Zig Struct Defaults

A struct default is a field value written in the struct definition that fills in when a creation omits that field. Defaults let a short creation still produce a complete value.

Use every default

Both fields of Settings have defaults. Creating an empty value fills both fields in.

const std = @import("std");
const Settings = struct { volume: i32 = 50, level: i32 = 1 };

pub fn main() !void {
    const out = std.io.getStdOut().writer();
    const s = Settings{};
    try out.print("{d} {d}\n", .{ s.volume, s.level });
}

Override one default

This creation names only the level field. The volume field keeps its default.

const std = @import("std");
const Settings = struct { volume: i32 = 50, level: i32 = 1 };

pub fn main() !void {
    const out = std.io.getStdOut().writer();
    const s = Settings{ .level = 3 };
    try out.print("{d} {d}\n", .{ s.volume, s.level });
}

Mix a required field with a default

The name field has no default, so the creation gives it. The lives field fills in on its own.

const std = @import("std");
const Player = struct { name: []const u8, lives: i32 = 3 };

pub fn main() !void {
    const out = std.io.getStdOut().writer();
    const p = Player{ .name = "Ana" };
    try out.print("{s} {d}\n", .{ p.name, p.lives });
}

Example

const std = @import("std");
const Item = struct { name: []const u8 = "pen", price: i32 = 5, stock: i32 = 0 };

pub fn main() !void {
    const out = std.io.getStdOut().writer();
    const a = Item{};
    const b = Item{ .stock = 12 };
    try out.print("{s} {d} {d}\n", .{ a.name, a.price, a.stock });
    try out.print("{s} {d} {d}\n", .{ b.name, b.price, b.stock });
}

Key points

Default: Write count: i32 = 0 to give a field a starting value.
Omit: Settings{} uses the default for every field that has one.
Override: Settings{ .level = 3 } replaces the default for that one field.
Keep: A field that is not named in a creation keeps its default value.
Mix: A struct can mix fields with defaults and fields without defaults.

In this example, the first Item uses all three defaults and shows a pen priced at 5 with no stock. The second Item overrides only stock to 12, so its name and price still come from the defaults.

Note

In Zig, a field without a default must be given a value in every creation.

Try it Yourself

Edit the code, then press Run.

Running is simulated

const std = @import("std");
const Item = struct { name: []const u8 = "pen", price: i32 = 5, stock: i32 = 0 };

pub fn main() !void {
    const out = std.io.getStdOut().writer();
    const a = Item{};
    const b = Item{ .stock = 12 };
    try out.print("{s} {d} {d}\n", .{ a.name, a.price, a.stock });
    try out.print("{s} {d} {d}\n", .{ b.name, b.price, b.stock });
}

Exercise

What does s.volume print when Settings has volume: i32 = 50 and the code creates const s = Settings{ .level = 3 }; ?

3 0 50 1