php小編魚仔為您揭示一個關于切片的問題：為什么通過通道和 goroutine 寫入切片最終為空？在Go語言中，通道和 goroutine 是并發編程的重要工具，但在某些情況下，使用它們寫入切片可能會出現意外結果。本文將詳細解釋這個問題的原因，并提供解決方案，幫助您更好地理解和處理這種情況。

問題內容

我運行這個函數：

func run() () {
    // this slice is going to be filled out by a channel and goroutine.
    vertices := make([]vertex, 0)

    var wg sync.waitgroup

    // obtain a writer to fill out the vertices.
    writer := writer(&wg, vertices)

    // run an arbitrary logic to send data to writer.
    logic(writer)

    // stop the writer reading on the channel.
    close(writer)

    // wait for the write to complete.
    wg.wait()

    // see if vertices slice is actually filled out.
    doublecheckvertices(vertices)
}

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但最終，我的 vertices 切片是空的：

func doublecheckvertices(vertices []vertex) () {
    // here i notice that `vertices` slice is actually empty :(

}

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返回 writer 的函數是這樣的：

func writer(wg *sync.waitgroup, vertices []vertex) (chan<- []*triangle3) {
    // external code writes to this channel.
    // this goroutine reads the channel and writes to vertices.
    writer := make(chan []*triangle3)

    // write by a goroutine.
    wg.add(1)
    go func() {
        defer wg.done()

        a := vertex{}

        // read from the channel and write them to vertices.
        for ts := range writer {
            for _, t := range ts {
                a.x = float32(t.v[0].x)
                a.y = float32(t.v[0].y)
                a.z = float32(t.v[0].z)
                vertices = append(vertices, a)
            }
        }
    }()

    return writer
}

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任何人都可以幫我弄清楚為什么我的 vertices 切片最終是空的嗎？

日志

日志表明 vertices 切片實際上已填充。但由于某種原因，傳遞給doublecheckvertices時為空。

                vertices = append(vertices, a)
                // This Log shows the slice is actually filled out:
                fmt.Printf("vertices len() is %v\n", len(vertices))

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解決方法

這看起來類似于“將切片作為函數參數傳遞，并修改原始切片”

如果您希望 goroutine 修改您在外部創建的切片，則需要一個指向該切片的指針：

func Writer(wg *sync.WaitGroup, vertices *[]Vertex) (chan<- []*Triangle3) {
    // External code writes to this channel.
    // This goroutine reads the channel and writes to vertices.
    writer := make(chan []*Triangle3)

    // Write by a goroutine.
    wg.Add(1)
    go func() {
        defer wg.Done()

        a := Vertex{}

        // Read from the channel and write them to vertices.
        for ts := range writer {
            for _, t := range ts {
                a.X = float32(t.V[0].X)
                a.Y = float32(t.V[0].Y)
                a.Z = float32(t.V[0].Z)
                *vertices = append(*vertices, a)  <=====
            }
        }
    }()

    return writer
}

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