How is the capacity of std::vector determined

Question

I know that in std::vector, the size will grow every time it runs out of room. Yet I'm not noticing a pattern in how it grows. Can someone please explain to me the pattern and why it was chosen.

#include <iostream>
using namespace std;

#include <iostream>
#include <vector>
int main()
{
    vector<int> myVector;
    for(int i =0 ; i < 100; ++i)
    {
        myVector.push_back(i);
        cout << myVector.capacity();
    cout << ", ";
    }
}

Result:

1, 2, 3, 4, 6, 6, 9, 9, 9, 13, 13, 13, 13, 19, 19, 19, 19, 19, 19, 28, 28, 28, 2
8, 28, 28, 28, 28, 28, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 42, 6
3, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 6
3, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 9
4, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 94, 141, 141, 141, 141, 141, 141

Answer 1

It depends on the implementation, so don't expect the same pattern when you switch the operating system, the compiler, etc.

The most common growth patterns are 1.5 * previous_capacity and 2 * previous_capacity. In your example, it seems that it's the former.

See https://stackoverflow.com/a/1100426/784668 for a possible explanation for why that factor was chosen. The point is that it apparently allows reusing free memory blocks that were previously used for storing the array.

Answer 2

It is an implementation detail, you are not supposed co care about it. In your case it seems to be something like

i += i/2

but somewhat more complex.