| _Defer_ is used to ensure that a function call is performed later in a program's execution, usually for purposes of cleanup. `defer` is often used where e.g. `ensure` and `finally` would be used in other languages. | ||
package main |
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import ( |
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"fmt" |
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"os" |
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) |
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| Suppose we wanted to create a file, write to it, and then close when we're done. Here's how we could do that with `defer`. | func main() { |
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| Immediately after getting a file object with `createFile`, we defer the closing of that file with `closeFile`. This will be executed at the end of the enclosing function (`main`), after `writeFile` has finished. | f := createFile("/tmp/defer.txt") |
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defer closeFile(f) |
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writeFile(f) |
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} |
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func createFile(p string) *os.File { |
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fmt.Println("creating") |
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f, err := os.Create(p) |
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if err != nil { |
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panic(err) |
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} |
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return f |
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} |
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func writeFile(f *os.File) { |
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fmt.Println("writing") |
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fmt.Fprintln(f, "data") |
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} |
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func closeFile(f *os.File) { |
creating writing |
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fmt.Println("closing") |
closing |
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err := f.Close() |
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| It's important to check for errors when closing a file, even in a deferred function. | if err != nil { |
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fmt.Fprintf(os.Stderr, "error: %v\n", err) |
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os.Exit(1) |
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} |
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} |