Question

When compiling this code on VS2008:

  #include <vector>

using namespace std;

class Vertex {

public: double X; 
        double Y;
        double Z;

        int id; // place of vertex in original mesh vertex list

        vector<Vertex*> neighbor; //adjacent vertices

        vector<Triangle*> face; // adjacent triangles

        float cost; // saved cost of collapsing edge

        Vertex *collapse; // 
};



 class Triangle {

public:
    Vertex * vertex[3]; 


};

I get the following error:

1>.\Triangle.cpp(15) : error C2065: 'Triangle' : undeclared identifier 

How can I fix this?

Answer 1

You use a forward declaration:

class Traingle;

class Vertex
{
    ...
};

class Triangle
{
    ...
};

A forward declaration of a type (e. g. class Triangle) allows you to declare pointers or references to that type, but not objects of the type. In other words

class Triangle;
class Vertex
{
  vector<Triangle*> face;
};

will compile, but

class Triangle;
class Vertex
{
  vector<Triangle> face;
};

will not compile.

Also, a forward declaration of a type does not let you access its members, because the compiler does not know about them yet. So the member functions that use objects of the forward declared type must be defined after the type is fully defined. In your case, after the definition of class Triangle.

Oh, and this is not at all specific to Visual Studio. This is just standard C++.