Question

I’ve only seen this caused by people not linking their class objects together at build time, but I have, and I’m unsure what the matter is.

the source:

//test.cpp
#include "Point.h"
#include "Sphere.h"
#include "Scene.h"


int main(){
    Sphere s;
    Scene sc;
    sc.img_plane.upper_left = Point(-1, 1, -3);
    sc.img_plane.upper_right = Point(1, 1, -3);
    sc.img_plane.lower_left = Point(-1, -1, -3);
    sc.img_plane.lower_right = Point(1, -1, -3);
    sc.width = 100;
    sc.height = 100;
    return 0;
}

the error:

test.cpp:(.text+0x17): undefined reference to `Sphere::Sphere()'
test.cpp:(.text+0x26): undefined reference to `Scene::Scene()'
test.cpp:(.text+0x192): undefined reference to `Scene::~Scene()'
test.cpp:(.text+0x1a1): undefined reference to `Sphere::~Sphere()'
test.cpp:(.text+0x1bf): undefined reference to `Scene::~Scene()'
test.cpp:(.text+0x1d3): undefined reference to `Sphere::~Sphere()'

the Makefile:

CC = g++
SRC = ./src
BUILD = ./build

main : main.o Point.o colort.o Sphere.o Scene.o Ray.o HitRecord.o
    $(CC) $(BUILD)/main.o $(BUILD)/Point.o -o $(BUILD)/main

main.o : $(SRC)/test.cpp
    $(CC) -c $< -o $(BUILD)/main.o

Point.o : $(SRC)/Point.cpp $(SRC)/Point.h
    $(CC) -c $< -o $(BUILD)/Point.o

colort.o : $(SRC)/colort.cpp $(SRC)/colort.h
    $(CC) -c $< -o $(BUILD)/colort.o

Sphere.o : $(SRC)/Sphere.cpp $(SRC)/Sphere.h $(SRC)/colort.cpp $(SRC)/Point.cpp
    $(CC) -c $< -o $(BUILD)/Sphere.o

ImagePlane.o : $(SRC)/ImagePlane.cpp $(SRC)/ImagePlane.h
    $(CC) -c $< -o $(BUILD)/ImagePlane.o

Scene.o : $(SRC)/Scene.cpp $(SRC)/Scene.h
    $(CC) -c $< -o $(BUILD)/Scene.o 

Ray.o : $(SRC)/Ray.cpp $(SRC)/Ray.h
    $(CC) -c $< -o $(BUILD)/Ray.o

HitRecord.o : $(SRC)/HitRecord.cpp $(SRC)/HitRecord.h
    $(CC) -c $< -o $(BUILD)/HitRecord.o     

clean :
    rm $(BUILD)/*

test : main 
    $(BUILD)/main

Scene constructor is the same as the Sphere:

Sphere.h:

#ifndef SPHERE_H_
#define SPHERE_H_

#include "colort.h"
#include "Point.h"
#include "Ray.h"

class Sphere {
public:
    Sphere();
    virtual ~Sphere();
    Point center;
    double radius;
    color_t diffuse, specular;
    double intersection(Ray r);
};

#endif /* SPHERE_H_ */

Sphere.cpp:

#include "Sphere.h"

#include <math.h>

Sphere::Sphere() {

}

Sphere::~Sphere() {

}

...other class functions

Answer 1

In your main rule:

main : main.o Point.o colort.o Sphere.o Scene.o Ray.o HitRecord.o
    $(CC) $(BUILD)/main.o $(BUILD)/Point.o -o $(BUILD)/main

Notice that although you mention Sphere.o as a dependency, you have not linked it. You need to include it on the build line, too:

main : main.o Point.o colort.o Sphere.o Scene.o Ray.o HitRecord.o
    $(CC) $(BUILD)/main.o $(BUILD)/Point.o $(BUILD)/Sphere.o -o $(BUILD)/main

You probably also want to add the others, based on the fact that you list them as dependencies.