Zig fat pointer example

main.zig

const std = @import("std");
const testing = std.testing;
const assert = std.debug.assert;

pub const Runner = struct {
    const Self = @This();
    const VTable = struct { add: *const fn (*anyopaque, *usize) void };

    vtable: *const VTable,
    ptr: *anyopaque,

    fn make(pointer: anytype) Self {
        const Ptr = @TypeOf(pointer);
        const ptr_info = @typeInfo(Ptr);
        const alignment = ptr_info.Pointer.alignment;

        assert(ptr_info == .Pointer);
        assert(ptr_info.Pointer.size == .One);
        assert(@typeInfo(ptr_info.Pointer.child) == .Struct);

        const gen = struct {
            fn addImpl(ptr: *anyopaque, ctx: *usize) void {
                const self = @ptrCast(Ptr, @alignCast(alignment, ptr));
                try @call(.{ .modifier = .always_inline }, std.meta.Child(Ptr).add, .{ self, ctx });
            }

            const vtable = VTable{
                .add = addImpl,
            };
        };

        return .{
            .ptr = pointer,
            .vtable = &gen.vtable,
        };
    }

    fn add(self: *@This(), ctx: *usize) void {
        self.vtable.add(self.ptr, ctx);
    }
};

test "basic add functionality" {
    const Runner1 = struct {
        const Self = @This();

        c: usize = 10,

        pub fn add(self: *Self, acc: *usize) !void {
            acc.* += self.c;
        }
    };

    var acc: usize = 0;
    var r = Runner1{};
    var r1 = Runner.make(&r);

    r1.add(&acc);
    try testing.expect(acc == 10);
    r1.add(&acc);
    try testing.expect(acc == 20);
}


// https://zig.news/kilianvounckx/zig-interfaces-for-the-uninitiated-an-update-4gf1