Authenticate with Firebase using Password-Based Accounts using C++

You can use Firebase Authentication to let your users authenticate with Firebase using their email addresses and passwords, and to manage your app's password-based accounts.

Before you begin

Add Firebase to your C++ project.
In your project-level build.gradle file, make sure to include Google's Maven repository in both your buildscript and allprojects sections.
If you haven't yet connected your app to your Firebase project, do so from the Firebase console.
Enable Email/Password sign-in:
1. In the Firebase console, open the Auth section.
2. On the Sign in method tab, enable the Email/password sign-in method and click Save.

Access the `firebase::auth::Auth` class

The Auth class is the gateway for all API calls.

Add the Auth and App header files:

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

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__)

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);
```

Create a password-based account

To create a new user account with a password, complete the following steps in your app's sign-in code:

When a new user signs up using your app's sign-up form, complete any new account validation steps that your app requires, such as verifying that the new account's password was correctly typed and meets your complexity requirements.
Create a new account by passing the new user's email address and password to Auth::CreateUserWithEmailAndPassword:
```
firebase::Future<firebase::auth::User*> result =
    auth->CreateUserWithEmailAndPassword(email, password);
```

If your program has an update loop that runs regularly (say at 30 or 60 times per second), you can check the results once per update with Auth::CreateUserWithEmailAndPasswordLastResult:

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

Or, if your program is event driven, you may prefer to register a callback on the Future.

Sign in a user with an email address and password

The steps for signing in a user with a password are similar to the steps for creating a new account. In your app's sign-in function, do the following:

When a user signs in to your app, pass the user's email address and password to firebase::auth::Auth::SignInWithEmailAndPassword:
```
firebase::Future<firebase::auth::User*> result =
    auth->SignInWithEmailAndPassword(email, password);
```

If your program has an update loop that runs regularly (say at 30 or 60 times per second), you can check the results once per update with Auth::SignInWithEmailAndPasswordLastResult:

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

Or, if your program is event driven, you may prefer to register a callback on the Future.

Register a callback on a Future

Some programs have Update functions that are called 30 or 60 times per second. For example, many games follow this model. These programs can call the LastResult functions to poll asynchronous calls. However, if your program is event driven, you may prefer to register callback functions. A callback function is called upon completion of the Future.

void OnCreateCallback(const firebase::Future<firebase::auth::User*>& result,
                      void* user_data) {
  // The callback is called when the Future enters the `complete` state.
  assert(result.status() == firebase::kFutureStatusComplete);

  // Use `user_data` to pass-in program context, if you like.
  MyProgramContext* program_context = static_cast<MyProgramContext*>(user_data);

  // Important to handle both success and failure situations.
  if (result.error() == firebase::auth::kAuthErrorNone) {
    firebase::auth::User* user = *result.result();
    printf("Create user succeeded for email %s\n", user->email().c_str());

    // Perform other actions on User, if you like.
    firebase::auth::User::UserProfile profile;
    profile.display_name = program_context->display_name;
    user->UpdateUserProfile(profile);

  } else {
    printf("Created user failed with error '%s'\n", result.error_message());
  }
}

void CreateUser(firebase::auth::Auth* auth) {
  // Callbacks work the same for any firebase::Future.
  firebase::Future<firebase::auth::User*> result =
      auth->CreateUserWithEmailAndPasswordLastResult();

  // `&my_program_context` is passed verbatim to OnCreateCallback().
  result.OnCompletion(OnCreateCallback, &my_program_context);
}

The callback function can also be a lambda, if you prefer.

void CreateUserUsingLambda(firebase::auth::Auth* auth) {
  // Callbacks work the same for any firebase::Future.
  firebase::Future<firebase::auth::User*> result =
      auth->CreateUserWithEmailAndPasswordLastResult();

  // The lambda has the same signature as the callback function.
  result.OnCompletion(
      [](const firebase::Future<firebase::auth::User*>& result,
         void* user_data) {
        // `user_data` is the same as &my_program_context, below.
        // Note that we can't capture this value in the [] because std::function
        // is not supported by our minimum compiler spec (which is pre C++11).
        MyProgramContext* program_context =
            static_cast<MyProgramContext*>(user_data);

        // Process create user result...
        (void)program_context;
      },
      &my_program_context);
}

Next steps

After a user signs in for the first time, a new user account is created and linked to the credentials—that is, the user name and password, phone number, or auth provider information—the user signed in with. This new account is stored as part of your Firebase project, and can be used to identify a user across every app in your project, regardless of how the user signs in.

In your apps, you can get the user's basic profile information from the firebase::auth::User object:

firebase::auth::User* user = auth->current_user();
if (user != nullptr) {
  std::string name = user->display_name();
  std::string email = user->email();
  std::string photo_url = user->photo_url();
  // The user's ID, unique to the Firebase project.
  // Do NOT use this value to authenticate with your backend server,
  // if you have one. Use firebase::auth::User::Token() instead.
  std::string uid = user->uid();
}

In your Firebase Realtime Database and Cloud Storage Security Rules, you can get the signed-in user's unique user ID from the auth variable, and use it to control what data a user can access.

You can allow users to sign in to your app using multiple authentication providers by linking auth provider credentials to an existing user account.

To sign out a user, call SignOut():

auth->SignOut();