Answer.

The defer keyword in Go postpones the execution of specified functions until the surrounding function exits — functions accumulate in a stack and are executed in reverse order (LIFO). This is often used for resource cleanup (files, mutexes, connections).

A feature is that all arguments of the functions passed to defer are evaluated immediately at the moment of declaration, not at execution time.

func test() {
    for i := 0; i < 3; i++ {
        defer fmt.Println(i) // Will print: 2, 1, 0
    }
}

Trick question.

What will the following code output?

func main() {
    for i := 0; i < 3; i++ {
        defer fmt.Println(i)
    }
}

Answer:

It will output:

2
1
0

Because during each iteration, the argument i is evaluated immediately (at the moment of defer), and all values are stored in the defer stack.

Examples of real errors due to ignorance of the intricacies of the topic.

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Story

In a file service, it was forgotten that defer is only called on normal exit from a function. Leaks appeared if the program crashed before the defer call.

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Story

In a data pipeline, there was a forgetting — defer was used in a loop to close connections, but they actually closed only after the entire function finished, not after each iteration. This led to resource exhaustion.

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Story

In a logger, defer was used with an anonymous function, expecting the argument to be evaluated at the time of the call. As a result, the log at the end contained outdated information, as the values were captured earlier.