Question

class item {
    int i;
};
vector<item> ls(3);   // line 1
item         i1(1);   // line 2

ls.push_back(i1);     // line 3

• line 1:
  • default ctr called once
  • copy ctr gets called 3 times
• line 3:
  • copy ctr gets called 4 times

My question is in line-3, what initial 3 copy ctr are doing?

Answer 1

vector ls(3);

Your Expectation:
You probably expected the default constructor for item to be called 3 times, Instead of the default constructor is being called once and the copy constructor 3 times.

What Actually happens:
It is creating a class item object and then copying it to each element of the vector. There are 3 elements and hence the 3 copy constructor calls.

Rationale:
One can not make any assumption how the elements are constructed by std::vector. The standard says only that there must be a default constructor and does not force the std::vector implementation to do anything else in specific in this regard.

---

ls.push_back(i1);

What Actually happens:
The vector was initially created for 3 elements so when you push a 4th element, the first first 3 elements have to be copied to new locations,so that the vector can accomodate the new element and yet have contigious allocation, this accounts for 3 copy constructor calls. The method push_back creates a copy of the elelemnt being added and then adds it to the vector, so that results in the 4th copy constructor call, In all this results in 4 copy constructor calls.