# C++ Vector Class

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I have a question about the following code:
#ifndef _VECTOR
#define _VECTOR
#include <math.h>

class Vector3D
{
private:
float x, y, z;
public:
//default constructor
Vector3D(float X = 0, float Y = 0, float Z = 0)
{
x = X;
y = Y;
z = Z;
}
~Vector3D(){};

//calculate and return the magnitude of this vector
float GetMagnitude()
{
return sqrtf(x * x + y * y + z * z);
}

//multiply this vector by a scalar
Vector3D operator*(float num) const
{
return Vector3D(x * num, y * num, z * num);
}

//pass in a vector, pass in a scalar, return the product
friend Vector3D operator*(float num, Vector3D const &vec)
{
return Vector3D(vec.x * num, vec.y * num, vec.z * num);
}

Vector3D operator+(const Vector3D &vec) const
{
return Vector3D(x + vec.x, y + vec.y, z + vec.z);
}

//subtract two vectors
Vector3D operator-(const Vector3D &vec) const
{
return Vector3D(x - vec.x, y - vec.y, z - vec.z);
}

//normalize this vector
void normalizeVector3D()
{
float magnitude = sqrtf(x * x + y * y + z * z);
x /= magnitude;
y /= magnitude;
z /= magnitude;
}

//calculate and return dot product
float dotVector3D(const Vector3D &vec) const
{
return x * vec.x + y * vec.y + z * vec.z;
}

//calculate and return cross product
Vector3D crossVector3D(const Vector3D &vec) const
{
return Vector3D(y * vec.z - z * vec.y,
z * vec.x - x * vec.z,
x * vec.y - y * vec.x);
}
};

#endif

1. I normally see the implementation split into a .cpp file. What are the implications of writing a class this way? More efficient? Bad code? No difference?

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It's largely an organization vs. (potential) performance tradeoff. If you put the implementation in a .cpp file, then client code doesn't get to see the implementation. I'm not talking about security, just about organization. If you're changing the implementation of some of the functions, this is nice, because the header doesn't change, so you don't have to recompile every file that includes it.

On the other hand, putting the code into the header makes it more likely that the compiler can inline the code. For very short functions (like the ones you've shown), the compiler probably will choose to inline them, given the chance. Note that some compilers can inline even if your code is in the .cpp file (using link-time-code-generation), but if you want to ensure that any compiler (gcc, etc...) could do so, you need to leave the code in the header.

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great explanation ty

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Functions defined inside a class body is implicitly inline. Inline doesn't however mean anything in and of itself, the compiler is free to ignore it. It does decrease code maintainability, you have to recompile all files including it if you change it. If the code were in a source file, only the source file need be recompiled. However, this isn't a huge problem for a vector class, because they are infrequently changed.

There are some issues with your class:
0) Don't put a empty destructor. The complier will generate one for you
1) As an optimisation, you can provide a "SquaredMagnitude" function, so that comparing the relative lengths of two vectors can avoid costly sqrt calls.
2) Personally, I think "crossVector3D" and "dotCrossVector3D" are overly verbose.

Consider the usage:
Vector3 one, two;float dot = one.dot(two);Vector3 cross = two.cross(one);

Is as readable (or more so, IMO) than:
Vector3 one, two;float dot = one.dotVector3(two);Vector3 cross = two.crossVector3(one);

In my code, I have dot and cross as free functions. But that is just how I choose to do it.

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ty for the advice. what is a free function? I don't have any experience using these functions but I'm adding the squaredMagnitude() for the day I do. I'll take your word for it.

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One not in a class. For example, I have:

float dot(const Vector &one, const Vector &two ){    return /* ... */;}Vector cross(const Vector &one, const Vector &two ){    return /* ... */;}// usage:void example(){    Vector3 one, two;    float d = dot(one,two);    Vector3 c = cross(two,one);}

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Do you put your dot and cross inside the vector.h?

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Yes. I define all the operators outside the class, because none of them access private data. The only things in the class body itself are the constructors and the length() and normalise() functions.

But that is just a personal choice I made. Keeping functions inside the class body is fine.

edit:

Looking again, I see that you have made your x, y and z members private. For a simple type like a vector, it is arguable that this is unnecessary (encapsulation adds nothing). I have these variables public.

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I have a question about normalizing vectors. The function I have changes the actual x, y, z. I would have to copy it's coordinates to another vector before normalizing it (which would overwrite it's position).

In the real world, you need to keep track of not only the normal of an object but also it's coordinates right? Or, would you have some other class keep track of where objects are in the world?

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I think you have "normalisation" mixed up with "normal". In this context, normalising a vector means making it of unit length. A normal, however, is a direction vector that's perpendicular to a plane. Normals are often normalised, meaning that they are of unit length (unit length means length of 1).

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