在go中不能用作赋值中的类型 [英] Cannot use as type in assignment in go

问题描述

当我编译我的代码时，我收到以下错误消息，不知道为什么会发生。有人可以帮我指出原因吗？
$ b

不能使用px.InitializePaxosInstance（val）（类型PaxosInstance）作为
类型*赋值中的PaxosInstance

 类型Paxos struct {
实例映射[int] * PaxosInstance 
 } 
 
类型PaxosInstance结构{
接口{} 
决定bool 
} 
 
 func（px * Paxos）InitializePaxosInstance（val接口{}} PaxosInstance {
返回PaxosInstance {决定：false，value：val} 
} 
 
 func（px * Paxos）PartAProcess（seq int，val interface {}）error {
 px.instance [seq] = px.InitializePaxosInstance（val）
 return nil 
  

}

解决方案

您的地图期待指向 PaxosInstance （ * PaxosInstance ），但是你传递一个struct值给它。改变你的Initialize函数返回一个指针。

pre $ func（px * Paxos）InitializePaxosInstance（val interface {}）* PaxosInstance { b $ b return& PaxosInstance {决定：false，value：val}
}

现在它返回一个指针。你可以使用& 和（需要的话）再次用 * 取消引用变量的指针， 。在诸如

  x：=& PaxosInstance {} 
  

或

  p：= PaxosInstance {} 
x ：=& p 
  

x 现在是 * PaxosInstance 。如果您需要（无论出于何种原因），您可以使用

$将其取消引用（使用指向实际值的指针）回到 PaxosInstance struct值b
$ b

  p = * x 
  

你通常不想将结构作为实际值传递，因为Go是按值传递的，这意味着它将复制整个事物。

至于阅读编译器错误，你可以看到它告诉你什么。类型 PaxosInstance 和类型 * PaxosInstance 不一样。

when I compile my code, I get the following error message, not sure why it happens. Can someone help me point why? Thank you in advance.

cannot use px.InitializePaxosInstance(val) (type PaxosInstance) as type *PaxosInstance in assignment

type Paxos struct {
    instance   map[int]*PaxosInstance
}    

type PaxosInstance struct {
    value        interface{}
    decided      bool
}    

func (px *Paxos) InitializePaxosInstance(val interface{}) PaxosInstance {
    return PaxosInstance {decided:false, value: val}
}

func (px *Paxos) PartAProcess(seq int, val interface{}) error {  
    px.instance[seq] = px.InitializePaxosInstance(val)
    return nil 

}

解决方案

Your map is expecting a pointer to a PaxosInstance (*PaxosInstance), but you are passing a struct value to it. Change your Initialize function to return a pointer.

func (px *Paxos) InitializePaxosInstance(val interface{}) *PaxosInstance {
    return &PaxosInstance {decided:false, value: val}
}

Now it returns a pointer. You can take the pointer of a variable using & and (should you ever need to) dereference it again with *. After a line like

x := &PaxosInstance{} 

or

p := PaxosInstance{}
x := &p

the value type of x is now *PaxosInstance. And if you ever need to (for whatever reason), you can dereference it (follow the pointer to the actual value) back into a PaxosInstance struct value with

p = *x

You usually do not want to pass structs around as actual values, because Go is pass-by-value, which means it will copy the whole thing.

As for reading the compiler errors, you can see what it was telling you. The type PaxosInstance and type *PaxosInstance are not the same.

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