Getting a pointer to a constant in Go

Published on 11/25/2024, 403 words, 2 minutes to read

From least to most hacky

In Go, sometimes you need to get a pointer to a constant value. This is normally easy, but only if you have a value, not a constant. Let's say you or a friend are dealing with the AWS S3 API and you need to pass a value to one of the parameters:

_, err = s3c.PutObject(ctx, &s3.PutObjectInput{
	Bucket:      "mah-bukkit",
	Key:         "something",
	Body:        bytes.NewReader(fileContent),
})

Doing this gets you a compile error, because you need a pointer to the string.

There's several ways to work around this. I'm going to go over them in order from least to most hacky.

Make those constants into values

You can make a pointer to a value, but not a constant. Lift the bucket name and key values into variables:

bucketName := "mah-bukkit"
key := "something"

_, err = s3c.PutObject(ctx, &s3.PutObjectInput{
	Bucket:      &bucketName,
	Key:         &key,
	Body:        bytes.NewReader(fileContent),
})

This works in most cases, but you have to declare variables every time. This can look odd.

The aws.String / aws.Type functions:

The aws package exposes some helper functions that do this conversion for you. You'll see these in the example code:

_, err = s3c.PutObject(ctx, &s3.PutObjectInput{
	Bucket:      aws.String("mah-bukkit"),
	Key:         aws.String("something"),
	Body:        bytes.NewReader(fileContent),
})

This works because function arguments are treated as values:

package aws

func String(val string) *string {
  return &val
}

Making your own generic pointer to anything function

Something else you can do is use Go generics to make a "get me the pointer of this" function:

func p[T any](val T) (*T) {
  return &val
}

Then you can use it as normal:

_, err = s3c.PutObject(ctx, &s3.PutObjectInput{
	Bucket:      p("mah-bukkit"),
	Key:         p("something"),
	Body:        bytes.NewReader(fileContent),
})

The Kubernetes trick

Making variables and passing things as arguments to functions aren't the only way to do this, there's also a trick I learned by reading Kubernetes source code. I'll paste an example and then explain how it works:

raised := &[]string{"foo"}[0]

This works by creating an anonymous string slice with one member "foo", grabs the first element of that slice, and gets the pointer to it. This makes the code look kinda cursed:

_, err = s3c.PutObject(ctx, &s3.PutObjectInput{
	Bucket:      &[]string{"mah-bukkit"}[0],
	Key:         &[]string{"something"}[0],
	Body:        bytes.NewReader(fileContent),
})

However every step in this is perfectly logical.

Facts and circumstances may have changed since publication. Please contact me before jumping to conclusions if something seems wrong or unclear.

Tags: