Opaque

opaque types in Zig have an unknown (albeit non-zero) size and alignment. Because of this these data types cannot be stored directly. These are used to maintain type safety with pointers to types that we don't have information about.

const Window = opaque {};
const Button = opaque {};

extern fn show_window(*Window) callconv(.C) void;

test "opaque" {
    var main_window: *Window = undefined;
    show_window(main_window);

    var ok_button: *Button = undefined;
    show_window(ok_button);
}

./test-c1.zig:653:17: error: expected type '*Window', found '*Button'
    show_window(ok_button);
                ^
./test-c1.zig:653:17: note: pointer type child 'Button' cannot cast into pointer type child 'Window'
    show_window(ok_button);
                ^

Opaque types may have declarations in their definitions (the same as structs, enums and unions).

const Window = opaque {
    fn show(self: *Window) void {
        show_window(self);
    }
};

extern fn show_window(*Window) callconv(.C) void;

test "opaque with declarations" {
    var main_window: *Window = undefined;
    main_window.show();
}

The typical usecase of opaque is to maintain type safety when interoperating with C code that does not expose complete type information.