C++: Elements just below the constructor call of a class

Question

In my teacher’s code, he passed the following class template, with the constructor defined below:

template<typename T>
class CircularBuffer {
    std::vector<T> _elements;
    size_t _first, _n;
public:
    CircularBuffer(size_t n);
    void push(T v);
    T front() const;
    void pop();
    size_t size() const;
};

template<typename T>
CircularBuffer<T>::CircularBuffer(size_t n)
    : _elements(n), _first(0), _n(0) //?
{
}

I would like to understand what is and the usefulness of the line of code I marked with a comment and question mark

Answer 1

The class attributes _first and _n are being initialized with the value 0.

The attribute _elements, it is a std::vector and is being built with initial size n, through one of its non-standard builders.

All this happens before the execution of the CircularBuffer and in the order in which they were specified.