Defer

Intermediate · Functions & Errors

defer schedules a call to run when the surrounding function returns. Deferred calls run in LIFO order and execute on every return path — the idiomatic way to guarantee cleanup (defer f.Close()).

• defer1 — LIFO order (and shaping a named return value)
• defer2 — guaranteed cleanup on every return path

Resources

• A Tour of Go: Defer
• Go by Example: Defer
• Effective Go: Defer

Exercises

defer1 test

Deferred calls run when the surrounding function returns, in LIFO order

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Defers run LIFO. With a named return seq, deferred appends mutate it:

defer func() { seq = append(seq, 1) }()
defer func() { seq = append(seq, 2) }()
defer func() { seq = append(seq, 3) }()  // runs first -> [3,2,1]

Source

defer2 test

defer is the idiomatic way to guarantee cleanup runs on every return path

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Defer the cleanup at the top so it runs on every return:

defer r.Close()

Source