test:zig

guide/language/opaque.zig

@@ -0,0 +1,45 @@
+const expect = @import("std").testing.expect;
+const print = @import("std").debug.print;
+
+// `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.
+// ```zig
+// 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).
+
+// XXX: This is a C function that is not defined in the Zig code.
+extern fn show_window(*Window) callconv(.C) void;
+
+const Window = opaque {
+    fn show(self: *Window) void {
+        show_window(self);
+    }
+};
+
+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.