Link Multiple Auth Providers to an Account using C++

Important: There is a known issue that prevents linkWithCredentials() from working correctly in some projects. See the issue report for a workaround and the status of a fix.

You can allow users to sign in to your app using multiple authentication providers by linking auth provider credentials to an existing user account. Users are identifiable by the same Firebase user ID regardless of the authentication provider they used to sign in. For example, a user who signed in with a password can link a Google account and sign in with either method in the future. Or, an anonymous user can link a Facebook account and then, later, sign in with Facebook to continue using your app.

Before you begin

Add support for two or more authentication providers (possibly including anonymous authentication) to your app.

Link auth provider credentials to a user account

To link auth provider credentials to an existing user account:

1. Sign in the user using any authentication provider or method.
2. Complete the sign-in flow for the new authentication provider up to, but not including, calling one of the firebase::auth::Auth::SignInWithCredential methods. For example, get the user's Google ID token, Facebook access token, or email and password.

3. Get a firebase::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);

4. Pass the firebase::auth::Credential object to the signed-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::AuthResult> result =
       current_user.LinkWithCredential(credential);

   The call to LinkWithCredential will fail if the credentials are already linked to another user account. In this situation, you must handle merging the accounts and associated data as appropriate for your app:

   // Gather data for the currently signed in User.
   firebase::auth::User current_user = auth->current_user();
   std::string current_email = current_user.email();
   std::string current_provider_id = current_user.provider_id();
   std::string current_display_name = current_user.display_name();
   std::string current_photo_url = current_user.photo_url();
   
   // Sign in with the new credentials.
   firebase::Future<firebase::auth::AuthResult> result =
       auth->SignInAndRetrieveDataWithCredential(credential);
   
   // To keep example simple, wait on the current thread until call completes.
   while (result.status() == firebase::kFutureStatusPending) {
     Wait(100);
   }
   
   // The new User is now active.
   if (result.error() == firebase::auth::kAuthErrorNone) {
     firebase::auth::User* new_user = *result.result();
   
     // Merge new_user with the user in details.
     // ...
     (void)new_user;
   }

If the call to LinkWithCredential succeeds, the user can now sign in using any linked authentication provider and access the same Firebase data.

Unlink an auth provider from a user account

You can unlink an auth provider from an account, so that the user can no longer sign in with that provider.

To unlink an auth provider from a user account, pass the provider ID to the Unlink method. You can get the provider IDs of the auth providers linked to a user by calling ProviderData.

// Unlink the sign-in provider from the currently active user.
firebase::auth::User current_user = auth->current_user();
firebase::Future<firebase::auth::AuthResult> result =
    current_user.Unlink(providerId);