将函数返回的值分配给指针。

huangapple go评论93阅读模式
英文:

Assign value returned from function to pointer

问题

在Go语言中,如何将函数调用返回的值赋给指针?

考虑以下示例,注意time.Now()返回的是一个time.Time值(而不是指针):

package main

import (
    "fmt"
    "time"
)

type foo struct {
    t *time.Time
}

func main() {
    var f foo 

    f.t = time.Now()  // 失败,第15行

    f.t = &time.Now() // 失败,第17行

    tmp := time.Now() // 解决方法
    f.t = &tmp

    fmt.Println(f.t)
}

这两种方式都会失败:

$ go build
# _/home/jreinhart/tmp/go_ptr_assign
./test.go:15: cannot use time.Now() (type time.Time) as type *time.Time in assignment
./test.go:17: cannot take the address of time.Now()

是否真的需要一个局部变量?这样做是否会产生不必要的复制?

英文:

In Go, how do you assign a value returned by a function call to a pointer?

Consider this example, noting that time.Now() returns a time.Time value (not pointer):

package main

import (
    "fmt"
    "time"
)

type foo struct {
    t *time.Time
}

func main() {
    var f foo 

    f.t = time.Now()  // Fail line 15

    f.t = &time.Now() // Fail line 17

    tmp := time.Now() // Workaround
    f.t = &tmp

    fmt.Println(f.t)
}

These both fail:

<!-- language: none -->

$ go build
# _/home/jreinhart/tmp/go_ptr_assign
./test.go:15: cannot use time.Now() (type time.Time) as type *time.Time in assignment
./test.go:17: cannot take the address of time.Now()

Is a local variable truly required? And doesn't that incur an unnecessary copy?

答案1

得分: 6

本地变量是必需的根据规范

要获取值的地址,调用函数必须将返回值复制到可寻址的内存中。这里有一个复制,但它并不是多余的。

Go程序通常使用time.Time值。

在某些情况下,会使用*time.Time,以便应用程序可以区分没有值和其他时间值。区分SQL NULL和有效时间就是一个例子。由于time.Time的零值在过去很久以前,通常可以使用零值来表示没有值。使用IsZero()方法来测试是否为零值。

英文:

The local variable is required per the specification.

To get the address of a value, the calling function must copy the return value to addressable memory. There is a copy, but it's not extra.

Go programs typically work with time.Time values.

A *time.Time is sometimes used situations where the application wants to distinguish between no value and other time values. Distinguishing between a SQL NULL and a valid time is an example. Because the zero value for a time.Time is so far in the past, it's often practical to use the zero value to represent no value. Use the IsZero() method to test for a zero value.

huangapple
  • 本文由 发表于 2017年2月27日 10:15:53
  • 转载请务必保留本文链接:https://go.coder-hub.com/42476803.html
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