vector push_back calling copy_constructor more than once?

Question

I am a bit confused with the way vector push_back behaves, with the following snippet I expected the copy constructor to be invoked only twice, but the output suggest otherw

Answer 1

You got it...it was the resizing. But I'll just point out that if you're doing some bean counting on your constructors, you might be interested in "emplacement":

#include 
#include 
using namespace std;

class Myint
{
private:
    int my_int;
public:
    explicit Myint(int value = 0) : my_int(value) 
    {
        cout << "Inside default " << endl;
    }
    Myint(const Myint& x) : my_int(x.my_int)
    {
        cout << "Inside copy with my_int = " << x.my_int << endl;
    }
    Myint(const Myint&& x) noexcept : my_int(x.my_int) {
        cout << "Inside move with my_int = " << x.my_int << endl;
    } 
};

int main() {
    vector myints;
    myints.reserve(2);

    myints.emplace_back(0);
    myints.emplace_back(1);

    // your code goes here
    return 0;
}

That should give you:

Inside default 
Inside default

And, due to the noexcept on the move constructor...if you delete the reserve you'd get a move, not a copy:

Inside default 
Inside default 
Inside move with my_int = 0

There's no real advantage with this datatype of a move over a copy. But semantically it could be a big difference if your data type was more "heavy weight" and had a way of "moving" its members that was more like transferring ownership of some pointer to a large data structure.