11 destructor and 2 constructor for 5 objects???

Question

#include <iostream>
#include <vector>
using namespace std;
class A {
public:
static int cnt;
int a;
A(int a)
{
A::a = a;
cout << "I am a " << cnt << " constructor!" << endl;
}
~A()
{
++cnt;
cout << "I am a " << cnt << " destructor!" << endl;

}
};
int A::cnt = 0;
int main()
{
vector<A> v(4, 1);
v.push_back(1);
cout << A::cnt << endl;

return 0;
}

This code ouputs:
I am a 0 constructor!
I am a 1 destructor!
I am a 1 constructor!
I am a 2 destructor!
I am a 3 destructor!
I am a 4 destructor!
I am a 5 destructor!
I am a 6 destructor!
6
I am a 7 destructor!
I am a 8 destructor!
I am a 9 destructor!
I am a 10 destructor!
I am a 11 destructor!

Why do we have so many destructors and only 2 constructors??

Answer 1

I didn't, it is from chellenge

Answer 2

thanks a lot, that should be usefull

Answer 3

one thing I know is when you use combine vector assignment then compiler will create one object temperary and use copy constructor to construct object you have specified.

Run below code to verify.

#include <iostream>
#include <vector>
using namespace std;
class A {
public:
static int cnt;
int a;
A(int a)
{
this->a = a;
cnt++;
cout << "I am a " << cnt << " constructor!" << endl;
}
~A()
{
++cnt;
cout << "I am a " << cnt << " destructor!" << endl;

}
};
int A::cnt = 0;
int main()
{
vector<A> v(1, 1);
cout << v.size()<< endl;

return 0;
}

Answer 4

hey, first of all you should keep in mind that for optimization purpose compiler create one object and use copy of that to construct others.

vector capacity is responsible for multiple destructor and constructor. Here I added some code to understand what is happening.

if count the constructor in output then it is 11.
https://code.sololearn.com/cht4WW98g0ot/?ref=app