Opened 13 months ago

#181 new enhancement

Add Support for Opaque Polymorphic Types

Reported by: ajbeach Owned by:
Priority: minor Component: cfa-cc
Version: 1.0 Keywords:


I have been trying to make some smart pointers and I hit a problem with the polymorphic data structures. All are polymorphic on a type T which is the type they point at (their dereference returns a T&).

One of the pointers points directly at a T object and it works as a pointer and T can be opaque. The other one wraps the T type in a new structure. This means it must have a sized assertion and in turn the pointer type must also include it, which means it cannot be used with an opaque.

To explain it here are the current definitions:

forall(dtype T | sized(T))
struct counter_data {
    unsigned int counter;
    T object;

forall(dtype T | sized(T) /* ! */)
struct counter_ptr {
    counter_data(T) * data;

The ! assertion is the one that means you can't use an opaque type. This conflicts with the traditional C use of a pointer to an opaque type if you wanted to pass around a memory managed pointer.

The problem is that all uses of a polymorphic type must carry its assertions, even if they aren't used here. There should be a way to drop sized from the assertion list when you just have a pointer to the type. The system doesn't have to know the size or layout as long as you don't dereference the pointer except for the assertion list.

Change History (0)

Note: See TracTickets for help on using tickets.