Reference offset of member variable within POD declaration

Question

Is it possible to have this code work somehow? (Current get a compile error stating MMVertex2F4B2F does not exist) I don't want to wrap the 2 within another struct as that will effect the usability of the class within my existing code.

I need this all to happen at compile time, essentially the TemplatedClass needs to hold data regarding it's outer class that I can access later by creating an instance of TemplatedClass.

struct MMVertex2F4B2F
{
  MMPoint vertex;
  MMColor4B col;
  MMPoint tex;

  struct TemplatedClass<offsetof(MMVertex2F4B2F, vertex)> {};
};

Regards, James

Answer 1

you could put the data of your main class into a sub-type:

struct MMVertex2F4B2F
{
  struct MMVertexData {
    MMPoint vertex;
    MMColor4B col;
    MMPoint tex;
  } Data;
  struct TemplatedClass<offsetof(MMVertexData, vertex)> {};
};

(if you access the data members via member methods, you only have to adapt those to this change instead of all of your code)

Alternatively, the TemplatedClass<> could be declared standalone (not a sub-type) similar to a traits template.

The fact that your code doesn't work shows that its design is flawed.

Answer 2

Note that

struct TemplatedClass<offsetof(MMVertex2F4B2F, vertex)> {};

isn't valid C++ code. Either this is a template specialization, which would be

template <> struct TemplatedClass<offsetof(MMVertex2F4B2F, vertex)> {};

(which doesn't seem like it would make much sense in this context) it's should be a declaration of an instance of the template, which would be

TemplatedClass<offsetof(MMVertex2F4B2F, vertex)> myInstanceVar;

For the rest, I agree with Walter's answer.

Answer 3

The offsetof macro is inherited from C, and it does work with POD structs, but it's not the best approach to referencing a member in C++. Parameterize the template on pointer-to-members.

template< typename client, MMPoint client ::* > // PTM parameter
struct TemplatedClass {
    …
};

struct MMVertex2F4B2F
{
  MMPoint vertex;
  MMColor4B col;
  MMPoint tex;

  // PTM argument acceptable despite incomplete class:
  TemplatedClass<MMVertex2F4B2F, & MMVertex2F4B2F::vertex> template_inst;
};

The template in this example accepts a pointer to member of type MMPoint within any specified class. You might use some other parameterization. Anything is better than byte offsets and casting through char * .

Answer 4

After sleeping on it I came to this solution which seems to work:

struct MMVertex2F4B2F
{
  MMPoint vertex;
  MMColor4B col;
  MMPoint tex;

  struct TemplatedClass;  //Use a forward declaration here
};

//The template will now work as MMVertex2F4B2F is fully declared
struct MMVertex2F4B2F::TemplatedClass
       : public VertDef <offsetof(MMVertex2F4B2F, vertex)> {};