英文:
Reference to another field with in a func in the same struct in golang
问题
我有一个类似这样的结构体：
type Job struct {
Action func()
Cron string
}
那么对于这个结构体的实例，Action 的实现代码是否可以获取到 Cron 的引用？谢谢。
英文:
I have a struct looks something like this:
type Job struct {
Action func()
Cron string
}
So for an instance of this struct, is it possible for the code inside the implementation of Action to get a reference to Cron? Thanks.
答案1
得分: 5
使用闭包来创建Action函数：
func MakeAction(job *Job) func() {
return func() {
// 使用变量job的函数逻辑
}
}
你可以像这样使用MakeAction函数：
j := &Job{ nil, "foo" }
j.Action = MakeAction(j)
请注意，当有人复制Job结构体时，这种方式可能会产生意外的结果——由MakeAction()创建的闭包内部的引用仍然指向原始实例（而不是副本）。
英文:
Use a closure to create the Action function:
func MakeAction(job *Job) func() {
return func() {
// function that uses the variable job
}
}
You can use the MakeAction function like this:
j := &Job{ nil, "foo" }
j.Action = MakeAction(j)
Notice that this will behave in possibly unexpected ways when somebody makes a copy of the Job structure—the reference inside the closure created by MakeAction() will still point to the original instance (instead of the copy).
答案2
得分: 3
你似乎必须将Job引用传递给Action函数，以便该函数可以访问Cron。
例如，参见以下代码：
package main
import "fmt"
type Job struct {
Action func(*Job)
Cron string
}
func main() {
j := &Job{f, "test"}
fmt.Println("Hello, playground", j)
j.Action(j)
}
func f(j *Job) {
fmt.Println(j.Cron)
}
输出结果为：
Hello, playground &{0x201a0 test}
test
在任何闭包示例中，都无法引用自己的结构体。
OP补充道：
外部框架期望一个这样的函数：
func()。
我将这个函数包装到了我自己的结构体Job中，并在Job结构体中放入了其他内容。
现在我希望Action能够访问Job中的其他字段，但似乎不可能实现。
稍后我将直接将Action传递给外部框架。
我确认，仅仅通过将func()（来自外部框架）包装在你自己的结构体中，并不意味着f通过闭包可以访问结构体字段。
你需要进行单独的初始化，就像FUZxxl的答案所示。
否则，那个函数（来自外部框架）怎么会知道Job的存在呢？
这只有在该函数期望一个具有Cron()函数的接口时才能起作用，而不是一个它对其一无所知、具有Cron字段的结构体。
就像这个例子中一样。
英文:
You seem to have to pass a Job reference to the Action function in order for said function to access Cron.
package main
import "fmt"
type Job struct {
Action func(*Job)
Cron string
}
func main() {
j := &Job{f, "test"}
fmt.Println("Hello, playground", j)
j.Action(j)
}
func f(j *Job) {
fmt.Println(j.Cron)
}
Output:
Hello, playground &{0x201a0 test}
test
None of the closures examples allows you to reference your own struct.
The OP adds:
> The external framework expects a function like this: func().
I'm wrapping this function into my homebrewed struct Job, and I put something else into the Job struct.
Now I hope the Action could access other fields in Job, but it doesn't seem to be possible.
Later I will pass the Action directly into the external framework.
I confirm that, just by wrapping func() (from external framework) in a struct of yours doesn't means f has access to the struct fields through closure.
You need a separate initialization, as FUZxxl's answer shows.
Otherwise, how that function (from an external framework) would even know about Job?
That could only work if that function expect an interface with a Cron() function, not a struct which it knows nothing about, with a Cron field.
As in this example.
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