Get a varargs-like vprintf or vfprintf in Go

This page was created on Mon Sep 28 2015 and last changed on Sat Oct 05 2024.

This is an example Go program demonstrating how to get a "varargs" effect like vprintf or vfprintf in C. The trick is to declare your "varargs" as ...interface{} then send them on to the next function as a.... This example demonstrates printing an error message with the file name and line number.

package main

import (
        "fmt"
        "os"
        "runtime"
)

func line_err(format string, a ...any) {
        _, file, line, _ := runtime.Caller(1)
        fmt.Fprintf(os.Stderr, "%s:%d: ", file, line)
        fmt.Fprintf(os.Stderr, format, a...)
        fmt.Fprintf(os.Stderr, "\n")
}

func main() {
        line_err("Try %d this %s out", 1, "moo")
}

(download)

The output of the example looks like this:

/share/websites/www.lemoda.net/go/varargs/v.go:17: Try 1 this moo out