objc2 0.6.4

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

// Note: In this case `NSUInteger` could be a type alias for `usize`, and
// actually that's already available as `objc2::ffi::NSUInteger`.
#[repr(transparent)]
struct NSUInteger {
    _inner: usize,
}

// SAFETY: `NSUInteger` has the same `repr` as `usize`.
unsafe impl Encode for NSUInteger {
    // Running `@encode(NSUInteger)` gives `Q` on 64-bit systems and `I` on
    // 32-bit systems. This corresponds exactly to `usize`, which is also how
    // we've defined our struct.
    const ENCODING: Encoding = usize::ENCODING;
}

// SAFETY: `&NSUInteger` has the same representation as `&usize`.
unsafe impl RefEncode for NSUInteger {
    // Running `@encode(NSUInteger*)` gives `^Q` on 64-bit systems and `^I` on
    // 32-bit systems. So implementing `RefEncode` as a plain pointer is
    // correct.
    const ENCODING_REF: Encoding = Encoding::Pointer(&NSUInteger::ENCODING);
}

fn main() {
    assert!(NSUInteger::ENCODING.equivalent_to_str("Q"));
    assert!(<&NSUInteger>::ENCODING.equivalent_to_str("^Q"));
    assert!(<&NSUInteger>::ENCODING.equivalent_to_str("r^Q"));
}