Getting multiple definition error even with include guard

Question

My assignment is to keep a circular dependency while breaking down a code to multiple files , the code is about a billiard simulation using object orientation , but my problem is not the code but the compilation.

I have wrestled with this and kind of resolved my issue using this post: Resolve build errors due to circular dependency amongst classes

but I keep getting compilation errors after "make" command that says I have defined somethings multiple times ; this is while I have used include guards that prevent the program from including definitions more than once, I somehow feel I have made a small mistake somewhere and I don't seem to find it .

These are my files

ball.cpp

#ifndef BALL_CPP
#define BALL_CPP
#include "ball.hpp"
#include <iostream>
using namespace std;
Ball::Ball(double _x, double _y, double _vx, double _vy, Table *t)
{
     cout << "Ball constructor called" << endl;
     table = t;
     set_location(_x, _y);
     set_speed(_vx, _vy);
     cout << "Initial position is: (" << x << ',' << y << ')' << endl;
}
void Ball::set_location(double _x, double _y)
{
     cout << "Ball::set_location called with (" << _x << ',' << _y << ')' << endl;
     if (!table->contains_point(_x, _y))
          abort();
     x = _x;
     y = _y;
}
void Ball::set_speed(double _vx, double _vy)
{
     cout << "Ball::set_speed called with (" << _vx << ',' << _vy << ')' << endl;
     vx = _vx;
     vy = _vy;
}

void Ball::move(double dt)
{
     cout << "Ball::move called" << endl;
     x += vx * dt;
     y += vy * dt;

     if (!table->contains_point(x, y))
          table->reflect(this);
     cout << "New position is: (" << x << ',' << y << ')' << endl;
}
#endif

ball.hpp

#ifndef BALL_HPP
#define BALL_HPP
#include "table.hpp"
class Ball
{
public:
     Ball(double _x, double _y, double _vx, double _vy, Table *t);
     void move(double dt);
     double get_x() { return x; }
     double get_y() { return y; }
     double get_vx() { return vx; }
     double get_vy() { return vy; }
     void set_location(double _x, double _y);
     void set_speed(double _vx, double _vy);

private:
     double x;
     double y;
     double vx;
     double vy;
     Table *table;
};
#endif

table.cpp:

#ifndef TABLE1CPP
#define TABLE1CPP
using namespace std;
#include <iostream>
#include "ball.hpp"
Table::Table(double w, double h)
{
     cout << "Table constructor called" << endl;
     if (w <= 0 || h <= 0)
          abort();
     width = w;
     height = h;
}
bool Table::contains_point(double x, double y)
{
     cout << "Table::contains_point called" << endl;
     return x >= 0 && x < width && y >= 0 && y < height;
}
void Table::reflect(Ball *b)
{
     cout << "Table::reflect called" << endl;
     double x = b->get_x();
     double y = b->get_y();
     double vx = b->get_vx();
     double vy = b->get_vy();

     while (!contains_point(x, y))
     {
          if (x < 0)
          {
               x = -x;
               vx = -vx;
          }
          if (x >= width)
          {
               x = 2 * width - x;
               vx = -vx;
          }
          if (y < 0)
          {
               y = -y;
               vy = -vy;
          }
          if (y >= height)
          {
               y = 2 * height - y;
               vy = -vy;
          }
     }
     b->set_location(x, y);
     b->set_speed(vx, vy);
}
#endif

table.hpp:

#ifndef TABLE_HPP
#define TABLE_HPP
class Ball;
class Table
{
public:
     Table(double w, double h);
     bool contains_point(double x, double y);
     void reflect(Ball *b);

private:
     double width;
     double height;
};
#endif

And my makefile looks like this :(I am very new to the multi file concept and very inexperienced so i might have a blunder here that I don't understand )

makefile :

output: main.o table.o ball.o
    g++ main.o ball.o table.o -o billiards
main.o: main.cpp
    g++ -c main.cpp
ball1.o:    ball.cpp ball.hpp
    g++ -c ball1.cpp
table1.o: table.cpp table.hpp
    g++ -c table.cpp
clean:
    rm * .o output

main.cpp :

#include <iostream>
#include <cstdlib>
#include "ball.cpp" // should be changed to "ball.hpp"
#include "table.cpp" // should be changed to "table.hpp"
using namespace std;
int main()
{
     Table t(100, 50);
     Ball b(10, 20, 25, 5, &t);
     b.move(10);
}

the errors :

/usr/bin/ld: ball.o: in function `Ball::Ball(double, double, double, double, Table*)':
ball.cpp:(.text+0x0): multiple definition of `Ball::Ball(double, double, double, double, Table*)'; main.o:main.cpp:(.text+0x0): first defined here
/usr/bin/ld: ball.o: in function `Ball::set_location(double, double)':
ball.cpp:(.text+0x10c): multiple definition of `Ball::set_location(double, double)'; main.o:main.cpp:(.text+0x10c): first defined here
/usr/bin/ld: ball.o: in function `Ball::set_speed(double, double)':
ball.cpp:(.text+0x1dc): multiple definition of `Ball::set_speed(double, double)'; main.o:main.cpp:(.text+0x1dc): first defined here
/usr/bin/ld: ball.o: in function `Ball::Ball(double, double, double, double, Table*)':
ball.cpp:(.text+0x0): multiple definition of `Ball::Ball(double, double, double, double, Table*)'; main.o:main.cpp:(.text+0x0): first defined here
/usr/bin/ld: ball.o: in function `Ball::move(double)':
ball.cpp:(.text+0x280): multiple definition of `Ball::move(double)'; main.o:main.cpp:(.text+0x280): first defined here
/usr/bin/ld: table.o: in function `Table::Table(double, double)':
table.cpp:(.text+0x0): multiple definition of `Table::Table(double, double)'; main.o:main.cpp:(.text+0x3be): first defined here
/usr/bin/ld: table.o: in function `Table::Table(double, double)':
table.cpp:(.text+0x0): multiple definition of `Table::Table(double, double)'; main.o:main.cpp:(.text+0x3be): first defined here
/usr/bin/ld: table.o: in function `Table::contains_point(double, double)':
table.cpp:(.text+0x7c): multiple definition of `Table::contains_point(double, double)'; main.o:main.cpp:(.text+0x43a): first defined here
/usr/bin/ld: table.o: in function `Table::reflect(Ball*)':
table.cpp:(.text+0x10a): multiple definition of `Table::reflect(Ball*)'; main.o:main.cpp:(.text+0x4c8): first defined here
collect2: error: ld returned 1 exit status
make: *** [Makefile:2: output] Error 1

Answer 1

Okay, so, this is what's happening: main.cpp is pulling ball.cpp and table.cpp, so the object file main.o will have the full implementations of all the functions and variables you implemented in ball.cpp and table.cpp. This is an issue, because you also separately compile ball.cpp and table.cpp, so you end up with all the duplications.

I'm guessing you put the include guards in the .cpp files to avoid this problem, however, this doesn't actually work: #define'd macros won't carry over between invocations of the compiler.

What you want to do here is #include the .hpp files instead: main.cpp will be able to see the declarations of the functionalities you're using, but it won't compile the implementations into main.o, so there won't be any conflicts with ball.o or table.o. Additionally, you should probably take the include guards out of the .cpp files: for exactly these reasons of duplication, #includeing C or C++ source files generally isn't done except in a few odd cases which aren't really relevant to your particular situation.