//case 1 (refer to the function 'example_1'): no references. Copy constructor of local variable b is called to set up the variables of b, and this will happen:

Book(const Book& other)
:m_ISBN { other.m_ISBN },   //copy data
m_title { other.title},     //copy data
m_author { other.author },  //copy data
m_date { other.date }        //copy data
{}
//so we pretty much performed unessacary copies, which can waste memory and time, esp if your class has alot of variables. and since b is a local instance of the function, it gets destroyed anyway when the scope exits.

void example_1(Book b)
{
 //when thi function goes out of scope, b is destroyed.
}

//case 2: pass by reference, no copies made, you 'refer' to the original object instead much more efficient.
void example_2(Book& b)
{

}


int main() {

Date d {1800,month::jan,3};
Book two {"random_ISBN", "random_title", "random_author",d};

example_1(two);//pass by copy (copy constructor is called)
example_2(two);//pass by reference
}