Authenticate with Firebase Anonymously Using C++

You can use Firebase Authentication to create and use temporary anonymous accounts to authenticate with Firebase. These temporary anonymous accounts can be used to allow users who haven't yet signed up to your app to work with data protected by security rules. If an anonymous user decides to sign up to your app, you can link their sign-in credentials to the anonymous account so that they can continue to work with their protected data in future sessions.

Before you begin

1. Add Firebase to your C++ project.
2. In your project-level build.gradle file, make sure to include Google's Maven repository in both your buildscript and allprojects sections.
3. If you haven't yet connected your app to your Firebase project, do so from the Firebase console.
4. Enable anonymous auth:
   1. In the Firebase console, open the Auth section.
   2. On the Sign-in Methods page, enable the Anonymous sign-in method.

Authenticate with Firebase anonymously

When a signed-out user uses an app feature that requires authentication with Firebase, sign in the user anonymously by completing the following steps:

1. Add the Auth and App header files:

   #include "firebase/app.h"
   #include "firebase/auth.h"
   

2. In your initialization code, create a firebase::App class.

   #if defined(__ANDROID__)
     firebase::App* app =
         firebase::App::Create(firebase::AppOptions(), my_jni_env, my_activity);
   #else
     firebase::App* app = firebase::App::Create(firebase::AppOptions());
   #endif  // defined(__ANDROID__)
   

3. Acquire the firebase::auth::Auth class for your firebase::App. There is a one-to-one mapping between App and Auth.

   firebase::auth::Auth* auth = firebase::auth::Auth::GetAuth(app);
   

firebase::Future<firebase::auth::User*> result = auth->SignInAnonymously();

firebase::Future<firebase::auth::User*> result =
    auth->SignInAnonymouslyLastResult();
if (result.status() == firebase::kFutureStatusComplete) {
  if (result.error() == firebase::auth::kAuthErrorNone) {
    firebase::auth::User* user = *result.result();
    printf("Sign in succeeded for `%s`\n", user->display_name().c_str());
  } else {
    printf("Sign in failed with error '%s'\n", result.error_message());
  }
}

Convert an anonymous account to a permanent account

When an anonymous user signs up to your app, you might want to allow them to continue their work with their new account—for example, you might want to make the items the user added to their shopping cart before they signed up available in their new account's shopping cart. To do so, complete the following steps:

1. When the user signs up, complete the sign-in flow for the user's authentication provider up to, but not including, calling one of the Auth::SignInWith methods. For example, get the user's Google ID token, Facebook access token, or email address and password.

2. Get an auth::Credential for the new authentication provider:

   Google Sign-In

   firebase::auth::Credential credential =
       firebase::auth::GoogleAuthProvider::GetCredential(google_id_token,
                                                         nullptr);
   

   Facebook Login

   firebase::auth::Credential credential =
       firebase::auth::FacebookAuthProvider::GetCredential(access_token);
   

   Email-password sign-in

   firebase::auth::Credential credential =
       firebase::auth::EmailAuthProvider::GetCredential(email, password);
   

3. Pass the auth::Credential object to the sign-in user's LinkWithCredential method:

   // Link the new credential to the currently active user.
   firebase::auth::User* current_user = auth->current_user();
   firebase::Future<firebase::auth::User*> result =
       current_user->LinkWithCredential(credential);
   

If the call to LinkWithCredential succeeds, the user's new account can access the anonymous account's Firebase data.

Next steps

Now that users can authenticate with Firebase, you can control their access to data in your Firebase database using Firebase rules.