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# memoffset #
[](https://crates.io/crates/memoffset)
C-Like `offset_of` functionality for Rust structs.
Introduces the following macros:
* `offset_of!` for obtaining the offset of a member of a struct.
* `span_of!` for obtaining the range that a field, or fields, span.
`memoffset` works under `no_std` environments.
## Usage ##
Add the following dependency to your `Cargo.toml`:
```toml
[dependencies]
memoffset = "0.5"
```
These versions will compile fine with rustc versions greater or equal to 1.19.
Add the following lines at the top of your `main.rs` or `lib.rs` files.
```rust,ignore
#[macro_use]
extern crate memoffset;
```
## Examples ##
```rust
#[macro_use]
extern crate memoffset;
#[repr(C, packed)]
struct Foo {
a: u32,
b: u32,
c: [u8; 5],
d: u32,
}
fn main() {
assert_eq!(offset_of!(Foo, b), 4);
assert_eq!(offset_of!(Foo, d), 4+4+5);
assert_eq!(span_of!(Foo, a), 0..4);
assert_eq!(span_of!(Foo, a .. c), 0..8);
assert_eq!(span_of!(Foo, a ..= c), 0..13);
assert_eq!(span_of!(Foo, ..= d), 0..17);
assert_eq!(span_of!(Foo, b ..), 4..17);
}
```
## Feature flags ##
### Usage in constants ###
`memoffset` has **experimental** support for compile-time `offset_of!` on a nightly compiler.
In order to use it, you must enable the `unstable_const` crate feature and several compiler features.
Cargo.toml:
```toml
[dependencies.memoffset]
version = "0.5"
features = ["unstable_const"]
```
Your crate root: (`lib.rs`/`main.rs`)
```rust,ignore
#![feature(ptr_offset_from, const_ptr_offset_from, const_maybe_uninit_as_ptr, const_raw_ptr_deref)]
```
If you intend to use `offset_of!` inside a `const fn`, also add the `const_fn` compiler feature.
### Raw references ###
Recent nightlies support [a way to create raw pointers](https://github.com/rust-lang/rust/issues/73394) that avoids creating intermediate safe references.
`memoffset` can make use of that feature to avoid what is technically Undefined Behavior.
Use the `unstable_raw` feature to enable this.
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