Exported and Unexported fields in Go Language

Question

I have a function in Go, the return value of which I hope to encode using gob. The return value is a struct pointer. However even though I do understand what exported variables are, I am not quite sure how to get it working.

Here is what my function is like

fun loadXYZ(root *structABC) *structABC{
    const once = "stateData.bin"
    rd, errr := ioutil.ReadFile(once)
    if errr!=nil{

        //Do some computation and store in "root"

        buf := &bytes.Buffer{}
        errr = gob.NewEncoder(buf).Encode(root)
        if errr != nil {
            panic(errr)
        }
        errr = ioutil.WriteFile(once, buf.Bytes(), 0666)
        if errr != nil {
            panic(errr)
        }
        return root
    }
    var d *structABC
    errr = gob.NewDecoder(bytes.NewReader(rd)).Decode(&d)
    if errr != nil {
         panic(errr)
    }
    return d
}

This is the error I get

panic: gob: type main.stateNode has no exported fields

I know why the error is occurring. But can someone help me solve it?

Answer 1

In go, fields and variables that start with an Uppercase letter are "Exported", and are visible to other packages. Fields that start with a lowercase letter are "unexported", and are only visible inside their own package.

The encoding/gob package depends on reflection to encode values, and can only see exported struct fields.

In order to make things encodable, capitalize the first letter of each field name in your stateNode struct that you want to be saved.