#define SOMETHING( whatever, change ) ((whatever)->(change))

Where change is a string?

or

#define SOMETHING( whatever, change ) ((whatever)->##change) ?

The C pre-processor is a straight-up text replacement system that doesn't know much about the C language at all.

If you:

#define foo(x,y) x(y)

When you use foo:

foo(blah, 3)

It generates:

blah(3)

That's all it does… various compilers add some extra convenience things for stuff like variable arguments, __FILE__, and other extensions.