Why Does My fmt.Fprintf Wrapper Print Unexpected Output?

Passing Arguments to Variadic Functions

Variadic functions in Go accept a variable number of arguments, which are stored as a slice of the specified type. When calling a variadic function, it's essential to understand how arguments are passed.

Problem: Extra Output in fmt.Fprintf Wrapper

Consider the following code that attempts to create a wrapper around fmt.Fprintf with variable-length arguments:

func Die(format string, args ...interface{}) {
    str := fmt.Sprintf(format, args)
    fmt.Fprintf(os.Stderr, "%v\n", str)
    os.Exit(1)
}

When calling Die("foo"), the output contains the unexpected "%%!(EXTRA []interface {}=[])".

Solution: Using Ellipsis ... to Pass Arguments

The issue arises from passing the args slice directly to fmt.Sprintf. To correctly pass arguments individually, the ellipsis operator ... must be used:

package main

import (
    "fmt"
    "os"
)

func Die(format string, args ...interface{}) {
    str := fmt.Sprintf(format, args...)
    fmt.Fprintf(os.Stderr, "%v\n", str)
    os.Exit(1)
}

func main() {
    Die("foo")
}

By using ..., each element of the args slice is passed as a separate argument to fmt.Sprintf, resolving the issue and producing the expected output:

foo

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