Golang cannot use select

In the Go language, select is a very useful language structure. It can wait to receive data on multiple channels. Once one of the channels has data to receive, the corresponding operation will be performed immediately. However, there are some cases where we cannot use the select statement in Go language. Let's analyze these situations below.

1. The channel itself cannot be used select

The first thing you need to understand is that when declaring a channel in the Go language, you need to specify the type of the channel, and only if the type can be Select can only be used when comparing, copying and closing. For example, the channels defined below:

c1 := make(chan int)
c2 := make(chan bool)

They can all be selected, because the int and bool types are basic types that can be compared.

However, if we define a channel of structure type, we cannot use select directly. For example:

type MyStruct struct {
    x int
    y string
}

c := make(chan MyStruct)

The reason why select cannot be used is because the MyStruct type cannot be compared, so it cannot be used in Use select on a channel whose channel type is MyStruct.

2. The default statement needs to be processed

There is a special case in select, the default statement, which will be executed when all cases cannot be executed immediately. If we use the default statement in select, the default statement will also be executed when there is data to receive in any case.

However, when using the default statement, we must consider any possible situation in the select, including those that have not been considered. If we don't consider all situations, we can't use default, otherwise there will be potential bugs. For example, the following code:

select {
case msg1 := <-c1:
    fmt.Println("received", msg1)
case msg2 := <-c2:
    fmt.Println("received", msg2)
default:
    fmt.Println("nothing received") // 未考虑到其他情况的default语句
}

In this example, we have not considered all other situations, which may cause some data to be ignored or processed incorrectly. Therefore, when we use the default statement, we must consider all possible situations.

3. Select cannot be used during synchronous communication

In addition to the default statement, there is also a situation of synchronous communication in the Go language, that is, the channel buffer is full or When it is full, data cannot be written and you need to wait for other goroutines to receive data. Select cannot be used in this case.

For example, the following code:

c := make(chan int, 1)
c <- 1
c <- 2 // 缓冲区已满，此处会堵塞

In this example, we define a channel with a buffer, and the buffer size is 1. First write data 1 to the channel, and then write data 2 to the channel. At this time, because the buffer is full, this operation will be blocked. If we try to receive data from this channel in select, this operation will also be blocked because the buffer is full, and no other operations can be performed.

Therefore, when using select, you must avoid deadlock situations and ensure that the read and write operations of the channel can be executed independently.

In short, it is very important to use select correctly in the Go language. We must consider all possible situations to avoid problems such as programming errors and deadlocks. At the same time, we also need to be aware of some special situations to avoid using select in inappropriate situations, causing a waste of time and resources.

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