defer

test_defer.zig

const std = @import("std");
const expect = std.testing.expect;
const print = std.debug.print;
// defer will execute an expression at the end of the current scope.
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);
}
// If multiple defer statements are specified, they will be executed in
// the reverse order they were run.
fn deferUnwindExample() void {
    print("\n", .{});
    defer {
        print("1 ", .{});
    }
    defer {
        print("2 ", .{});
    }
    if (false) {
        // defers are not run if they are never executed.
        defer {
            print("3 ", .{});
        }
    }
}
test "defer unwinding" {
    deferUnwindExample();
}

Shell

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

test_invalid_defer.zig

// Inside a defer expression the return statement is not allowed.
fn deferInvalidExample() !void {
    defer {
        return error.DeferError;
    }
    return error.DeferError;
}

Shell

$ zig test test_invalid_defer.zig
docgen_tmp/test_invalid_defer.zig:4:9: error: cannot return from defer expression
        return error.DeferError;
        ^~~~~~~~~~~~~~~~~~~~~~~
docgen_tmp/test_invalid_defer.zig:3:5: note: defer expression here
    defer {
    ^~~~~

test_errdefer.zig

const std = @import("std");
const print = std.debug.print;
// The errdefer keyword is similar to defer, but will only execute if the
// scope returns with an error.
//
// This is especially useful in allowing a function to clean up properly
// on error, and replaces goto error handling tactics as seen in c.
fn deferErrorExample(is_error: bool) !void {
    print("\nstart of function\n", .{});
    // This will always be executed on exit
    defer {
        print("end of function\n", .{});
    }
    errdefer {
        print("encountered an error!\n", .{});
    }
    if (is_error) {
        return error.DeferError;
    }
}
// The errdefer keyword also supports an alternative syntax to capture the
// generated error.
//
// This is useful for printing an additional error message during clean up.
fn deferErrorCaptureExample() !void {
    errdefer |err| {
        std.debug.print("the error is {s}\n", .{@errorName(err)});
    }
    return error.DeferError;
}
test "errdefer unwinding" {
    deferErrorExample(false) catch {};
    deferErrorExample(true) catch {};
    deferErrorCaptureExample() catch {};
}

Shell

$ zig test test_errdefer.zig
1/1 test.errdefer unwinding...
start of function
end of function
start of function
encountered an error!
end of function
the error is DeferError
OK
All 1 tests passed.

See also:

• Errors