Why Does Global Error Variable Initialization Fail in Go?

Global Error Variable Initialization Failure

In a Go program, initializing an error variable globally poses a peculiar issue where it remains nil in other functions within the same package. To unravel this mystery, let's delve into a specific example:

package main</p>
<p>import (</p>
<pre class="brush:php;toolbar:false">"os"
"fmt"

)

var loadErr error

func main() {

f, loadErr := os.Open("asdasd")
if loadErr != nil {
    checkErr()
}
if f != nil {
    fmt.Println(f.Name())
}

}

// panic won't be called because loadErr is nil
func checkErr() {

if loadErr != nil {
    panic(loadErr)
}

}

In this scenario, you'd naturally expect the code to panic when the file cannot be opened. However, contrary to expectations, it remains silent due to the loadErr variable being nil. To resolve this, a crucial distinction must be made.

In Go, when using the := operator, a new local variable is created within the scope of the function. In this case, the line:

f, loadErr := os.Open("asdasd")

essentially constructs a local variable named loadErr, distinct from the globally declared variable. Unfortunately, the global variable remains unaffected, leading to the nil value conundrum.

To remedy this issue, the := operator must be replaced with the standard assignment operator =. This ensures that the global variable loadErr is referenced and initialized with the value returned by os.Open():

func main() {</p>
<pre class="brush:php;toolbar:false">_, loadErr = os.Open("asdasd")

By making this subtle change, the global error variable is correctly set, and the panic function will now behave as intended.

Furthermore, if you require two return values from os.Open(), the second variable used in the assignment must be explicitly declared beforehand:

var f *os.File<br>f, loadErr = os.Open("asdasd")<br>

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