C++ linker problems with static method

问题:

I'm writing a Vector3D class that calls a static method on a VectorMath class to perform a calculation. When I compile, I get this:

bash-3.1$ g++ VectorMath.cpp Vector3D.cpp
/tmp/cc5cAPia.o: In function `main':
Vector3D.cpp:(.text+0x4f7): undefined reference to 'VectorMath::norm(Vector3D*)'
collect2: ld returned 1 exit status

The code:

VectorMath.h:

#ifndef VECTOR3D_H
#include "Vector3D.h"
#endif

class VectorMath {
    public:
    static Vector3D* calculatePerpendicularVector(Vector3D*, Vector3D*);
    static Vector3D* norm(Vector3D*);
    static double length(Vector3D*);
};

VectorMath.cpp

#include "VectorMath.h"
Vector3D* norm(Vector3D* vector) { // can't be found by linker
    // do vector calculations
    return new Vector3D(xHead, yHead, zHead, xTail, yTail, zTail);
}
// other methods

Vector3D.cpp

#include "Vector3D.h"
#include "VectorMath.h"
// ...
// vector implementation
// ...
int main(void) {
    Vector3D* v = new Vector3D(x, y, z);
    Vector3D* normVector = VectorMath::norm(v); // error here
}        

Why can't the linker find the VectorMath::norm method? At first glance I'd think that I'd need to declare norm like this:

Vector3D* VectorMath::norm(Vector3D* vector) {

but that doesn't help either...

回答1:

You're missing this:

//VectorMath.cpp
#include "VectorMath.h"

             |
             V - here
Vector3D* VectorMath::norm(Vector3D* vector)
{
    ...
}

The norm function is part of VectorMath::. Without that, you just have a free function.

---

This is more about your design, but why are you using pointers to everything? This is much cleaner:

class VectorMath {
    public:
    static Vector3D norm(const Vector3D&);
};

Take references, you're in C++ so don't write C code. What happens when I call this?

VectorMath::norm(0); // null

It will either crash, you have to put in a check, in which case, what should it return? This is all cleaned up by using references.

Also, why not just make these members of the Vector3D class?

Vector3D* v = new Vector3D(x, y, z);
v->norm(); // normalize would be better, in my opinion

Lastly, stack-allocate things. Your code right now has a memory leak:

int main(void) {
    Vector3D* v = new Vector3D(x, y, z);
    Vector3D* normVector = VectorMath::norm(v); 

    // delete v;
    // ^ you're not deleting it!
}        

Change it to this, and use RAII concepts:

int main(void) {
    Vector3D v(x, y, z);
    Vector3D* normVector = VectorMath::norm(v);

    // delete v;
    // ^ you're not deleting it!
}    

And by making norm a member function you end up with the very clean code:

int main(void) {
    Vector3D v(x, y, z);
    Vector3D normVector(v.norm());
}    

No pointers, no leaks, all sexy.

回答2:

You haven't defined Vector3D::norm method in VectorMath.cpp. Instead you've defined a global function named norm. What you need to do is qualify the method name in the definition:

Vector3D* Vector3D::norm(Vector3D* vector)

回答3:

Vector3D* VectorMath::norm(Vector3D* vector) { // can't be found by linker
    // do vector calculations
    return new Vector3D(xHead, yHead, zHead, xTail, yTail, zTail);
}