Go Language Basics: Notes on Using nil

definition

nilNot a golang keyword, just a variable name . defined buildin/buildin.goin

// nil 是一个预先声明的标识符，表示指针、通道、函数、接口、映射或切片类型的零值。
var nil Type // 类型必须是指针、通道、函数、接口、映射或切片类型
​
// Type在这里仅用于文档目的。它是任何 Go 类型的替代品，但代表任何给定函数调用的相同类型。
type Type int
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nilWe can even override it by declaring a variable named by ourselves, of course, this is not recommended.

As seen in the definition's comments, the zero valuenil used to represent a pointer, channel, function, interface, map, or slice type .

zero value

When declaring a variable in golang, if we do not explicitly initialize it, the variable will be assigned a zero value.

type of data	zero value
boolean type	false
Numeric type	0
string	""
reference type	nil

Structures and arrays recursively initialize their fields or elements, whose initial values ​​depend on the element or field.

Panic

assign a value to a null pointer

var i *int
*i = 1    // panic
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Solution

Allocate a block of memory to a null pointer

var i *int
i = new(int)
*i = 1    // panic
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get value from null pointer

var i *int
a := *i    // panic
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Solution

Before assigning, check whether the pointer is empty

var i *int
var a int
if i != nil {
    a = *i
}
// a = 0
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Interface Type

There is a famous example in golang nil != nil:

var x *int
var y interface{}
fmt.Println(x == nil)    // true
fmt.Println(y == nil)    // true
fmt.Println(x == y)      // false
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This result is related interfaceto the implementation principle of , and will not be explained in detail here. In short, one interfaceneeds to determine the value through typeand two attributes.value

Add a comment to the above code to make it clear:

var x *int            // type = *int, value = nil
var y interface{}     // type = nil, value = nil
fmt.Println(x == y)   // (*int, nil) == (nil, nil) => false
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Error handling pit

type MyError struct {}
​
func (m MyError) Error() string { return "my error" }
​
func test() error {
   var err *MyError
   return err
}
​
func main() {
   err := test()
   fmt.Println(err)           // <nil>
   fmt.Println(err == nil)    // false
}
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We will find that when the value of err is directly output, it is indeed <nil>. When nilcomparing this to , the result is again false.

With the above example, we know that it was test()changed in the erroriginal type, resulting in the final judgment result not meeting expectations.