Defer
Defer is a language feature that schedules an operation to execute when the surrounding function or scope exits, including during exceptional unwinding.
Go in Practice, Second Edition
A deferred function is a function call scheduled by Go to execute when its surrounding function returns, including during panic unwinding.
- How it is used
- Schedules cleanup after acquiring HTTP bodies, files, or network connections, and runs panic-recovery handlers during unwinding.
- How deep it goes
- Explains execution on return or panic, closure scope, recovery mechanics, and the `os.Exit` exception without detailing runtime implementation.
- Examples
- Uses an HTTP response body, printed execution order, a recovering closure, closure-scope examples, and a network log client.
- What you can do after
- Practice/perform deferred resource cleanup and panic recovery while avoiding `log.Fatal*` when cleanup must run.
100 Go Mistakes and How to Avoid Them
A defer statement schedules a function call to execute when the surrounding function returns, including during panic unwinding.
- How it is used
- Uses deferred calls to centralize resource cleanup and status reporting across returns.
- How deep it goes
- Traces function-return timing and immediate argument evaluation, then works through deferred cleanup errors rather than runtime internals.
- Examples
- Contrasts file-opening loops with per-file functions, and shows status calls receiving an unexpectedly empty string.
- What you can do after
- Diagnose/debug with it: prevent loop-bound descriptor leaks and deferred calls that receive stale values.
Read how 100 Go Mistakes and How to Avoid Them explains defer →
Learn Go with Pocket-Sized Projects
defer is a Go language feature that schedules a function call to execute when the surrounding function returns.
- How it is used
- Uses deferred cleanup to keep file-resource management next to opening operations and protect it across return paths.
- How deep it goes
- Defines deferred execution, demonstrates return-time ordering, and applies it to file closing without examining runtime internals.
- Examples
- Demonstrates deferred print statements and defers file.Close immediately after os.Open succeeds.
- What you can do after
- Apply defer to schedule cleanup reliably when writing Go functions that open files.
Read how Learn Go with Pocket-Sized Projects explains defer →
Related concepts
- close functionuses · In Go, close is a built-in function that marks a channel closed, preventing further sends and allowing receivers to detect completion.
- closeuses · Close is an operation that terminates access to an associated resource or interface and releases or finalizes related resources or pending work.
- functionuses · A function is a callable program unit that performs an operation, may accept inputs, and can produce a result or side effect.
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