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golang when is nil not nil
source link: https://www.tuicool.com/articles/QzU3qeF
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在开发中，我们经常会遇到一个nil值不等于nil，先看一下下面这个例子
type itest struct {
a string
}
func printA() *itest{
return nil
}
func main() {
var i interface{} = printA()
fmt.Printf("i is nil %v, i = %v", i == nil, i)
}
最后输出结果是 i is nil false, i = <nil>
这是因为interface是由两个元素type:T 和value: V所表示。V是具体值，比如int, struct, pointer的具体值。var i interface = 3, i的type是int， value是3。
只有当type和value都没有设置（T=nil, V 没有赋值）时interface才等于nil。可是在上面例子，i的type是*itest，所以i不是nil。具体解释可以参考 官方FAQ
解决办法：
func main() {
var i = printA()
fmt.Printf("i is nil %v, i = %v", i == nil, i)
}
最后输出结果是 i is nil true, i = <nil>
不将i转换成interface类型，i这个时候是*itest类型对应的nil值。
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