zig/doc/langref/change_active_union_field.zig

const std = @import("std");

const Foo = union {
    float: f32,
    int: u32,
};

pub fn main() void {
    var f = Foo{ .int = 42 };
    bar(&f);
}

fn bar(f: *Foo) void {
    f.* = Foo{ .float = 12.34 };
    std.debug.print("value: {}\n", .{f.float});
}

// exe=succeed