std::vector and copy constructors

Question

vector<X> v;
X x;
v.push_back(x); v.push_back(x); v.push_back(x);

Why this code calls the copy constructor of a class X 6 times? (using g++ 4.7.2 STL)

Please, I'd like to know it precisely what happens under hood with this particular STL.

Answer 1

When you insert x with push_back(), the memory is reallocated eventually to make room for the new element. The already inserted members must then be copied around using the copy constructor X(const X&).

If you insert

v.reserve(3);

reallocation is prevented for at least the first three push_back()s and, as a result, there will be only three calls to X(const X&)

Answer 2

You can use vector reserve to create space in the vector before hand to speed up adding elements to a vector as well as stopping this from happening.