Go

What do you think the output of the following code would be? package main import "fmt" func main() { input := "hello" TestDefer(&input) } func TestDefer(input *string) { defer fmt.Println(*input) *input = "world" fmt.Println(*input) } Given how defer-ed functions are executed just before the parent function exits, I expected the output to be world world But, on execution it actually prints world hello This is because the arguments are evaluated when the defer is encountered, and not when the deferred function is actually called. Effective Go even has a line specifically about this behavior (which I discovered later). ...

Recently, I started randomly going through the Go standard library, mostly to satiate my curiosity and to find out what goes on behind the curtains. While checking out the testing package, I found this interesting little snippet of code in src/testing/testing.go: // TB is the interface common to T and B. type TB interface { Cleanup(func()) Error(args ...interface{}) Errorf(format string, args ...interface{}) Fail() FailNow() Failed() bool Fatal(args ...interface{}) Fatalf(format string, args ...interface{}) Helper() Log(args ...interface{}) Logf(format string, args ...interface{}) Name() string Skip(args ...interface{}) SkipNow() Skipf(format string, args ...interface{}) Skipped() bool TempDir() string // A private method to prevent users implementing the // interface and so future additions to it will not // violate Go 1 compatibility. private() } This seems pretty evident once you see it. It makes sense for the Go standard library where the private function enables them to circumvent the compatibility promise by ensuring that no one would be able to use this interface outside of the standard library because of the private function. This gives them the flexibility to add functionality later without breaking anything. ...