Add public version of Firebase C++ style guide

AGENTS.md

@@ -306,6 +306,9 @@ API documentation.
 
 ## Coding Style
 
+*   **Firebase C++ Style Guide**: For specific C++ API design and coding
+    conventions relevant to this SDK, refer to the
+    [STYLE_GUIDE.md](STYLE_GUIDE.md).
 *   **Google C++ Style Guide**: Adhere to the
     [Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html)
     as mentioned in `CONTRIBUTING.md`.

STYLE_GUIDE.md

@@ -0,0 +1,386 @@
+# C++ API Guidelines
+
+**WIP** - *please feel free to improve*
+
+Intended for Firebase APIs, but also applicable to any C++ or Game APIs.
+
+# Code Style
+
+Please comply with the
+[Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html)
+as much as possible. Refresh your memory of this document before you start :)
+
+# C++ API Design
+
+### Don't force any particular usage pattern upon the client.
+
+C++ is a huge language, with a great variety of ways in which things can be
+done, compared to other languages. As a consequence, C++ projects can be very
+particular about what features of the language they use or don't use, how they
+represent their data, and structure their code.
+
+An API that forces the use of a feature or structure the client doesn't use
+will be very unwelcome. A good API uses only the simplest common denominator
+data types and features, and will be useable by all. This can generally be
+done with minimal impact on your API’s simplicity, or at least should form
+the baseline API.
+
+Examples of typical Do's and Don'ts:
+
+* Don't force the use of a particular data structure to supply or receive
+  data. Typical examples:
+    * `std::vector<T>`: If the client doesn't have the data already in a
+      `std::vector` (or only wants to use part of a vector), they are forced
+      to copy/allocate a new one, and C++ programmers don't like unnecessary
+      copies/allocations.
+      Instead, your primary interface should always take a
+      `(const T *, size_t)` instead. You can still supply an optional helper
+      method that takes a `std::vector<T> &` and then calls the former if
+      you anticipate it to be called very frequently.
+    * `std::string`: Unlike Java, these things aren't pooled, they're mutable
+      and copied. A common mistake is to take a `const std::string &`
+      argument, which forces all callers that supply a `const char *` to go
+      thru a strlen+malloc+copy that is possibly of no use to the callee.
+      Prefer to take a `const char *` instead for things that are
+      names/identifiers, especially if they possibly are compile-time
+      constant. If you're unsetting a string property, prefer to pass
+      nullptr rather than an empty string. (There are
+      [COW implementations](https://en.wikipedia.org/wiki/Copy-on-write),
+      but you can't rely on that).
+    * `std::map<K,V>`: This is a costly data structure involving many
+      allocations. If all you wanted is for the caller to supply a list of
+      key/value pairs, take a `const char **` (yes, 2 stars!). Or
+      `const SimpleStruct *` instead, which allows the user to create this
+      data statically.
+* Per-product configuration should be accomplished using an options struct
+  passed to the library's `firebase::<library>::Initialize` function.
+  Default options should be provided by the options struct's default
+  constructor. The `Initialize` function should be overloaded with a version
+  that does not take the options struct (which is how the Google style guide
+  prefers that we pass default parameters).
+
+  For example,
+
+```c++
+    struct LibraryOptions {
+      LibraryOptions() : do_the_thing(false) {}
+
+      bool do_the_thing;
+    };
+
+    InitResult Initialize(const App& app) {
+      return Initialize(app, LibraryOptions());
+    }
+    InitResult Initialize(const App& app, const LibraryOptions& options);
+```
+
+* Don't make your client responsible for data you allocate or take ownership
+  of client data. Typical C++ APIs are written such that both the client
+  and the library own their own memory, and they take full responsibility
+  for managing it, regardless of what the other does. Any data exchanged is
+  typically done through weak references and copies.
+  An exception may be a file loading API where buffers exchanged may be
+  really big. If you are going to pass ownership, make this super obvious
+  in all comments and documentation (C++ programmers typically won't
+  expect it), and document which function should be used to free the data
+  (free, delete, or a custom one).
+  Alternatively, a simple way to pass ownership of a large new buffer to
+  the client is to ask the client to supply a std::string *, which you then
+  resize(), and write directly into its owned memory. This somewhat
+  violates the rule about use of std::string above, though.
+
+* Don't use exceptions. This one is worth mentioning separately. Though
+  exceptions are great in theory, in C++ hardly any libraries use them, and
+  many code-bases disallow them entirely. They also require the use of RTTI
+  which some environments turn off. Oh, yes, also don't use RTTI.
+
+* Go easy on templates when possible. Yes, they make your code more general,
+  but they also pull a lot of implementation detail into your API, lengthen
+  compile times and bloat binaries. In C++ they are glorified macros, so
+  they result in hard to understand errors, and can make correct use of
+  your API harder to understand.
+
+* Utilize C++11 features where appropriate. This project has adopted C++11,
+  and features such as `std::unique_ptr`, `std::shared_ptr`,
+  `std::make_unique`, and `std::move` are encouraged to improve code safety
+  and readability. However, avoid features from C++14 or newer standards.
+
+* Go easy on objectifying everything, and prefer value types. In languages
+  like Java it is common to give each "concept" your API deals with its own
+  class, such that methods on it have a nice home. In C++ this isn't
+  always desirable, because objects need to be managed, stored and
+  allocated, and you run into ownership/lifetime questions mentioned above.
+  Instead:
+
+    * For simple data, prefer their management to happen in the parent
+      class that owns them. Actions on them are methods in the parent. If
+      at all possible, prefer not to refer to them by pointer/reference
+      (which creates ownership and lifetime issues) but by index/id, or
+      string if not performance sensitive (for example, when referring to
+      file resources, since the cost of loading a file dwarfs the cost of
+      a string lookup).
+
+    * If you must create objects, and objects are not heavyweight (only
+      scalars and non-owned pointers), make use of these objects by value
+      (return by value, receive by const reference). This makes ownership
+      and lifetime management trivial and efficient.
+
+* If at all possible, don't depend on external libraries. C++ compilation,
+  linking, dependency management, testing (especially cross platform) are
+  generally way harder than any other language. Every dependency is a
+  potential source of build complexity, conflicts, efficiency issues, and
+  in general more dependencies means less adoption.
+
+    * Don't pull in large libraries (e.g. BOOST) just for your
+      convenience, especially if their use is exposed in headers.
+
+    * Only use external libraries that have hard to replicate essential
+      functionality (e.g. compression, serialization, image loading,
+      networking etc.). Make sure to only access them in implementation
+      files.
+
+    * If possible, make a dependency optional, e.g. if what your API does
+      benefits from compression, make the client responsible for doing so,
+      or add an interface for it. Add sample glue code or an optional API
+      for working with the external library that is by default off in the
+      build files, and can be switched on if desired.
+
+* Take cross-platform-ness seriously: design the API to work on ALL
+  platforms even if you don't intend to supply implementations for all.
+  Hide platform issues in the implementation. Don't ever include platform
+  specific headers in your own headers. Have graceful fallback for
+  platforms you don't support, such that some level of building / testing
+  can happen anywhere.
+
+* If your API is meant to be VERY widespread in use, VERY general, and very
+  simple (e.g. a compression API), consider making at least the API (if
+  not all of it) in C, as you'll reach an even wider audience. C has a
+  more consistent ABI and is easier to access from a variety of systems /
+  languages. This is especially useful if the library implementation is
+  intended to be provided in binary.
+
+* Be careful not to to use idioms from other languages that may be foreign
+  to C++.
+
+    * An example of this is a "Builder" API (common in Java). Prefer to
+      use regular constructors, with additional set_ methods for less
+      common parameters if the number of them gets overwhelming.
+
+* Do not expose your own UI to the user as part of an API. Give the
+  developer the data to work with, and let them handle displaying it to
+  the user in the way they see fit.
+
+    * Rare exceptions can be made to this rule on a case-by-case basis.
+      For example, authentication libraries may need to display a sign-in
+      UI for the user to enter their credentials. Your API may work with
+      data owned by Google or by the user (e.g. the user's contacts) that
+      we don't want to expose to the app; in those cases, it is
+      appropriate to expose a UI (but to limit the scope of the UI to the
+      minimum necessary).
+
+    * In these types of exceptional cases, the UI should be in an isolated
+      component, separate from the rest of the API. We do allow UIs to be
+      exposed to the user UI-specific libraries, e.g. FirebaseUI, which
+      should be open-source so developers can apply any customizations
+      they need.
+
+# Game API Design
+
+### Performance matters
+
+Most of this is already encoded in C++ API design above, but it bears
+repeating: C++ game programmers can be more fanatic about performance than
+you expect.
+
+It is easy to add a layer of usability on top of fast code, it is very
+hard to impossible to "add performance" to an API that has performance
+issues baked into its design.
+
+### Don't rely on state persisting for longer than one frame.
+
+Games have an interesting program structure very unlike apps or web pages:
+they do all processing (and rendering) of almost all functionality of the
+game within a *frame* (usually 1/60th of a second), and then start anew
+for the next frame.
+
+It is common for all or part of the state of a game to be wiped out from
+one frame to the next (e.g when going into the menu, loading a new level,
+starting a cut-scene..).
+
+The consequence of this is that the state kept between frames is the only
+record of what is currently going on, and that managing this state is a
+source of complexity, especially when part of it is reflected in external
+code:
+
+* Prefer API design that is stateless, or if it is stateful, is so only
+  within a frame (i.e. between the start of the frame and the start of
+  the next one). This really simplifies the client's use of your API:
+  they can't forget to "reset" your API's state whenever they change
+  state themselves.
+
+* Prefer not to use cross-frame callbacks at all (non-escaping callbacks
+  are fine). Callbacks can be problematic in other contexts, but they're
+  even more problematic in games. Since they will execute at a future
+  time, there's no guarantee that the state that was there when the
+  callback started will still be there. There's no easy way to robustly
+  "clear" pending callbacks that don't make sense anymore when state
+  changes. Instead, make your API based on *polling*.
+  Yes, everyone learned in school that polling is bad because it uses
+  CPU, but that's what games are based on anyway: they check a LOT of
+  things every frame (and only at 60hz, which is very friendly compared
+  to busy-wait polling). If your API can be in various states of a state
+  machine (e.g. a networking based API), make sure the client can poll
+  the state you're in. This can then easily be translated to user
+  feedback.
+  If you have to use asynchronous callbacks, see the section on async
+  operations below.
+
+* Be robust to the client needing to change state. If work done in your
+  API involves multiple steps, and the client never gets to the end of
+  those steps before starting a new sequence, don't be "stuck", but deal
+  with this gracefully. If the game's state got reset, it will have no
+  record of what it was doing before. Try to not make the client
+  responsible for knowing what it was doing.
+
+* Interaction with threading:
+
+    * If you are going to use threading at all, make sure the use of that
+      is internal to the library, and any issues of thread-safety don't
+      leak into the client. Allow what appears to be synchronous access
+      to a possibly asynchronous implementation. If the asynchronous
+      nature will be externally visible, see the section on async
+      operations below.
+
+    * Games are typically hard to thread (since it’s hard to combine with
+      its per-frame nature), so the client typically should have full
+      control over it: it is often better to make a fully synchronous
+      single-threaded library and leave threading it to the client. Do
+      not try to make your API itself thread-safe, as your API is
+      unlikely the threading boundary (if your client is threaded, use
+      of your library is typically isolated to one thread, and they do
+      not want to pay for synchronization cost they don't use).
+
+    * When you do spin up threads to reduce a workload, it is often a
+      good idea to do that once per frame, as avoid the above mentioned
+      state based problems, and while starting threads isn't cheap, you
+      may find it not a problem to do 60x per second. Alternatively you
+      can pool them, and make sure you have an explicit way to wait for
+      their idleness at the end of a frame.
+
+* Games typically use their own memory allocator (for efficiency, but
+  also to be able to control and budget usage on memory constrained
+  systems). For this reason, most game APIs tend to provide allocation
+  hooks that will be used for all internal allocation. This is even more
+  important if you wish to be able to transfer ownership of memory.
+
+* Generally prefer solutions that are low on total memory usage. Games
+  are always constrained on memory, and having your game be killed by
+  the OS because the library you use has decided it is efficient to
+  cache everything is problematic.
+
+    * Prefer to recompute values when possible.
+
+    * When you do cache, give the client control over total memory used
+      for this purpose.
+
+    * Your memory usage should be predictable and ideally have no peaks.
+
+# Async Operations
+
+### Application Initiated Async Operations
+
+* Use the Future / State Pattern.
+* Add a `*LastResult()` method for each async operation method to allow
+  the caller to poll and not save state.
+
+e.g.
+
+```c++
+    // Start async operation.
+    Future<SignInWithCrendentialResult> SignInWithCredential(...);
+    // Get the result of the pending / last async operation for the method.
+    Future<SignInWithCrendentialResult> SignInWithCredentialLastResult();
+
+    Usage examples:
+    // call and register callback
+    auto& result = SignInWithCredential();
+    result.set_callback([](result) { if (result == kComplete) { do_something_neat(); wake_up(); } });
+    // wait
+
+    // call and poll #1 (saving result)
+    auto& result = SignInWithCredential();
+    while (result.value() != kComplete) {
+      // wait
+    }
+
+    // call and poll #2 (result stored in API)
+    SignInWithCredential();
+    while (SignInWithCredentialLastResult().value() != kComplete) {
+    }
+```
+
+### API Initiated Async Event Handling
+
+* Follow the
+  [listener / observer pattern](https://en.wikipedia.org/wiki/Observer_pattern)
+  for API initiated (i.e where the caller doesn't initiate the event)
+  async events.
+* Provide a queued interface to allow users to poll for events.
+
+e.g.
+
+```c++
+    class GcmListener {
+     public:
+      virtual void OnDeletedMessage() {}
+      virtual void OnMessageReceived(const MessageReceivedData* data) {}
+    };
+
+    class GcmListenerQueue : private GcmListener {
+     public:
+      enum EventType {
+        kEventTypeMessageDeleted,
+        kEventTypeMessageReceived,
+      };
+
+      struct Event {
+        EventType type;
+        MessageReceivedData data;  // Set when type == kEventTypeMessageReceived
+      };
+
+      // Returns true when an event is retrieved from the queue.
+      bool PollEvent(Event *event);
+    };
+
+    // Wait for callbacks
+    class MyListener : public GcmListener {
+     public:
+      virtual void OnDeletedMessage() { /* do stuff */ }
+      virtual void OnMessageReceived() { /* display message */ }
+    };
+    MyListener listener;
+    gcm::Initialize(app, &listener);
+
+    // Poll
+    GcmListenerQueue queued_listener;
+    gcm::Initialize(app, &queued_listener);
+    GcmListenerQueue::Event event;
+    while (queued_listener(&event)) {
+      switch (event.type) {
+        case kEventTypeMessageDeleted:
+          // do stuff
+          break;
+        case kEventTypeMessageReceived:
+          // display event.data
+          break;
+      }
+    }
+```
+
+# Data Types
+
+* Time: [Milliseconds since the epoch](https://en.wikipedia.org/wiki/Unix_time)
+
+# More on C++ and games
+
+* Performance Oriented C++ for Game Developers (very incomplete).