Boost Circular Buffer push_back inserts data at front

Question

I have below code:

#include "boost/circular_buffer.hpp"

int main()
{
    // Create a circular buffer with a capacity for 3 integers.
    boost::circular_buffer<int> cb(3);

    // Insert threee elements into the buffer.
    cb.push_back(1);


    int a = cb[0];  // a == 1

}

As per documentation push_back API means "Insert a new element at the end of the circular_buffer" - then how cb[0] is 1? Shouldn't cb[2] should store value 1?

=======================================UPDATE=============================

// Insert threee elements into the buffer.
cb.push_front(11);
cb.push_back(2);
cb.push_back(3);

int a = cb[0];  // a == 11
int b = cb[1];  // b == 2
int c = cb[2];  // c == 3

It seems that in case there are 0 elements in an buffer it takes up the first but if the same has one then it puts it at the end

Answer 1

No. The buffer has capacity for three elements, but only contains one.

When you do your first push_back, you push to the end of a series of zero elements. So the new element becomes the first.

If you want to fill it up completely on construction, you can do that! Just like you would with a vector:

boost::circular_buffer<int> cb(3, 0);

But then, your first push_back will still alter cb[0], because it's a circular buffer! That means it wraps around to the front when full. cb[3] is out of range. There is no cb[3].