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是什么？为什么？

如何利用unsafe包修改私有成员？

结构体会被分配到一块连续的内存，结构体的地址也代表第一个成员的地址。

如何利用unsafe包获取slice和map的长度？

// 利用unsafe包修改私有成员
type S struct {name     stringlanguage stringage      int
}func main() {s := S{name:     "张三",language: "英语",age:      4,}name := (*string)(unsafe.Pointer(&s))*name = "qcrao"lang := (*string)(unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.language)))*lang = "Golang"age := (*int)(unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.age)))*age = 1fmt.Println(s)
}

// 利用unsafe获取切片和map的长度.
func main() {s := make([]int, 9, 20)var Len = *(*int)(unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + uintptr(8)))fmt.Println(Len, len(s)) // 9 9var Cap = *(*int)(unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + uintptr(16)))fmt.Println(Cap, cap(s)) // 20 20mp := make(map[string]int)mp["qcrao"] = 100mp["stefno"] = 18count := **(**int)(unsafe.Pointer(&mp))fmt.Println(count, len(mp)) // 2 2
}