Get a varargs-like vprintf or vfprintf in Go

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...interface{}) {
        _, 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


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