 1,3,3
This thing doesn't happen and that's why I'm here asking you.
But, when I have **std::allocator<string> myVar** instead of **std::allocator<int> myVar** it works.
Then the input: one, two, three and output is: one, three
Here's the code using std::allocator<int>:
#include <iostream>
#include <memory>
using namespace std;
int main()
{
allocator<int> a1;
int* arr = a1.allocate(3);
for (int i = 0; i < 3; i++)
a1.construct(arr + i, i + 1);
a1.destroy(arr + 1);
a1.construct(arr + 1, 3);
for (int i = 0; i < 3; i++)
cout << arr[i] << " ";
cout << endl;
a1.deallocate(arr, 3);
return 0;
}
And here's the code with std::allocator<string>:
#include <iostream>
#include <memory>
#include <string>
using namespace std;
int main()
{
allocator<string> a1;
string* wrd = a1.allocate(3);
a1.construct(wrd, "one");
a1.construct(wrd + 1, "two");
a1.construct(wrd + 2, "three");
a1.destroy(wrd + 1);
cout << wrd[0] << " " << wrd[1] << " " << wrd[2] << endl;
a1.deallocate(wrd, 3);
return 0;
}

Answer 1

When you call allocator::destroy, it just destroys the object in the memory -- it doesn't do anything to the memory (its still there) or move anything. When you later try to do somthing with that memory, you get undefined behavior, but in the case of your strings, the "undefined behavior" turns out to be "act as if it was an empty string", so nothing gets printed.
If you were to call a1.construct(wrd+1, "three"); after the call to a1.destroy ins your string code (to make it the same as your int code), you would see it prints one three three