示例：

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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
 
package proxy
 
import (
"net"
)
 
type direct struct{}
 
// Direct is a direct proxy: one that makes network connections directly.
var Direct = direct{}
 
func (direct) Dial(network, addr string) (net.Conn, error) {
return net.Dial(network, addr)
}

上面是 golang.org/x/net/proxy/direct.go 里面的代码

  1. 我们显然是可以在proxy包外访问到Direct变量的
  2. 我们显然是无法在proxy包外new 一个 direct 结构变量的
  3. 由于2，而且我们无法改变Direct的变量类型，所以，我们做不到给Direct进行重新赋值