Zig doesn’t have a traditional macro system or complex template syntax like C++. Instead, it allows you to run Zig code during compilation. This feature is called comptime.

Running Code at Compile Time

You can use the comptime keyword to force a block of code to execute while your program is being compiled.

const result = comptime {
    var sum = 0;
    var i = 0;
    while (i < 10) : (i += 1) {
        sum += i;
    }
    return sum;
};

Since result is computed at compile time, it’s just a constant 45 in the final binary.

Generic Types

comptime is also how Zig handles generics. A generic type is just a function that returns a type at compile time.

fn List(comptime T: type) type {
    return struct {
        items: []T,
        len: usize,
    };
}

const IntList = List(i32);
const FloatList = List(f64);

This approach is incredibly powerful because it uses the same language syntax you already know, rather than a separate template language.

Inline Loops

You can even use inline for to unroll loops at compile time:

inline for (.{ "hello", "world" }) |word| {
    // This code is generated twice, once for "hello" and once for "world"
    std.debug.print("{s}\n", .{word});
}

comptime turns metaprogramming from a dark art into standard programming.