Create a Storage Reference with Cloud Storage for C++

Your files are stored in a Google Cloud Storage bucket. The files in this bucket are presented in a hierarchical structure, just like the file system on your local hard disk, or the data in the Firebase Realtime Database. By creating a reference to a file, your app gains access to it. These references can then be used to upload or download data, get or update metadata or delete the file. A reference can either point to a specific file or to a higher level node in the hierarchy.

If you've used the Firebase Realtime Database, these paths should seem very familiar to you. However, your file data is stored in Google Cloud Storage, not in the Realtime Database.

Create a Reference

Create a reference to upload, download, or delete a file, or to get or update its metadata. A reference can be thought of as a pointer to a file in the cloud. References are lightweight, so you can create as many as you need. They are also reusable for multiple operations.

References are created from the storage service on your Firebase app by calling the GetReferenceFromUrl() method and passing in a URL of the form gs://<your-cloud-storage-bucket>. You can find this URL in the Storage section of the Firebase console.

// Get a reference to the storage service, using the default Firebase App
Storage* storage = Storage::GetInstance(app);

// Create a storage reference from our storage service
StorageReference storage_ref = storage->GetReferenceFromUrl("gs://<your-cloud-storage-bucket>");

You can create a reference to a location lower in the tree, say 'images/space.jpg', by using the child method on an existing reference.

// Create a child reference
// images_ref now points to "images"
StorageReference images_ref = storage_ref.Child("images");

// Child references can also take paths delimited by '/'
// space_ref now points to "images/space.jpg"
// images_ref still points to "images"
StorageReference space_ref = storage_ref.Child("images/space.jpg");

// This is equivalent to creating the full reference
StorageReference space_ref = storage.GetReferenceFromUrl("gs://<your-cloud-storage-bucket>/images/space.jpg");

You can also use the Parent and Root methods to navigate up in our file hierarchy. Parent navigates up one level, while Root navigates all the way to the top.

// Parent allows us to move to the parent of a reference
// images_ref now points to 'images'
StorageReference images_ref = space_ref.Parent();

// Root allows us to move all the way back to the top of our bucket
// root_ref now points to the root
StorageReference root_ref = space_ref.Root();

Child, Parent, and Root can be chained together multiple times, as each returns a reference. The exception is the Parent of Root, which is an invalid StorageReference.

// References can be chained together multiple times
// earth_ref points to "images/earth.jpg"
StorageReference earth_ref = space_ref.Parent().Child("earth.jpg");

// null_ref is null, since the parent of root is an invalid StorageReference
StorageReference null_ref = space_ref.Root().Parent();

Reference Methods

You can inspect references to better understand the files they point to using the full_path, name, and bucket methods. These methods get the file's full path, name, and bucket.

// Reference's path is: "images/space.jpg"
// This is analogous to a file path on disk
space_ref.full_path();

// Reference's name is the last segment of the full path: "space.jpg"
// This is analogous to the file name
space_ref.name();

// Reference's bucket is the name of the storage bucket where files are stored
space_ref.bucket();

Limitations on References

Reference paths and names can contain any sequence of valid Unicode characters, but certain restrictions are imposed including:

  1. Total length of reference.fullPath must be between 1 and 1024 bytes when UTF-8 encoded.
  2. No Carriage Return or Line Feed characters.
  3. Avoid using #, [, ], *, or ?, as these do not work well with other tools such as the Firebase Realtime Database or gsutil.

Full Example

Storage* storage = Storage::GetInstance(app);

// Points to the root reference
StorageReference storage_ref = storage->GetReferenceFromUrl("gs://<your-bucket-name>");

// Points to "images"
StorageReference images_ref = storage_ref.Child("images");

// Points to "images/space.jpg"
// Note that you can use variables to create child values
std::string filename = "space.jpg";
StorageReference space_ref = images_ref.Child(filename);

// File path is "images/space.jpg"
std::string path = space_ref.full_path()

// File name is "space.jpg"
std::string name = space_ref.name()

// Points to "images"
StorageReference images_ref = space_ref.Parent();

Next Steps

Next, let's learn how to upload files to Cloud Storage.