GO by example, part 5

5-go-by-example/44-defer.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+// Defer is used to ensure that a function call is performed later in a program’s execution, usually for purposes of cleanup
+func main() {
+	// suppose you wanted to create a file, write to it, and then close when done
+	// immediately after getting a file object with createFile, 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")
+	defer closeFile(f)
+	writeFile(f)
+}
+
+func createFile(p string) *os.File {
+	fmt.Println("creating")
+	f, err := os.Create(p)
+	if err != nil {
+		panic(err)
+	}
+	return f
+}
+
+func writeFile(f *os.File) {
+	fmt.Println("writing")
+	fmt.Fprintln(f, "data")
+
+}
+
+// it’s important to check for errors when closing a file, even in a deferred function
+func closeFile(f *os.File) {
+	fmt.Println("closing")
+	err := f.Close()
+
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+		os.Exit(1)
+	}
+}