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Created December 1, 2020 04:35
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Nil in Go

What is nil in Go

nil in Go has several meanings:

  • It represents "null" in Go. This means two things: 1. It does not have type. 2. Its value is "null".
  • It is a predeclared identifier in Go, which means you can use it without declaring it.
  • It represents zero values (and default values) of some types in Go, including:
    • interface types
    • pointer types
    • slice types
    • map types
    • channel types
    • function types

Using nil as Zero Values

nil represents zero values (and default values) of some types in Go.

Example: https://gist.github.com/d1c9fa6273aa11c087fb304ed395fe7d

Using nil in Comparison

Two nil Values of Two Different Types Are Not Comparable

Example: https://gist.github.com/d86f6b1d85e6194d2617ea5fe6b290b1

These code will fail to compile as they are trying to compare nil values of two different types.

Two nil Values of The Same Type May Not Be Comparable

Example: https://gist.github.com/327a0625cf92c10cde3985f83ba7368c

Take var sb = (map[string]bool)(nil) == (map[string]bool)(nil) as an example, the reason why two nil values of a same type (map[string]bool) are not comparable is because Go does not support comparison in slice, map and function types. You can see that we are comparing two values of a non-comparable type in this case. That is why it fails.

But the following code works and results are true:

https://gist.github.com/f158575db9b1813f9369533815028c0f

Take var sb = (map[string]bool)(nil) == nil as an example, (map[string]bool)(nil) declares a map[string]bool temporary variable which value is nil and (map[string]bool)(nil) == nil detects whether the variable's value is nil and then assigns the results to sb. You can see that we are comparing the value of a non-comparable type with its zero value (nil) in this case. That's why it works.

Two nil Values of The Same Type Can Be Comparable Only When This Type Supports Comparision

Example: https://gist.github.com/63ca0182dca2c8e9c89a89d63dc00edf

Be Careful in nil Comparision When Interface Values Are Involved

The following code will not cause any compiler failure but the result is false other than true.

https://gist.github.com/42523b5f818e2a0dbf0da1df1a205120

Explanation:

  • An interface value consists of a dynamic type and a dynamic value. interface{}(nil) declares an interface value with {type: nil, value: nil}.
  • The non-interface value is converted to the type of the interface value before making the comparison with an interface value. In this example, (*int)(nil) is converted to an interface value with {type: *int, value: nil}.
  • Two nil interface values are equivalent only when they carry the same type. In this case, the converted interface value {type: *int, value: nil} has a concrete dynamic type but the other interface value has not. That is why the comparison result is false.

A more interesting example: https://gist.github.com/a67a6e1cea706a787e017f0786479e13

Explanation:

  • An interface value equals to nil only when its type and value are both nil. In example, w is an io.Writer interface value with {type: *bytes.Buffer, value: nil} after the w = b assignment. Therefore, w == nil is false as it carries *bytes.Buffer other than nil as its concrete dynamic type.

Summary

  • nil is and a pre-declared identifier which can be used to represent the zero values of some types in Go.
  • Be careful when using nil in comparison, especially when interface values are involved. You need to understand what you are comparing: types, or values, or both.
  • (a thing)(nil) may not equal to nil, depends on what that thing is (a pointer or an interface). This means Go is a strong-type language and it also applies to nil even though nil itself does not have default type (sarcasm).
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