Question

I’ve run into this problem time & again, and I’ve always had to resolve it using a forward declaration and a pointer.

It seems wrong that C++ requires a work around to get objects to co-operate. Is there any way to get this to compile WITHOUT making Mesh* a pointer in class Shape (or, the vector<Tri> into vector<Tri*>?

Shape.h

#ifndef S
#define S

#include "Mesh.h"
//class Mesh ; //delete line above, uncomment this to make it work
class Shape // a mathematical shape
{
    Mesh mesh ; // the poly mesh repr' this math shape
    //Mesh *mesh ; // make it work

    // a shape must be intersectable by a ray
    //virtual Intersection intersects( const Ray& ray )=0 ;
} ;

#endif

Mesh.h

#ifndef M
#define M

#include <vector>
using namespace std;
#include "Triangle.h"

struct Mesh
{
    vector<Triangle> tris ; // a mesh is a collection of triangles
} ;

#endif

Triangle.h

#ifndef T
#define T
#include "Shape.h"
class Triangle : public Shape
{
    // a triangle must be intersectable by a ray
    // a triangle's mesh is simple but it is still a mesh (1 poly)
} ;
#endif

Answer 1

It works for me without a pointer (there are, naturally, pointers used inside std::vector). You just have to analyze your dependencies carefully. Triangle inherits Shape, therefore Shape‘s definition is above Triangle‘s. Shape contains a Mesh, therefore Mesh‘s definition precedes Shape‘s. This gives the order: Mesh, Shape, Triangle, which compiles without errors.

Naturally, some meshes will have to have empty vectors, since every triangle inside the vector itself requires a mesh.