Default-Constructor when splitting file - c++

Question

got a quick question. Why does the first programm work, but not the one when i split it into header/source file? How can i fix this? (error is that there is no default-constructor for "Stud s [line 6 main.cpp]")
Here is my code:
The working one:
#include <iostream>
#include <string>
using namespace std;
class Stud {
private:
string pren, surn;
unsigned int number;
public:
Stud(string p = "", string s = "", unsigned int n = 0) : pren(p), surn(s), number(n)
{
return;
}
friend ostream& operator<<(ostream& os, Stud& st) {
os << st.number << ", " << st.pren << " " << st.surn;
return os;
}
};
int main() {
Stud s;
Stud test("x", "y", 123);
cout << s << endl;
cout << test << endl;
}
Now the splitted one:
stud.hpp:
#ifndef STUD_HPP
#define STUD_HPP
#include <iostream>
#include <string>
class Stud {
private:
std::string pren, surn;
unsigned int number;
public:
Stud(std::string, std::string, unsigned int);
friend std::ostream& operator<<(std::ostream&, const Stud&);
};
#endif
stud.cpp:
#include "stud.hpp"
Stud::Stud(std::string p = "", std::string s = "", unsigned int n = 0) : pren(p), surn(s), number(n)
{
return;
}
std::ostream& operator<<(std::ostream& os, const Stud& st) {
os << st.number << ", " << st.pren << " " << st.surn;
return os;
}
main.cpp
#include <iostream>
#include "stud.hpp"
using namespace std;
int main() {
Stud s;
Stud test("x", "y", 123);
cout << s << endl;
cout << test << endl;
}