Pointer as parameter is null

[fafner]

In my raytracer-project I have the following function:

bool hitObjects(Ray &ray, std::vector<Object *> &objects, Vector3& normal, Material *mat) {
	double tmin = K_HUGE_VALUE;
	double t;
	bool hit;
	for(int i = 0; i < objects.size(); i++) {
		if (objects.at(i)->mesh->hit(ray, t, normal) && t < tmin) {
			tmin = t;
			mat = objects.at(i)->material;
			hit = true;
		}
		else {
			hit = false;
		}
	}
	if (hit && mat == NULL) {
		std::cout << "Object was hit, but material is still NULL\n";
	}
	return hit;
}

This is then being called by

if (hitObjects(ray, objects, normal, mat)) {
	if (mat == NULL) {
		std::cout << "Material pointer is null\n";
	}

When I try to run this, I only get the message "Material pointer is null". So, somehow the pointer is null after the hitObjects-function is called, even though it isn't inside that function. Does anyone know what I'm doing wrong?


Fafner

[kbw]

That's weird, but we need to see the declaration of Material and the declaration of mat in the caller.

[fafner]

Sure.

class Material {
public:
	virtual Vector3 shade(std::vector<Light*> lights, Vector3& normal) = 0;
};

class PhongMaterial : public Material {
private:
	Vector3 specular_reflectance;
	Vector3 diffuse_reflectance;
	Vector3 ambient_reflectance;
	float specular_exponent;

public:
	PhongMaterial(Vector3 s = 0, Vector3 d = 0, Vector3 a = 0, float e = 0.0) :
		specular_reflectance(s), diffuse_reflectance(d),
		ambient_reflectance(a), specular_exponent(e) {}

	Vector3 shade(std::vector<Light*> lights, Vector3& normal) {
		return Vector3(1.0, 0.0, 0.0); //TODO
	}
};

I forgot to add, mat is just a pointer declared right before hitObjects is called, like so

Material *mat = NULL;
if (hitObjects(ray, objects, normal, mat)) {
	if (mat == NULL) {
		std::cout << "Material pointer is null\n";
	}
...

[fafner]

The Object-class is just super-simple, like this

class Object {
public:
	Geometry *mesh;
	Material *material;

	Object(Geometry *m, Material *mat) : mesh(m), material(mat) {}
};

[coder777]

The parameter mat is the copy of the pointer. Any change isn't propagated to the caller. If you want that you either provide pointer to the pointer Material ** mat or refernce Material *& mat

[fafner]

Wow, thanks! Can't say I fully understand why that works yet, but it does! :P Thank you:)

[coder777]

Can't say I fully understand why that works yet, but it does!

The keyword here is copy. You're passing the pointer by value hence you have a copy of the pointer. Modifing the copy [of the pointer] leaves the original [pointer] unchanged

You may read this:
http://stackoverflow.com/questions/373419/whats-the-difference-between-passing-by-reference-vs-passing-by-value