dictionary - 检测映射中的键是否存在结构

Question

根据 map 上的 Golang 文档，

If the requested key doesn't exist, we get the value type's zero value. In this case the value type is int, so the zero value is 0:

j := m["root"] // j == 0

所以我要确定给定字符串是否存在一个结构，我该如何确定呢？我会检查一个带有零值的空结构吗？这里的比较会是什么样子？

type Hello struct{}
structMap := map[string]Hello{}
j := structMap["example"]
if(j==?) {
 ...
}

Answer 1

使用特殊的“comma, ok”形式来告知是否在 map 中找到了键。去Spec: Index Expressions:

An index expression on a map a of type map[K]V used in an assignment or initialization of the special form

v, ok = a[x]
v, ok := a[x]
var v, ok = a[x]

yields an additional untyped boolean value. The value of ok is true if the key x is present in the map, and false otherwise.

所以在你的代码中:

type Hello struct{}
structMap := map[string]Hello{}
if j, ok := structMap["example"]; !ok {
    // "example" is not in the map
}