Assertions

An assertion checks a condition that should always be true. You can use them to catch bugs and to serve as documentation for other programmers.

std.debug.assert asserts that its argument is true:

const std = @import("std");
const print = std.debug.print;

pub fn main() void {
    std.debug.assert(1 + 1 == 2);
    print("OK\n", .{});
}

$ zig run assert.zig
OK

The unreachable keywords asserts that a location in the code will never be reached:

const std = @import("std");
const print = std.debug.print;

pub fn main() void {
    if (1 + 1 == 2) {
        print("OK\n", .{});
        return;
    }
    unreachable;
}

$ zig run unreachable.zig
OK

If an assertion fails in Debug or ReleaseSafe mode, we get an error message and a stack trace.

In the other modes, a failing assertion is effectively illegal behavior – the idea is that before you enable ReleaseFast or ReleaseSmall you’ve done enough testing to be sure there will be no assertion failures.

Next example: If Statements.