`sync.WaitGroup` 的方法集是什么？

Question

我在下面有这个简单的程序

package main

import (
    "fmt"
    "sync"
    "time"
)

var wg sync.WaitGroup

func main() {
    wg.Add(1)

    go func() {
        fmt.Println("starting...")
        time.Sleep(1 * time.Second)
        fmt.Println("done....")
        wg.Done()
    } ()

    wg.Wait()

}

请注意，我var wg sync.WaitGroup用作值，而不是指针。但是同步包的页面指定Add,Done和Wait函数采用*sync.WaitGroup.

为什么/这是如何工作的？

Answer 1

的设定方法的sync.WaitGroup是空的方法集：

wg := sync.WaitGroup{}
fmt.Println(reflect.TypeOf(wg).NumMethod())

输出（在Go Playground上试试）：

0

这是因为所有的方法sync.WaitGroup都有指针接收者，所以它们都是该*sync.WaitGroup类型的方法集的一部分。

当你这样做时：

var wg sync.WaitGroup

wg.Add(1)
wg.Done()
// etc.

This is actually a shorthand for (&wg).Add(1), (&wg).Done() etc.

This is in Spec: Calls:

If x is addressable and &x's method set contains m, x.m() is shorthand for (&x).m().

So when you have a value that is addressable (a variable is addressable), you may call any methods that have pointer receiver on non-pointer values, and the compiler will automatically take the address and use that as the receiver value.

See related question:

Calling a method with a pointer receiver by an object instead of a pointer to it?