Wrong destructor called by vector erase - c++

Question

I have a vector with some objects.
I noticed that when I remove one element from vector with erase method, I get the destructor call to the wrong element (always to the last element).
Here a minimal example that produce a bad output.
// Example program
#include <iostream>
#include <string>
#include <vector>
using namespace std;
class Test {
public:
Test(string value) {
_value = value;
cout << "Constructor:" << _value << endl;
}
Test(const Test &t) {
_value = t._value;
cout << "Copied:" << _value << endl;
}
~Test() {
cout << "Destructor:" << _value << endl;
}
string print() {
return _value;
}
string _value;
};
int main()
{
vector<Test> vec;
vec.reserve(3);
cout << "Creation" << endl << endl;
vec.push_back(Test("1"));
vec.push_back(Test("2"));
vec.push_back(Test("3"));
cout << endl << "Deleting" << endl << endl;
vec.erase(vec.begin()); // Here is called the element with value "3"
vec.erase(vec.begin()); // Here is called the element with value "3"
cout << endl << "Log" << endl << endl;
// But the final log print "3"
for (unsigned i = 0; i < vec.size(); i++) {
cout << vec[i].print()<<endl;
}
return 0;
}
The output is:
Creation
Constructor:1
Copied:1
Destructor:1
Constructor:2
Copied:2
Destructor:2
Constructor:3
Copied:3
Destructor:3
Deleting
Destructor:3 <-- WRONG, NEED TO BE 1
Destructor:3 <-- WRONG, NEED TO BE 2
Log
3
Destructor:3
I would solve this problem without change the container vector.

Answer 1

vec.erase(vec.begin()); does not destruct the first element. It overwrites it by shifting all of the subsequent ones by one place, using either the move- or copy-assignment operator. What remains of the last element after it has been moved from is then destructed, which is what you're observing.