[c++] Przeciążanie operatorów

#include <iostream>

class Book
{
private:
    int a;
    int b;
    int c;
public:
    Book();
    ~Book();
    Book operator +( Book & bo );
    void show();
   
};
Book::Book()
{
    a = 10;
    b = 20;
    c = a + b;
}
Book::~Book()
{
   
}
Book Book::operator +( Book & bo )
{
    Book bum = bo + bo;
    return bum;
}
void Book::show()
{
    std::cout << c << std::endl;
}

int main()
{
    Book bo, ok, bk;
    bk = bo + ok;
    bk.show();
   
    std::cin.get();
    std::cin.get();
    return 0;
}

Book Book::operator +( Book & bo )
{
    Book bum;
    bum.c = bo.a + bo.b;
    return bum;
}