Use File Metadata with Cloud Storage for C++

After uploading a file to Cloud Storage reference, you can also get and update the file metadata, for example to update the content type. Files can also store custom key/value pairs with additional file metadata.

Get File Metadata

File metadata contains common properties such as name, size, and content_type (often referred to as MIME type) in addition to some less common ones like content_disposition and time_created. This metadata can be retrieved from a Cloud Storage reference using the GetMetadata method.

// Create reference to the file whose metadata we want to retrieve
StorageReference forest_ref = storage_ref.Child("images/forest.jpg");

// Get metadata properties
Future future = forest_ref.GetMetadata();

// Wait for Future to complete...

if (future.Error() != firebase::storage::kErrorNone) {
  // Uh-oh, an error occurred!
} else {
  // We can now retrieve the metadata for 'images/forest.jpg'
  Metadata* metadata = future.Result();
}

Update File Metadata

You can update file metadata at any time after the file upload completes by using the UpdateMetadata method. Refer to the full list for more information on what properties can be updated. Only the properties specified in the metadata are updated, all others are left unmodified.

// Create reference to the file whose metadata we want to change
firebase::storage::StorageReference forest_ref = storage_ref.child("images/forest.jpg");

// Create file metadata to update
Metadata new_metadata;
newMetadata.set_cache_control("public,max-age=300");
newMetadata.set_content_type("image/jpeg");

// Update metadata properties
Future future = forest_ref.UpdateMetadata(new_metadata);

// Wait for Future to complete...

if (future.Error() != firebase::storage::kErrorNone) {
  // Uh-oh, an error occurred!
} else {
  // We can now retrieve the updated metadata for 'images/forest.jpg'
  Metadata* metadata = future.Result();
}

You can delete writable metadata properties by passing the empty string:

// Create file metadata with property to delete
StorageMetadata new_metadata;
new_metadata.set_content_type("");

// Delete the metadata property
Future future = forest_ref.UpdateMetadata(new_metadata);

// Wait for Future to complete...

if (future.Error() != 0) {
  // Uh-oh, an error occurred!
} else {
  // metadata.content_type() should be an empty string
  Metadata* metadata = future.Result();
}

Handle Errors

There are a number of reasons why errors may occur on getting or updating metadata, including the file not existing, or the user not having permission to access the desired file. More information on errors can be found in the Handle Errors section of the docs.

Custom Metadata

You can specify custom metadata as an std::map containing std::string properties.

std::map* custom_metadata = metadata.custom_metadata();
custom_metadata->insert(std::make_pair("location", "Yosemite, CA, USA");
custom_metadata->insert(std::make_pair("activity", "Hiking");

You can store app-specific data for each file in custom metadata, but we highly recommend using a database (such as the Firebase Realtime Database) to store and synchronize this type of data.

File Metadata Properties

A full list of metadata properties on a file is available below:

Property Type Writable
bucket const char* NO
generation const char* NO
metageneration const char* NO
full_path const char* NO
name const char* NO
size int64_t NO
time_created int64_t NO
updated int64_t NO
cache_control const char* YES
content_disposition const char* YES
content_encoding const char* YES
content_language const char* YES
content_type const char* YES
download_urls std::vector<std::string> NO
custom_metadata std::map<std::string, std::string> YES

Next Steps

Uploading, downloading, and updating files is important, but so is being able to remove them. Let's learn how to delete files from Cloud Storage.

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