Defer

Defer is used to execute a statement while exiting the current block.

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

test "defer" {
    var x: i16 = 5;
    {
        defer x += 2;
        try expect(x == 5);
    }
    try expect(x == 7);
}

When there are multiple defers in a single block, they are executed in reverse order.

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

test "multi defer" {
    var x: f32 = 5;
    {
        defer x += 2;
        defer x /= 2;
    }
    try expect(x == 4.5);
}

Defer is useful to ensure that resources are cleaned up when they are no longer needed. Instead of needing to remember to manually free up the resource, you can add a defer statement right next to the statement which allocates that resource.