Help on *const ()

1

let mut x:*const ();
x=&1234;

If i want to chang integer pointer to float it dosent compile..

x = &44.4;

  1. () type is general type or tuple type?
    2.how to assign any type to x.
2

If you are using raw pointers, then you can cast between different pointer types. Generally all variables must have the same type for its entire lifetime.

3

if you box the variable you can use the Any trait, but in most cases that's a sign of poor design.

4
5

It is not general -- it is specifically the unit type. You can think of it like an empty tuple, but it has other properties too.

1 Like
6

It looks like you are coming from C and you expect *const () to be the equivalent of void *. This expectation of yours is not going to be fulfilled on three levels:

  • Low-level primitive constructs have very different semantics in C and in Rust. Rust has a lot of constraints and a richer type system even around raw pointers. The languages also have radically different idioms and approaches – trying to directly map concepts from C to Rust is not going to help you, it would only cause endless grief.
  • () in Rust is not the same as C's void type. In Rust's real, algebraic type system, () is a proper type, the empty tuple (product) type, which is the canonical singleton type with exactly one possible value (also spelled ()) and thus no information content. Similarly to a void-returning C function, Rust functions that can't return any other meaningful value return (). However, unlike (), the C void "type" is not really a type. It's a keyword that has different meanings in different contexts.
  • Consequently, the void * type in C is not really first-class, its behavior is inconsistent with that of other pointer types: it's not really typed (you can assign any pointer type to a value of type void * and vice versa), and it can't be dereferenced (as it doesn't point to any particular type). In contrast, Rust's () type does compose with pointers, so *const () is a proper type, which points and dereferences to a – useless and zero-sized, but valid – value of type (), and you can't just assign a pointer-to-any-other-type to a variable with a declared type of *const (), at least not without an explicit conversion.

At this point I have to ask: what are you trying to do by assigning either a pointer-to-int or a pointer-to-float to the same variable? What higher-level goal do you have?

  • If you are trying to do dynamic dispatch, you can create a trait object (e.g. &dyn Display).
  • If you are trying to emulate a tagged union, you can create an enum type with some associated data in each variant.
  • If you are just trying to write code that works with many types sharing some common properties, you can use a trait and write code that is generic over that trait.
2 Likes
7

And if you're trying to convert int bits to a float, use

8

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