objc2 0.6.4

Objective-C interface and runtime bindings
Documentation 
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use objc2::encode::{Encoding, RefEncode};

// We choose in this case to represent `NSDecimal` as an opaque struct because
// we don't know much about the internals.
//
// Therefore we do not implement `Encode`.
#[repr(C)]
struct NSDecimal {
    // Note: This should be an [extern type][rfc-1861] instead, when that
    // becomes possible, for now we use this as a workaround.
    //
    // [rfc-1861]: https://rust-lang.github.io/rfcs/1861-extern-types.html
    _priv: [u8; 0],
}

// SAFETY: `&NSDecimal` is a valid pointer, and the encoding is correct.
unsafe impl RefEncode for NSDecimal {
    const ENCODING_REF: Encoding = Encoding::Pointer(&Encoding::Struct("?", &[]));
}

fn main() {
    // Running `@encode` on `NSDecimal*` on my 64-bit system gives
    // `^{?=cCCC[38C]}`, but empty structs are treated as equivalent to all
    // other structs by `objc2`.
    assert!(NSDecimal::ENCODING_REF.equivalent_to_str("^{?=cCCC[38C]}"));
    // Does not compile:
    // println!("{:?}", NSDecimal::ENCODING);
}