Defer
This Go code illustrates the use of defer
statements to ensure that certain actions, like closing a file, are performed even if an error occurs. Let's go through the code with inline comments and explanations:
// Importing necessary packages.
import (
"fmt"
"os"
)
// The main function, where the execution of the program begins.
func main() {
// Creating a file and deferring the closing of the file until the surrounding function returns.
f := createFile("/tmp/defer.txt")
defer closeFile(f)
// Writing data to the file.
writeFile(f)
}
// createFile function creates a file and returns a pointer to the os.File.
func createFile(p string) *os.File {
fmt.Println("creating")
// Attempting to create the file.
f, err := os.Create(p)
if err != nil {
// If an error occurs during file creation, the program panics.
panic(err)
}
return f
}
// writeFile function writes data to the provided os.File.
func writeFile(f *os.File) {
fmt.Println("writing")
// Writing the data to the file.
fmt.Fprintln(f, "data")
}
// closeFile function closes the provided os.File, handling errors.
func closeFile(f *os.File) {
fmt.Println("closing")
// Closing the file and checking for errors.
err := f.Close()
// Handling errors if the file cannot be closed successfully.
if err != nil {
fmt.Fprintf(os.Stderr, "error: %v\n", err)
os.Exit(1)
}
}
Output
creating
panic: open /tmp/defer.txt: The system cannot find the path specified.
goroutine 1 [running]:
main.createFile({0x622f83, 0xe})
..:/...../..../..../defer.go:19 +0x93
main.main()
..:/...../..../..../defer.go:10 +0x2b
exit status 2
Explanation:
createFile(p string) *os.File
:This function creates a file at the specified path
p
.If an error occurs during file creation, the function panics with the error message.
The created file is returned as a pointer to
os.File
.
writeFile(f *os.File)
:This function writes the string "data" to the provided file (
os.File
).
closeFile(f *os.File)
:This function closes the provided file (
os.File
).If an error occurs during file closing, it prints an error message to
os.Stderr
and exits the program.
defer closeFile(f)
:The
defer
statement ensures that thecloseFile
function is called when the surrounding function (main
in this case) returns, regardless of whether it returns normally or panics.In this example, it ensures that the file is closed even if an error occurs during the execution of
main
.
Using defer
is a convenient way to ensure that cleanup actions are performed, especially when dealing with resources like files, to avoid resource leaks.
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