CODEWITHSUNDEEP
pointersreflectiongo
Zoltan
Zoltan

Reputation: 315

Golang - Get a pointer to a field of a struct through an interface

I'm at a very early stage of understanding interfaces in Go. I'm writing some logical simulations and have something like the following code (I heavily simplify here):

Please see the comments for my question:

type LogicNode struct {
    Input   *bool
    Output   *bool
    Operator string
    Next     Node
}

func (n *LogicNode) Run() {
    // do some stuff here
    n.Next.Run()
}

type Node interface {
    Run()
}

func main() {

    nodes := make([]Node, 1000)

    for i := 0; i < 1000; i++ {
        n := LogicNode{
            //assign values etc.
        }
        nodes[i] = &n
    }

    for i, node := range nodes {
        // I need to access LogicNode's Output pointer here, as a *bool.
        // so I can set the same address to other Node's Input thereby "connecting" them.
        // but I could only get something like this:

        x := reflect.ValueOf(node).Elem().FieldByName("Output")

        // which is <*bool Value>
        // I couldn't find a way to set a new *bool to the underlying (LogicNode) Struct's Input or Output..
    }
}

The reason I'm using interfaces is because there are other node types FloatNode MathNode etc. which have different fields, but they implement their own run method.

I've successfully used Value's SetString or SetBool methods, but can't set a pointer there... Thanks in advance.

Upvotes: 5

Views: 13311

Answers (1)

Alvivi
Alvivi

Reputation: 3331

You can use the Set generic version to update the field value:

package main

import (
    "fmt"
    "reflect"
)

type LogicNode struct {
    Input   *bool
    Output   *bool
    Operator string
    Next     Node
}

func (n *LogicNode) Run() {
    // do some stuff here
    // n.Next.Run()
    fmt.Printf("LogicNode.Input = %v (%v)\n", *n.Input, n.Input)
}

type Node interface {
    Run()
}

func main() {
    input := false
    input2 := true
    fmt.Printf("Input1 = %v (%v)\n", input, &input)
    fmt.Printf("Input2 = %v (%v)\n", input2, &input2)
    var node Node = &LogicNode{Input: &input} // Remember, interfaces are pointers
    node.Run()
    x := reflect.ValueOf(node).Elem().FieldByName("Input")
    x.Set(reflect.ValueOf(&input2))
    node.Run()
}

Outputs:

Input1 = false (0x10500168)
Input2 = true (0x10500170)
LogicNode.Input = false (0x10500168)
LogicNode.Input = true (0x10500170)

Playground here.

Upvotes: 11

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