defer
- defer

defer

Executes an expression unconditionally at scope exit.

test_defer.zig

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

fn deferExample() !usize {
    var a: usize = 1;

    {
        defer a = 2;
        a = 1;
    }
    try expect(a == 2);

    a = 5;
    return a;
}

test "defer basics" {
    try expect((try deferExample()) == 5);
}

Shell

$ zig test test_defer.zig
1/1 test_defer.test.defer basics... OK
All 1 tests passed.

Defer expressions are evaluated in reverse order.

defer_unwind.zig

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

test "defer unwinding" {
    print("\n", .{});

    defer {
        print("1 ", .{});
    }
    defer {
        print("2 ", .{});
    }
    if (false) {
        // defers are not run if they are never executed.
        defer {
            print("3 ", .{});
        }
    }
}

Shell

$ zig test defer_unwind.zig
1/1 defer_unwind.test.defer unwinding...
2 1 OK
All 1 tests passed.

Inside a defer expression the return statement is not allowed.

test_invalid_defer.zig

fn deferInvalidExample() !void {
    defer {
        return error.DeferError;
    }

    return error.DeferError;
}

Shell

$ zig test test_invalid_defer.zig
docgen_tmp/test_invalid_defer.zig:3:9: error: cannot return from defer expression
        return error.DeferError;
        ^~~~~~~~~~~~~~~~~~~~~~~
docgen_tmp/test_invalid_defer.zig:2:5: note: defer expression here
    defer {
    ^~~~~