What Does `...interface{}` Mean in Go Variadic Functions?

Understanding the Meaning of ...interface{} in Go

In Go, a parameter type prefixed with three dots (...) is known as a variadic parameter. This means that a function can accept any number of arguments for that particular parameter.

Consider the following function:

func DPrintf(format string, a ...interface{}) (n int, err error) {
  if Debug > 0 {
    n, err = fmt.Printf(format, a...)
  }
  return
}

The parameter a is of type ...interface{}, indicating that it can accept zero or more arguments of any type. These arguments are then received by the function as a slice of the type declared for the parameter, which in this case is []interface{}.

The Go Specification provides the following explanation:

"The final parameter in a function signature may have a type prefixed with .... A function with such a parameter is called variadic and may be invoked with zero or more arguments for that parameter."

Therefore, a parameter of type ...interface{} is equivalent to a parameter of type []interface{} within the function. The difference lies in how arguments are passed to the function. They can be passed either individually or as a single slice, in which case the slice value must be suffixed with the three dots (...).

For example, the following two calls to the fmt.Println function will produce the same result:

fmt.Println("First", "Second", "Third")

s := []interface{}{"First", "Second", "Third"}
fmt.Println(s...)

In both cases, the slice s is passed to the Println function with the three dots, indicating that it should be treated as a variadic argument.

This mechanism provides flexibility by allowing functions to accept a variable number of arguments, making them more versatile and reusable.

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