using extern, please help

Hi, I'm trying to use extern for a struct.

The struct is for vertices and is first used in a vertex class

Code:

///////////////////////////////////////////
//file vertices.h
///////////////////////////////////////////
struct vertex{
float x, y, z;
};

class VertexClass
{
vertex *vert; 
...
void MakeVertices(int howmany);//make vertices
};




///////////////////////////////////////////
//file vertexData.h
///////////////////////////////////////////

//trying to use extern of above vertex struct
extern vertex *DataForVertex;

class VData
{
vertex *DataForVertex;
...
...
//I'm trying to pass one of the vertice structs from the other 
//class into this 

void MakeVertex()//initialise the external vertex 
void GetVertexData(vertex *data)//get data from the vertices in above class


};

Basicly I'm making vertex objects in the first class and in the second class I want to pass those objects (in a loop), to do calculations on them. I think if I'm trying to pass a pointer to a whole vertex instance at once , then I also need the same type to "recieve" the data? What I'm actrually trying to pass is a VertexClass->vertex[n].

I have tried this but I get an undefined variable error for the vertex struct.

Can any one help me with this?
Thanks

You need to declare the variable in a .cpp file as well without the "extern" keyword.

Thanks for the replay, I just tried that but I'm getting a "no apprpriate constructor available" error.

I have used extern before but I can't find the project I used it in now.
The problem might be in how I'm trying to use it, are you allowed to use extern for a pointer?

I just thought, does extern have to refer to an already instantiated object, or is it just telling the compiler that the definition is somewhere else?