推迟现场分配是竞争条件吗？

Question

Consider the following code:

type foo struct {
  bar string
}

func f() *foo {
  ret := &foo{"before"}
  defer func() { ret.bar = "after" }()
  return ret
}

func main() {
  fmt.Println(f()) // prints "&{after}"
}

The motivation is having to return a struct but some of the fields need to be set only before returning (e.g. a timestamp of when the function completed).

Is deferring the field assignation a race condition? Is it idiomatic? Is there a better way?

Answer 1

The main benefit of using defer statement versus invoking something before returning from a function or routine is defer will run the statement even when a panic occurs before returning.

Thus it is commonly used to clean up resources (such as closing a file or network connection) rather than to set state.

The function below will not print or return "hello"

func f() string {
        panic("omg")
        fmt.Println("hello")
        return "hello"
}

This code will print but won't return "hello"

func f() string {
        defer fmt.Println("ello")
        panic("omg")
        return "hello"
}

To answer your question: No, it won't result in a race. Apart from the above difference, it is equivalent to calling something before the return statement.