无法在go中用作类型分配

Question

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 

}

Answer 1

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.