Question

I have two classes Triangle and ALine and I want to assign new ALine instances to properties of Triangle in its constructor.
But I am getting this error

Undefined symbols for architecture x86_64:
  "ALine::ALine()", referenced from:
      Triangle::Triangle(triangle) in Triangle.o
ld: symbol(s) not found for architecture x86_64

Below is code I wrote:

ALine.cpp

#include "Geometry.h"
#include "ALine.h"



ALine::ALine(point a, point b)
{       
        double tmpy = a.y - b.y;
        double tmpx = a.x - b.x;
        double tmpk;
        if(equals(tmpy, 0))
        {
            tmpk = 0;
        }
        else
        {
            tmpk = tmpx/tmpy;
        }
        double tmpq = a.y - tmpk*a.x;
        if(equals(tmpx, 0))
        {
            if(equals(a.x, 0))
            {
                if(equals(b.x, 0)) tmpk = 0;
                else tmpk = (b.y-tmpq)/b.x;
            }
            else
            {
                tmpk = (a.y-tmpq)/a.x;
            }
        }
     

    
        a = a;
        b = b;
        k = tmpk;
        q = tmpq;
    
};

ALine.h

class ALine{
    
private:
    double k;
    double q;
    point a;
    point b;
    
    double length;
    
    void calculateLength();
    
public:
    
    ALine(point a, point b);
    ALine();
    
    
    
    point getK();
    point getQ();
    
    
    static bool areinline(point a, point b, point c);
    
    
};

Triangle.cpp

#include "Triangle.h"
#include "Geometry.h"
#include "ALine.h"


    


    Triangle::Triangle(triangle t)
    {
        triangle itself = t;
        a = *new ALine(itself.a, itself.b);
        b = *new ALine(itself.b, itself.a);
        c = *new ALine(itself.c, itself.a);
    };

Note, the classes are not complete, I pasted here only relevant code to my problem (if not, I can add more).

Answer 1

You’re declaring a default constructor but not defining one.

// in ALine.h
class ALine
{
private:
    void calculateLength(); // this is a declaration
public:
    ALine(point a, point b); // this is another
    ALine(); // this is a declaration as well

...

// in ALine.cpp
ALine::ALine(point a, point b) // this is a definition
{
    double tmpy = a.y - b.y;
    double tmpx = a.x - b.x;
    double tmpk;
    ...

You’re telling the compiler that the function exists, so it’s letting you use it. This is legal. However, when the linker tries to process the compiled code, it looks for ALine::ALine() and can’t find it, because you never said what it was.

Add to your ALine.cpp something like the following:

ALine::ALine()
{
}

That being said

You are not using dynamic allocation correctly. As it currently is, you allocate a new ALine on the heap, copy it into an ALine on the stack, and then discard the address of the heap ALine (not once but 3 times). This is a memory leak, the ALines never go away and as long as your program is running, they will always be there.

You would be much better off changing Triangle::Triangle(triangle other) to the following:

Triangle::Triangle(const triangle& t) : a(t.a, t.b), b(t.b, t.c), c(t.c, t.a) {}

This uses the syntax of Class::Class(args ...) : member(...), member2(...), ... { /* body */ } to initialize the members of a class in a constructor. One benefit of this is that their default constructors will not be called, as they would (implicitly) in your code.