C++ undefined reference to `Point::display()'|

Question

class_studying.h
#ifndef CLASS_STUDYING_H_INCLUDED
#define CLASS_STUDYING_H_INCLUDED

class Point
{
public:
Point(double x=0, double y=0):X(x),Y(y){}
void display();
private:
double X;
double Y;
};

class Circle:public Point
{
public:
Circle(double r=0):radius(r){Point();}
void display();
private:
double radius;
};

class Cylinder:public Circle
{
public:
Cylinder(double h):height(h){Circle();}
void display();
private:
double height;
};

#endif // CLASS_STUDYING_H_INCLUDED

class_studying.cpp
#include"class_studying.h"

using namespace std;

void Point::display()
{
cout<<"("<< X <<","<< y <<")"<<endl;
}

void Circle::display()
{
Point::display();
cout<<"radius\t"<<radius<<endl;
}

void Cylinder::display()
{
Circle::display();
cout<<"height\t"<<height<<endl;
}

main.cpp

#include "class_studying.h"

int main()
{
Point p (2,3);
Circle c(7);
Cylinder cy(4);
p.display();
c.display();
cy.display();

return 0;

}

||=== Build: Debug in Homework (compiler: GNU GCC Compiler) ===|
obj\Debug\main.o||In function main':|
E:\Code blocks\Homework\main.cpp|8|undefined reference toPoint::display()'|
E:\Code blocks\Homework\main.cpp|9|undefined reference to Circle::display()'|
E:\Code blocks\Homework\main.cpp|10|undefined reference toCylinder::display()'|
||error: ld returned 1 exit status|
||=== Build failed: 4 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

Answer 1

 main.cpp
#include "class_studying.h"

->
#include "class_studying.cpp"