Pointer, if pointed type is unknown?

[seany via Digitalmars-d-learn]

In my previous post 
(http://forum.dlang.org/thread/jcaomfduqeofobculgli@forum.dlang.org) 
I asked for an array of pointers to different types, as a stack 
for my own, where I can generate and insert runtime variables.

However, seems like I need to know the type of variable the 
pointer is pointing to. That is,

double pi = 3.14159;
size_t ptr = cast(ulong)π // so far oky

double pi_2 = *ptr; // this is where things break down

won't work.

What can I do if the type of the variable is not known?