go - 如果某物不在 map 中，返回值取什么值？

Question

好的，按照这个:

How to check if a map contains a key in go?

if val, ok := m["foo"]; ok {
    //do something here
}

很好，但我们为什么不能这样做:

val, ok := m["foo"]

if val == nil {   // cannot compare val to nil

}

我收到一个编译错误，提示我无法将 val 与 nil 进行比较，但是 val 有什么值呢？我可以将它与什么进行比较，以确定它是否存在？

m的类型是这样的:

type m map[string]struct{}

Answer 1

The Go Programming Language Specification

Index expressions

For a of map type M: if the map is nil or does not contain such an entry, a[x] is the zero value for the element type of M.

The zero value

When storage is allocated for a variable, either through a declaration or a call of new, or when a new value is created, either through a composite literal or a call of make, and no explicit initialization is provided, the variable or value is given a default value. Each element of such a variable or value is set to the zero value for its type: false for booleans, 0 for numeric types, "" for strings, and nil for pointers, functions, interfaces, slices, channels, and maps.

---

The Go Programming Language Specification

Composite literals

Composite literals construct values for structs, arrays, slices, and maps and create a new value each time they are evaluated. They consist of the type of the literal followed by a brace-bound list of elements. Each element may optionally be preceded by a corresponding key. For struct literals the following rules apply:

A literal may omit the element list; such a literal evaluates to the zero value for its type.

对于您的示例，键入 struct{}，从复合文字 struct{}{} 中省略元素列表作为零值。

例如，

package main

import "fmt"

func main() {
    m := map[string]struct{}{}
    val, ok := m["foo"]
    fmt.Printf("%T %v\n", val, val)
    if val == struct{}{} {
        fmt.Println("==", val, ok)
    }
}

Playground :https://play.golang.org/p/44D_ZfFDA77

输出:

struct {} {}
== {} false

---

The Go Programming Language Specification

Variable declarations

A variable declaration creates one or more variables, binds corresponding identifiers to them, and gives each a type and an initial value.

If a list of expressions is given, the variables are initialized with the expressions following the rules for assignments. Otherwise, each variable is initialized to its zero value.

If a type is present, each variable is given that type. Otherwise, each variable is given the type of the corresponding initialization value in the assignment.

在您的示例中，您可以声明一个没有初始值的 struct{} 类型的变量，该变量将被初始化为 struct{} 类型的零值.

例如，

package main

import "fmt"

func main() {
    m := map[string]struct{}{}
    val, ok := m["foo"]
    fmt.Printf("%T %v\n", val, val)
    var zeroValue struct{}
    if val == zeroValue {
        fmt.Println("==", val, ok)
    }
}

Playground :https://play.golang.org/p/_XcSCEeEKJV

输出:

struct {} {}
== {} false