optional-pointer.md

Optional Pointer

Created on 2024-03-19; last updated on 2024-04-09

The type ?*T describes an optional single-item pointer to exactly one value of type T.

The statements below generalize to optional many-item pointer types but not optional slice types because slices are not pointers.

Any optional pointer may have the value null, which is guaranteed to represent the zero memory address. Indeed, the types ?*T and *allowzero T coerce to one another, although the primitive null can't be assigned to a variable of type *allowzero T.

It is an error to attempt memory access through a null pointer (an optional pointer that has the value null).

The type ?*T is guaranteed to have the same size as the type *T.

It is possible to initialize a variable of type ?*allowzero T. This type seems to have limited practical utility beyond testing the compiler's casting behavior.

The type ?*allowzero T is larger than the type ?*T because extra space is needed to store the redundant information.

The type ?*T coerces to the type ?*allowzero T but not vice versa.

What about C pointers?

• The primitive null can be assigned to a variable of type [*c]T;
• ?[*c]T is to [*c]T as ?*allowzero T is to *allowzero T, and
• the compiler refuses to construct the type [*c]allowzero T because all C pointer types are necessarilly allowzero.

Zig doesn't account for the fact that on some targets the value of the null pointer depends on its address space.