去全局变量不能反映正确的值

Question

I have the following code

package main

import ( 
    "fmt"
    "flag"
)

var outputOnly bool


func something() string {
    if outputOnly {
        fmt.Println("outputting only")
    } else {
        fmt.Println("executing commands")
    }   
    return "blah"
}

func main() {

    vmoutputonlyPtr     := flag.Bool("outputonly",false,"If set it will only output the commands it would execute, naturally without the correct parameter values set.")
    flag.Parse()
    outputOnly          := *vmoutputonlyPtr 
    if outputOnly {
        fmt.Println("outputonly commands will not execute")
    }

    var blah string
    blah = something()
    fmt.Println("blah is " + blah)
}

But the output is this:

$ ./se -outputonly                       
outputonly commands will not execute
executing commands

ie. it appears that the function something() is aware of the global variable, but does not reflect its true value. This is my first attempt at golang. What am I doing wrong?

Answer 1

The problem is this line in main.

outputOnly := *vmoutputonlyPtr 

:= declares a new variable on the left, outputOnly, of the type of the expression on the right, *vmoutputonlyPtr and assigns the expression to it. It's equivalent to...

var outputOnly bool = *vmoutputonlyPtr

This new outputOnly "shadows" your global outputOnly in its scope. So all the code after outputOnly := *vmoutputonlyPtr in main refers to this outputOnly local to main. While something() refers to the global outputOnly.

See Redeclaration and Reassignment in Effective Go for more about variable shadowing in Go.

If you want to assign to an existing variable, just use =.

outputOnly = *vmoutputonlyPtr