How can it be that sub-class in compositioin is initialized in constructor initializer?

Question

How can the birthday class be initialized in Person class constructor although
there's no such Birthday constructor that has its object as a parameter?
-- please refer to the code bd(b) in Person class's constructor.
( Please anyone tell me the way how mechanism works )

class Person {
public:
Person(string n, Birthday b)
: name(n),
bd(b)
{
}
void printInfo()
{
cout << name << endl;
bd.printDate();
}
private:
string name;
Birthday bd;
};

Answer 1

you have created an object of birthday class inside person class.
Suppose your birthday class contains two variables
int date;
int month;
So, when you write bd(b), it invokes constructor of birthday class where you initialise date and month variables.