Defers

Defers are a programming mechanism for scheduling a piece of code to run later, typically when the surrounding function or scope exits. They are commonly used to guarantee resource cleanup and to separate cleanup logic from the main control flow, making code easier to read and less error-prone.

In languages that implement explicit defer constructs, the deferred action is registered and then executed when the surrounding function returns or the scope is exited. In Go, for example, the statement defer f(args) records a call to f with the given arguments; the arguments are evaluated at the time of the defer, and the call itself is executed in last-in, first-out order as the function returns. If a function contains multiple defers, they run in reverse order of their registration. In Swift, defer blocks work similarly, running when the scope exits, including in cases of early returns or thrown errors.

Defers are closely related to other cleanup mechanisms such as finally blocks in imperative languages or scope-based destructors in others. They are often used for resource management tasks like closing files, releasing locks, or cleaning up network resources, ensuring that cleanup runs even when errors or early exits occur.

Common considerations include the order of execution (usually LIFO), the evaluation timing of arguments to the deferred call, and how defers interact with exceptions or panics. While defers provide reliable cleanup, they can add complexity if overused or if the deferred actions have side effects that are hard to follow during unwinding. Overall, defers offer a robust pattern for ensuring necessary cleanup in a predictable, scope-bound manner.