overloading operator+ confusion

Hi
Can someone explain me how this code works ?

#include <iostream>
using namespace std;
class test{
private:
	int num;
	int den;
public:
	test();
	test(int);
	test(int,int);
	void print();
	friend test operator+(const test&,const test&);
};

test::test(){}

test::test(int nnum) {
	num=nnum;
	den=1;
}

test::test(int nnum,int nden){
	num=nnum;
	den=nden;
}
void test::print(){
	cout<<"num: "<<num<<"  den: "<<den;
}
test operator+(const test& one,const test& two) {
	test toreturn(one.num+two.num,one.den+two.den);
	return toreturn;
}
	
int main() {
	test obj1(5);
	test obj2;
	obj2=obj1+1; //here
	obj1.print();
	cout<<endl;
	obj2.print();


	cin.ignore();
	return 0;
}

At line 37,class test hasn't got a operator+ with parameter test and int,also

test operator+(const test&,int)

.
How does this work? The class has just one operator+ that takes two objects.Is the int converted to a test object? How?
If the class wouldn't have a constructor which has as parameters a int,the code wouldn't work...
Also is the constructor used here ? :| How does it apply ? Is there a rule or something ?

Thank you,have a nice evening