Return nil for a struct in Go

Question

I am connecting to a database, getting a row and then sending it back to the user. What I want to do though is have a return statement if I can't find that row or if I have an error.

Because I'm returning a struct I can't return nil and I get that error cannot use nil as type Item in return argument (Item is my struct)

I read online that if I use a pointer in the return statement and return *Item instead of Item then I'd be able to pass nil, when I try to create item := *Item{} I get the following error invalid indirect of Item literal (type Item)

I think there are a few solutions to this but I can't find any, what I really want to know is:

• How to return a pointer *Item instead of Item
• Is there another way to return nil for a struct?

Here's my code:

package main

import (
    "fmt"
    "github.com/aws/aws-lambda-go/lambda"

  "github.com/aws/aws-sdk-go/aws"
  "github.com/aws/aws-sdk-go/aws/session"
  "github.com/aws/aws-sdk-go/service/dynamodb"
  "github.com/aws/aws-sdk-go/service/dynamodb/dynamodbattribute"
)

type Request struct {
    Name string `json:"name"`
}

type Item struct {
  Name string `json:"name"`
  Stock int `json:"stock"`
  Price float64 `json:"price"`
}

func Handler(request Request) (Item, error) {
  sess, err := session.NewSession(&aws.Config{
    Region: aws.String("us-west-2")},
  )

  // Create DynamoDB client
  svc := dynamodb.New(sess)

  result, err := svc.GetItem(&dynamodb.GetItemInput{
    TableName: aws.String("Inventory"),
    Key: map[string]*dynamodb.AttributeValue{
      "name": {
          S: aws.String(request.Name),
      },
    },
  })

  if err != nil {
    fmt.Println(err.Error())
//     return nil, err
  }

  item := Item{}

  err = dynamodbattribute.UnmarshalMap(result.Item, &item)

  if err != nil {
    panic(fmt.Sprintf("Failed to unmarshal Record, %v", err))
//     return nil, err
  }

  if item.Name == "" {
      fmt.Println("Could not find item")
//       return nil, nil
  }

  fmt.Println("Found item:")
  fmt.Println("Name:  ", item.Name)
  fmt.Println("Stock: ", item.Stock)
  fmt.Println("Price:  ", item.Price)

    return item, nil
}

func main() {
    lambda.Start(Handler)
}

Answer 1

You're assigning your item variable wrong. You said you tried item := *Item{}, whereas the way to create a pointer is either through the use of the new builtin, or to create a literal, and the address-of operator (&). The latter is the approach you'll most commonly see in golang. There are some cases where one would use new, but in this case, I'd go for the second approach:

So either:

item := &Item{}
// or
item := new(Item)

Lastly, you can keep the code as-is, and just return a pointer at the end:

item := Item{}
// some code here
return &item, nil

In case where you have to return an error, you can still have return nil, err

So putting everything together:

// return *Item instead of Item
func Handler(request Request) (*Item, error) {
   // your code here, eg:
   item := Item{}
   if err := dynamodbattribute.UnmarshalMap(result.Item, &item); err != nil {
        return nil, err
    }
    return &item, nil
}

Alternatively, assign item as a pointer from the start

func Handler(request Request) (*Item, error) {
   // your code here, eg:
   item := &Item{}
   if err := dynamodbattribute.UnmarshalMap(result.Item, item); err != nil {
        return nil, err
    }
    return item, nil
}