Declaration scopes in Go

This post is about declaration scopes and shadowing in Go.

package main
import "fmt"
func f(x int) {
	for x := 0; x < 10; x++ {
		fmt.Println(x)
	}
}
var x int
func main() {
	var x = 200
	f(x)
}

This program declares x four times. All four are different variables because they exist in different scopes.

package main
import "fmt"
func f() {
	x := 200
	fmt.Println("inside f: x =", x)
}
func main() {
	x := 100
	fmt.Println("inside main: x =", x)
	f()
	fmt.Println("inside main: x =", x)
}

In Go the scope of a declaration is bound to the closest pair of curly braces, { and }. In this example, we declare x to be 100 inside main, and 200 inside f.

What do you expect this program will print?

package main
import "fmt"
func main() {
	x := 100
	for i := 0; i < 5; i++ {
		x := i
		fmt.Println(x)
	}
	fmt.Println(x)
}

There are several scopes in a Go program; block scope, function scope, file scope, package scope, and universe scope. Each scope encompasses the previous. What you are seeing is called shadowing.

var x = 100
func main() {
 var x = 200
 fmt.Println(x)
}

Most developers are comfortable with a function scoped variable shadowing a package scoped variable.

func f() {
 var x = 99
 if x > 90 {
 x := 60
 fmt.Println(x)
 }
}

But a block scoped variable shadowing a function scoped variable may be surprising.

The justification for a declaration in one scope shadowing another is consistency, prohibiting just block scoped declarations from shadowing another scope, would be inconsistent.

本文来自:Dave Cheney

感谢作者:Dave Cheney

查看原文:Declaration scopes in Go

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