Please help with this c++ vector<T>-question :-)

Question

Hey fellow learners,
a few hours ago I stumbled upon this question in a c++ challenge and I just can figure out why the answer is what it is. Maybe one of you c++-wizards would care to enlighten me ;-)

//assume necessary includes

class A {
public:
static int cnt;
A(int a){}
~A() {
++cnt;
}
};
int A::cnt = 0;

int main() {
vector<A> v(5,1);
cout << A::cnt << endl; // output is 1
return 0;
}

As I understand the vector v is created with 5 fields for A-objects, that are each instantiated with the value 1. As the static cnt is only incremented on object-destruction, I can't understand why it's output is 1 on the second line of main(). Can anyone please explain why?

Thx so much.

- code artist

Answer 1

It is just my guess, but I think that this is how it works in this constructor:

vector(size, value)
{
T tmp(value);
for(int i = 0; i < size; ++i)
{
this->push_back(tmp);
}
}

As you can see we create one temporary object which we copy 5 times to our vector. Then this temporary object is destroyed incrementing our static value.
It's just a wild guess, it could be a lot simplier solution there, but I cannot really think of one other than that.

Answer 2

Thanks Jakub, I think you're right.
After a little bit more digging I found a decent decription of the vector-constructor used in the example. Turns out it's exactly as you guessed.
The 2nd param is used to create a temp-object that is then copied to all the fields in the vector and destroyed afterwards, hence the dtor-call :-)