Regarding default constructor an object initialization/usage in C++ OOP - c++

Question

I have recently started learning OOP in C++ and I started solving example tasks regarding it. I want to instantiate an object of the class CStudent after having created a default constructor for it. However the compiler cannot compile the code. I would like to ask why is that?

Answer 1

When you write inside your class:
CStudent();
CStudent(string name, string fn);
...you only declare two constructors, one default (taking no-argument) and one taking two strings.
After declaring them, you need to define them, the same way you defined the methods getName or getAverage:
// Outside of the declaration of the class
CStudent::CStudent() { }
// Use member initializer list if you can
CStudent::CStudent(std::string name, string fn) :
name(std::move(name)), fn(std::move(fn)) { }
In C++, you can also define these when declaring them inside the class:
class CStudent {
// ...
public:
CStudent() { }
CStudent(std::string name, string fn) :
name(std::move(name)), fn(std::move(fn)) { }
// ...
};
Since C++11, you can let the compiler generate the default constructor for you:
// Inside the class declaration
CStudent() = default;

Answer 2

This should work, As commented by Holt, You need to define constructor, You have just declared it.
#include <iostream>
#include <string>
#include <list>
using namespace std;
class CStudent {
string name = "Steve";
list<int> scores;
string fn;
public:
CStudent() {};
CStudent(string name, string fn);
string getName();
double getAverage();
void addScore(int);
};
string CStudent::getName() {
return name;
}
double CStudent::getAverage() {
int av = 0;
for (auto x = scores.begin(); x != scores.end(); x++) {
av += *x;
}
return av / scores.size();
}
void CStudent::addScore(int sc) {
scores.push_back(sc);
}
int main()
{
CStudent stud1;
cout<< stud1.getName()<< endl;
return 0;
}