英文:
When `User` is struct, what is `(*User)(nil)`?
问题
这是一个Go语言的代码片段。(*User)(nil)是一个类型转换表达式，将nil转换为*User类型的指针。在这个代码中，v被声明为*User类型的指针，并被赋值为nil。在打印v的时候，它将输出<nil>，表示该指针为空指针。
英文:
This compiles:
package main
import (
"fmt"
)
type User struct {
ID int64
}
func main() {
v := (*User)(nil)
fmt.Println(v)
}
Here, what is (*User)(nil)?
I encountered this notation at go-pg, and had no clue to find an answer because it was very hard to search on google.
答案1
得分: 5
如果User是一个类型，*User是另一种类型，即指向User的指针类型。
(*User)(nil)是一种类型转换：它将未类型化的nil预声明标识符转换为(*User)。你必须将*User放在括号中，否则表达式将尝试将nil转换为User（如果User是一个结构体，则会在编译时出错），然后对其进行解引用。
因此，v将是一个类型为*User的变量，保存着nil指针值。
v := (*User)(nil)表达式是一种短变量声明，它等同于以下变量声明的简写形式：
var v *User = nil
当然，这与以下代码是相同的：
var v *User
因为如果初始化表达式缺失，变量将被初始化为其零值，对于所有指针类型来说，零值是nil。
英文:
If User is a type, *User is another type, a pointer type, a pointer to User.
(*User)(nil) is a type conversion: it converts the untyped nil predeclared identifier to (*User). You must put *User into parenthesis, else the expression would try to convert nil to User (which is a compile-time error if User is a struct), and then dereference it.
So v will be a variable of type *User, holding the nil pointer value.
The v := (*User)(nil) expression is a short variable declaration and it is equivalent (shorthand) to the following variable declaration:
var v *User = nil
Which is of course the same as
var v *User
Because if the initialization expression is missing, the variable will be initialized to its zero value which is nil for all pointer types.
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