mojo/public/cpp/bindings/README.md

# Mojo C++ Bindings API
This document is a subset of the [Mojo documentation](/mojo/README.md).

[TOC]

## Overview
The Mojo C++ Bindings API leverages the
[C++ System API](/mojo/public/cpp/system/README.md) to provide a more natural
set of primitives for communicating over Mojo message pipes. Combined with
generated code from the
[Mojom IDL and bindings generator](/mojo/public/tools/bindings/README.md), users
can easily connect interface clients and implementations across arbitrary intra-
and inter-process boundaries.

This document provides a detailed guide to bindings API usage with example code
snippets. For a detailed API references please consult the headers in
[//mojo/public/cpp/bindings](https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/README.md).

For a simplified guide targeted at Chromium developers, see [this
link](/docs/mojo_and_services.md).

## Getting Started

When a Mojom IDL file is processed by the bindings generator, C++ code is
emitted in a series of `.h` and `.cc` files with names based on the input
`.mojom` file. Suppose we create the following Mojom file at
`//services/db/public/mojom/db.mojom`:

```
module db.mojom;

interface Table {
  AddRow(int32 key, string data);
};

interface Database {
  CreateTable(Table& table);
};
```

And a GN target to generate the bindings in
`//services/db/public/mojom/BUILD.gn`:

```
import("//mojo/public/tools/bindings/mojom.gni")

mojom("mojom") {
  sources = [
    "db.mojom",
  ]
}
```

Ensure that any target that needs this interface depends on it, e.g. with a line like:

```
   deps += [ '//services/db/public/mojom' ]
```

If we then build this target:

```
ninja -C out/r services/db/public/mojom
```

This will produce several generated source files, some of which are relevant to
C++ bindings. Two of these files are:

```
out/gen/services/db/public/mojom/db.mojom.cc
out/gen/services/db/public/mojom/db.mojom.h
```

You can include the above generated header in your sources in order to use the
definitions therein:

``` cpp
#include "services/business/public/mojom/factory.mojom.h"

class TableImpl : public db::mojom::Table {
  // ...
};
```

This document covers the different kinds of definitions generated by Mojom IDL
for C++ consumers and how they can effectively be used to communicate across
message pipes.

*** note
**NOTE:** Using C++ bindings from within Blink code is typically subject to
special constraints which require the use of a different generated header.
For details, see [Blink Type Mapping](#Blink-Type-Mapping).
***

## Interfaces

Mojom IDL interfaces are translated to corresponding C++ (pure virtual) class
interface definitions in the generated header, consisting of a single generated
method signature for each request message on the interface. Internally there is
also generated code for serialization and deserialization of messages, but this
detail is hidden from bindings consumers.

### Basic Usage

Let's consider a new `//sample/logger.mojom` to define a simple logging
interface which clients can use to log simple string messages:

``` cpp
module sample.mojom;

interface Logger {
  Log(string message);
};
```

Running this through the bindings generator will produce a `logger.mojom.h`
with the following definitions (modulo unimportant details):

``` cpp
namespace sample {
namespace mojom {

class Logger {
  virtual ~Logger() {}

  virtual void Log(const std::string& message) = 0;
};

}  // namespace mojom
}  // namespace sample
```

### Remote and PendingReceiver

In the world of Mojo bindings libraries these are effectively strongly-typed
message pipe endpoints. If a `Remote<T>` is bound to a message pipe
endpoint, it can be dereferenced to make calls on an opaque `T` interface. These
calls immediately serialize their arguments (using generated code) and write a
corresponding message to the pipe.

A `PendingReceiver<T>` is essentially just a typed container to hold the other
end of a `Remote<T>`'s pipe -- the receiving end -- until it can be
routed to some implementation which will **bind** it. The `PendingReceiver<T>`
doesn't actually *do* anything other than hold onto a pipe endpoint and carry
useful compile-time type information.

![Diagram illustrating Remote and PendingReceiver on either end of a message pipe](/docs/images/mojo_pipe.png)

So how do we create a strongly-typed message pipe?

### Creating Interface Pipes

One way to do this is by manually creating a pipe and wrapping each end with a
strongly-typed object:

``` cpp
#include "sample/logger.mojom.h"

mojo::MessagePipe pipe;
mojo::Remote<sample::mojom::Logger> logger(
    mojo::PendingRemote<sample::mojom::Logger>(std::move(pipe.handle0), 0));
mojo::PendingReceiver<sample::mojom::Logger> receiver(std::move(pipe.handle1));
```

That's pretty verbose, but the C++ Bindings library provides a more convenient
way to accomplish the same thing. [remote.h](https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/remote.h)
defines a `BindNewPipeAndPassReceiver` method:

``` cpp
mojo::Remote<sample::mojom::Logger> logger;
auto receiver = logger.BindNewPipeAndPassReceiver();
```

This second snippet is equivalent to the first one.

*** note
**NOTE:** In the first example above you may notice usage of the
`mojo::PendingRemote<Logger>`. This is similar to a `PendingReceiver<T>`
in that it merely holds onto a pipe handle and cannot actually read or
write messages on the pipe. Both this type and `PendingReceiver<T>` are safe
to move freely from sequence to sequence, whereas a bound `Remote<T>` is bound
to a single sequence.

A `Remote<T>` may be unbound by calling its `Unbind()` method,
which returns a new `PendingRemote<T>`. Conversely, an `Remote<T>` may
bind (and thus take ownership of) an `PendingRemote<T>` so that interface
calls can be made on the pipe.

The sequence-bound nature of `Remote<T>` is necessary to support safe
dispatch of its [message responses](#Receiving-Responses) and
[connection error notifications](#Connection-Errors).
***

Once the `PendingRemote<Logger>` is bound we can immediately begin calling `Logger`
interface methods on it, which will immediately write messages into the pipe.
These messages will stay queued on the receiving end of the pipe until someone
binds to it and starts reading them.

``` cpp
logger->Log("Hello!");
```

This actually writes a `Log` message to the pipe.

![Diagram illustrating a message traveling on a pipe from Remote<Logger> to PendingReceiver<Logger>](/docs/images/mojo_message.png)

But as mentioned above, `PendingReceiver` *doesn't actually do anything*, so
that message will just sit on the pipe forever. We need a way to read messages
off the other end of the pipe and dispatch them. We have to
**bind the pending receiver**.

### Binding a Pending Receiver

There are many different helper classes in the bindings library for binding the
receiving end of a message pipe. The most primitive among them is `mojo::Receiver<T>`.
A `mojo::Receiver<T>` bridges an implementation of `T`
with a single bound message pipe endpoint (via a `mojo::PendingReceiver<T>`),
which it continuously watches for readability.

Any time the bound pipe becomes readable, the `Receiver` will schedule a task to
read, deserialize (using generated code), and dispatch all available messages to
the bound `T` implementation. Below is a sample implementation of the `Logger`
interface. Notice that the implementation itself owns a `mojo::Receiver`. This is
a common pattern, since a bound implementation must outlive any `mojo::Receiver`
which binds it.

``` cpp
#include "base/logging.h"
#include "sample/logger.mojom.h"

class LoggerImpl : public sample::mojom::Logger {
 public:
  // NOTE: A common pattern for interface implementations which have one
  // instance per client is to take a PendingReceiver in the constructor.

  explicit LoggerImpl(mojo::PendingReceiver<sample::mojom::Logger> receiver)
      : receiver_(this, std::move(receiver)) {}
  Logger(const Logger&) = delete;
  Logger& operator=(const Logger&) = delete;
  ~Logger() override {}

  // sample::mojom::Logger:
  void Log(const std::string& message) override {
    LOG(ERROR) << "[Logger] " << message;
  }

 private:
  mojo::Receiver<sample::mojom::Logger> receiver_;
};
```

Now we can construct a `LoggerImpl` over our `PendingReceiver<Logger>`, and the
previously queued `Log` message will be dispatched ASAP on the `LoggerImpl`'s
sequence:

``` cpp
LoggerImpl impl(std::move(receiver));
```

The diagram below illustrates the following sequence of events, all set in
motion by the above line of code:

1. The `LoggerImpl` constructor is called, passing the `PendingReceiver<Logger>` along
   to the `Receiver`.
2. The `Receiver` takes ownership of the `PendingReceiver<Logger>`'s pipe endpoint and
   begins watching it for readability. The pipe is readable immediately, so a
   task is scheduled to read the pending `Log` message from the pipe ASAP.
3. The `Log` message is read and deserialized, causing the `Receiver` to invoke
   the `Logger::Log` implementation on its bound `LoggerImpl`.

![Diagram illustrating the progression of binding a pending receiver, reading a pending message, and dispatching it](/docs/images/mojo_receiver_and_dispatch.png)

As a result, our implementation will eventually log the client's `"Hello!"`
message via `LOG(ERROR)`.

*** note
**NOTE:** Messages will only be read and dispatched from a pipe as long as the
object which binds it (*i.e.* the `mojo::Receiver` in the above example) remains
alive.
***

### Receiving Responses

Some Mojom interface methods expect a response. Suppose we modify our `Logger`
interface so that the last logged line can be queried like so:

``` cpp
module sample.mojom;

interface Logger {
  Log(string message);
  GetTail() => (string message);
};
```

The generated C++ interface will now look like:

``` cpp
namespace sample {
namespace mojom {

class Logger {
 public:
  virtual ~Logger() {}

  virtual void Log(const std::string& message) = 0;

  using GetTailCallback = base::OnceCallback<void(const std::string& message)>;

  virtual void GetTail(GetTailCallback callback) = 0;
}

}  // namespace mojom
}  // namespace sample
```

As before, both clients and implementations of this interface use the same
signature for the `GetTail` method: implementations use the `callback` argument
to *respond* to the request, while clients pass a `callback` argument to
asynchronously `receive` the response. The parameter `GetTailCallback` passed to
the implementation of `GetTail` is sequence-affine. It must be invoked on the
same sequence that `GetTail` is called on. A client's `callback` runs on the
same sequence on which they invoked `GetTail` (the sequence to which their
`logger` is bound). Here's an updated implementation:

```cpp
class LoggerImpl : public sample::mojom::Logger {
 public:
  // NOTE: A common pattern for interface implementations which have one
  // instance per client is to take a PendingReceiver in the constructor.

  explicit LoggerImpl(mojo::PendingReceiver<sample::mojom::Logger> receiver)
      : receiver_(this, std::move(receiver)) {}
  ~Logger() override {}
  Logger(const Logger&) = delete;
  Logger& operator=(const Logger&) = delete;

  // sample::mojom::Logger:
  void Log(const std::string& message) override {
    LOG(ERROR) << "[Logger] " << message;
    lines_.push_back(message);
  }

  void GetTail(GetTailCallback callback) override {
    std::move(callback).Run(lines_.back());
  }

 private:
  mojo::Receiver<sample::mojom::Logger> receiver_;
  std::vector<std::string> lines_;
};
```

And an updated client call:

``` cpp
void OnGetTail(const std::string& message) {
  LOG(ERROR) << "Tail was: " << message;
}

logger->GetTail(base::BindOnce(&OnGetTail));
```

Behind the scenes, the implementation-side callback is actually serializing the
response arguments and writing them onto the pipe for delivery back to the
client. Meanwhile the client-side callback is invoked by some internal logic
which watches the pipe for an incoming response message, reads and deserializes
it once it arrives, and then invokes the callback with the deserialized
parameters.

### Connection Errors

If a pipe is disconnected, both endpoints will be able to observe the connection
error (unless the disconnection is caused by closing/destroying an endpoint, in
which case that endpoint won't get such a notification). If there are remaining
incoming messages for an endpoint on disconnection, the connection error won't
be triggered until the messages are drained.

Pipe disconnection may be caused by:
* Mojo system-level causes: process terminated, resource exhausted, etc.
* The bindings close the pipe due to a validation error when processing a
  received message.
* The peer endpoint is closed. For example, the remote side is a bound
  `mojo::Remote<T>` and it is destroyed.

Regardless of the underlying cause, when a connection error is encountered on
a receiver endpoint, that endpoint's **disconnect handler** (if set) is
invoked. This handler is a simple `base::OnceClosure` and may only be invoked
*once* as long as the endpoint is bound to the same pipe. Typically clients and
implementations use this handler to do some kind of cleanup or -- particuarly if
the error was unexpected -- create a new pipe and attempt to establish a new
connection with it.

All message pipe-binding C++ objects (*e.g.*, `mojo::Receiver<T>`,
`mojo::Remote<T>`, *etc.*) support setting their disconnect handler
via a `set_disconnect_handler` method.

We can set up another end-to-end `Logger` example to demonstrate disconnect handler
invocation. Suppose that `LoggerImpl` had set up the following disconnect
handler within its constructor:

``` cpp
LoggerImpl::LoggerImpl(mojo::PendingReceiver<sample::mojom::Logger> receiver)
    : receiver_(this, std::move(receiver)) {
  receiver_.set_disconnect_handler(
      base::BindOnce(&LoggerImpl::OnError, base::Unretained(this)));
}

void LoggerImpl::OnError() {
  LOG(ERROR) << "Client disconnected! Purging log lines.";
  lines_.clear();
}

mojo::Remote<sample::mojom::Logger> logger;
LoggerImpl impl(logger.BindNewPipeAndPassReceiver());
logger->Log("OK cool");
logger.reset();  // Closes the client end.

```

As long as `impl` stays alive here, it will eventually receive the `Log` message
followed immediately by an invocation of the bound callback which outputs
`"Client disconnected! Purging log lines."`. Like all other receiver callbacks, a disconnect handler will
**never** be invoked once its corresponding receiver object has been destroyed.

The use of `base::Unretained` is *safe* because the error handler will never be
invoked beyond the lifetime of `receiver_`, and `this` owns `receiver_`.

### A Note About Endpoint Lifetime and Callbacks
Once a `mojo::Remote<T>` is destroyed, it is guaranteed that pending
callbacks as well as the connection error handler (if registered) won't be
called.

Once a `mojo::Receiver<T>` is destroyed, it is guaranteed that no more method
calls are dispatched to the implementation and the connection error handler (if
registered) won't be called.

### Best practices for dealing with process crashes and callbacks
A common situation when calling mojo interface methods that take a callback is
that the caller wants to know if the other endpoint is torn down (e.g. because
of a crash). In that case, the consumer usually wants to know if the response
callback won't be run. There are different solutions for this problem, depending
on how the `Remote<T>` is held:
1. The consumer owns the `Remote<T>`: `set_disconnect_handler`
   should be used.
2. The consumer doesn't own the `Remote<T>`: there are two helpers
   depending on the behavior that the caller wants. If the caller wants to
   ensure that an error handler is run, then
   [**`mojo::WrapCallbackWithDropHandler`**](https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/callback_helpers.h?l=46)
   should be used. If the caller wants the callback to always be run, then
   [**`mojo::WrapCallbackWithDefaultInvokeIfNotRun`**](https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/callback_helpers.h?l=40)
   helper should be used. With both of these helpers, usual callback care should
   be followed to ensure that the callbacks don't run after the consumer is
   destructed (e.g. because the owner of the `Remote<T>` outlives the
   consumer). This includes using
   [**`base::WeakPtr`**](https://cs.chromium.org/chromium/src/base/memory/weak_ptr.h?l=5)
   or
   [**`base::RefCounted`**](https://cs.chromium.org/chromium/src/base/memory/ref_counted.h?l=246).
   It should also be noted that with these helpers, the callbacks could be run
   synchronously while the Remote<T> is reset or destroyed.

### A Note About Ordering

As mentioned in the previous section, closing one end of a pipe will eventually
trigger a connection error on the other end. However it's important to note that
this event is itself ordered with respect to any other event (*e.g.* writing a
message) on the pipe.

This means that it's safe to write something contrived like:

``` cpp
LoggerImpl::LoggerImpl(mojo::PendingReceiver<sample::mojom::Logger> receiver,
      base::OnceClosure disconnect_handler)
    : receiver_(this, std::move(receiver)) {
  receiver_.set_disconnect_handler(std::move(disconnect_handler));
}

void GoBindALogger(mojo::PendingReceiver<sample::mojom::Logger> receiver) {
  base::RunLoop loop;
  LoggerImpl impl(std::move(receiver), loop.QuitClosure());
  loop.Run();
}

void LogSomething() {
  mojo::Remote<sample::mojom::Logger> logger;
  bg_thread->task_runner()->PostTask(
      FROM_HERE, base::BindOnce(&GoBindALogger, logger.BindNewPipeAndPassReceiver()));
  logger->Log("OK Computer");
}
```

When `logger` goes out of scope it immediately closes its end of the message
pipe, but the impl-side won't notice this until it receives the sent `Log`
message. Thus the `impl` above will first log our message and *then* see a
connection error and break out of the run loop.

## Types

### Enums

[Mojom enums](/mojo/public/tools/bindings/README.md#Enumeration-Types) translate
directly to equivalent strongly-typed C++11 enum classes with `int32_t` as the
underlying type. The typename and value names are identical between Mojom and
C++. Mojo also always defines a special enumerator `kMaxValue` that shares the
value of the highest enumerator: this makes it easy to record Mojo enums in
histograms and interoperate with legacy IPC.

For example, consider the following Mojom definition:

```cpp
module business.mojom;

enum Department {
  kEngineering,
  kMarketing,
  kSales,
};
```

This translates to the following C++ definition:

```cpp
namespace business {
namespace mojom {

enum class Department : int32_t {
  kEngineering,
  kMarketing,
  kSales,
  kMaxValue = kSales,
};

}  // namespace mojom
}  // namespace business
```

### Structs

[Mojom structs](mojo/public/tools/bindings/README.md#Structs) can be used to
define logical groupings of fields into a new composite type. Every Mojom struct
elicits the generation of an identically named, representative C++ class, with
identically named public fields of corresponding C++ types, and several helpful
public methods.

For example, consider the following Mojom struct:

```cpp
module business.mojom;

struct Employee {
  int64 id;
  string username;
  Department department;
};
```

This would generate a C++ class like so:

```cpp
namespace business {
namespace mojom {

class Employee;

using EmployeePtr = mojo::StructPtr<Employee>;

class Employee {
 public:
  // Default constructor - applies default values, potentially ones specified
  // explicitly within the Mojom.
  Employee();

  // Value constructor - an explicit argument for every field in the struct, in
  // lexical Mojom definition order.
  Employee(int64_t id, const std::string& username, Department department);

  // Creates a new copy of this struct value
  EmployeePtr Clone();

  // Tests for equality with another struct value of the same type.
  bool Equals(const Employee& other);

  // Equivalent public fields with names identical to the Mojom.
  int64_t id;
  std::string username;
  Department department;
};

}  // namespace mojom
}  // namespace business
```

Note when used as a message parameter or as a field within another Mojom struct,
a `struct` type is wrapped by the move-only `mojo::StructPtr` helper, which is
roughly equivalent to a `std::unique_ptr` with some additional utility methods.
This allows struct values to be nullable and struct types to be potentially
self-referential.

Every generated struct class has a static `New()` method which returns a new
`mojo::StructPtr<T>` wrapping a new instance of the class constructed by
forwarding the arguments from `New`. For example:

```cpp
mojom::EmployeePtr e1 = mojom::Employee::New();
e1->id = 42;
e1->username = "mojo";
e1->department = mojom::Department::kEngineering;
```

is equivalent to

```cpp
auto e1 = mojom::Employee::New(42, "mojo", mojom::Department::kEngineering);
```

Now if we define an interface like:

```cpp
interface EmployeeManager {
  AddEmployee(Employee e);
};
```

We'll get this C++ interface to implement:

```cpp
class EmployeeManager {
 public:
  virtual ~EmployeManager() {}

  virtual void AddEmployee(EmployeePtr e) = 0;
};
```

And we can send this message from C++ code as follows:

```cpp
mojom::EmployeManagerPtr manager = ...;
manager->AddEmployee(
    Employee::New(42, "mojo", mojom::Department::kEngineering));

// or
auto e = Employee::New(42, "mojo", mojom::Department::kEngineering);
manager->AddEmployee(std::move(e));
```

### Unions

Similarly to [structs](#Structs), tagged unions generate an identically named,
representative C++ class which is typically wrapped in a `mojo::StructPtr<T>`.

Unlike structs, all generated union fields are private and must be retrieved and
manipulated using accessors. A field `foo` is accessible by `get_foo()` and
settable by `set_foo()`. There is also a boolean `is_foo()` for each field which
indicates whether the union is currently taking on the value of field `foo` in
exclusion to all other union fields.

Finally, every generated union class also has a nested `Tag` enum class which
enumerates all of the named union fields. A Mojom union value's current type can
be determined by calling the `which()` method which returns a `Tag`.

For example, consider the following Mojom definitions:

```cpp
union Value {
  int64 int_value;
  float float_value;
  string string_value;
};

interface Dictionary {
  AddValue(string key, Value value);
};
```

This generates the following C++ interface:

```cpp
class Value {
 public:
  ~Value() {}
};

class Dictionary {
 public:
  virtual ~Dictionary() {}

  virtual void AddValue(const std::string& key, ValuePtr value) = 0;
};
```

And we can use it like so:

```cpp
ValuePtr value = Value::NewIntValue(42);
CHECK(value->is_int_value());
CHECK_EQ(value->which(), Value::Tag::kIntValue);

value->set_float_value(42);
CHECK(value->is_float_value());
CHECK_EQ(value->which(), Value::Tag::kFloatValue);

value->set_string_value("bananas");
CHECK(value->is_string_value());
CHECK_EQ(value->which(), Value::Tag::kStringValue);
```

Finally, note that if a union value is not currently occupied by a given field,
attempts to access that field will DCHECK:

```cpp
ValuePtr value = Value::NewIntValue(42);
LOG(INFO) << "Value is " << value->string_value();  // DCHECK!
```

### Sending Interfaces Over Interfaces

We know how to create interface pipes and use their Remote and PendingReceiver endpoints
in some interesting ways. This still doesn't add up to interesting IPC! The
bread and butter of Mojo IPC is the ability to transfer interface endpoints
across other interfaces, so let's take a look at how to accomplish that.

#### Sending Pending Receivers

Consider a new example Mojom in `//sample/db.mojom`:

``` cpp
module db.mojom;

interface Table {
  void AddRow(int32 key, string data);
};

interface Database {
  AddTable(pending_receiver<Table> table);
};
```

As noted in the
[Mojom IDL documentation](/mojo/public/tools/bindings/README.md#Primitive-Types),
the `pending_receiver<Table>` syntax corresponds
precisely to the `PendingReceiver<T>` type discussed in the sections above, and
in fact the generated code for these interfaces is approximately:

``` cpp
namespace db {
namespace mojom {

class Table {
 public:
  virtual ~Table() {}

  virtual void AddRow(int32_t key, const std::string& data) = 0;
}

class Database {
 public:
  virtual ~Database() {}

  virtual void AddTable(mojo::PendingReceiver<Table> table);
};

}  // namespace mojom
}  // namespace db
```

We can put this all together now with an implementation of `Table` and
`Database`:

``` cpp
#include "sample/db.mojom.h"

class TableImpl : public db::mojom:Table {
 public:
  explicit TableImpl(mojo::PendingReceiver<db::mojom::Table> receiver)
      : receiver_(this, std::move(receiver)) {}
  ~TableImpl() override {}

  // db::mojom::Table:
  void AddRow(int32_t key, const std::string& data) override {
    rows_.insert({key, data});
  }

 private:
  mojo::Receiver<db::mojom::Table> receiver_;
  std::map<int32_t, std::string> rows_;
};

class DatabaseImpl : public db::mojom::Database {
 public:
  explicit DatabaseImpl(mojo::PendingReceiver<db::mojom::Database> receiver)
      : receiver_(this, std::move(receiver)) {}
  ~DatabaseImpl() override {}

  // db::mojom::Database:
  void AddTable(mojo::PendingReceiver<db::mojom::Table> table) {
    tables_.emplace_back(std::make_unique<TableImpl>(std::move(table)));
  }

 private:
  mojo::Receiver<db::mojom::Database> receiver_;
  std::vector<std::unique_ptr<TableImpl>> tables_;
};
```

Pretty straightforward. The `pending_receiver<Table>` Mojom parameter to `AddTable` translates to
a C++ `mojo::PendingReceiver<db::mojom::Table>`, which we know is just a
strongly-typed message pipe handle. When `DatabaseImpl` gets an `AddTable` call,
it constructs a new `TableImpl` and binds it to the received `mojo::PendingReceiver<db::mojom::Table>`.

Let's see how this can be used.

``` cpp
mojo::Remote<db::mojom::Database> database;
DatabaseImpl db_impl(database.BindNewPipeAndPassReceiver());

mojo::Remote<db::mojom::Table> table1, table2;
database->AddTable(table1.BindNewPipeAndPassReceiver());
database->AddTable(table2.BindNewPipeAndPassReceiver());

table1->AddRow(1, "hiiiiiiii");
table2->AddRow(2, "heyyyyyy");
```

Notice that we can again start using the new `Table` pipes immediately, even
while their `mojo::PendingReceiver<db::mojom::Table>` endpoints are still in transit.

#### Sending Remotes

Of course we can also send `Remote`s:

``` cpp
interface TableListener {
  OnRowAdded(int32 key, string data);
};

interface Table {
  AddRow(int32 key, string data);

  AddListener(pending_remote<TableListener> listener);
};
```

This would generate a `Table::AddListener` signature like so:

``` cpp
  virtual void AddListener(mojo::PendingRemote<TableListener> listener) = 0;
```

and this could be used like so:

``` cpp
mojo::PendingRemote<db::mojom::TableListener> listener;
TableListenerImpl impl(listener.InitWithNewPipeAndPassReceiver());
table->AddListener(std::move(listener));
```

## Other Interface Binding Types

The [Interfaces](#Interfaces) section above covers basic usage of the most
common bindings object types: `Remote`, `PendingReceiver`, and `Receiver`.
While these types are probably the most commonly used in practice, there are
several other ways of binding both client- and implementation-side interface
pipes.

### Self-owned Receivers

A **self-owned receiver** exists as a standalone object which owns its interface
implementation and automatically cleans itself up when its bound interface
endpoint detects an error. The
[**`MakeSelfOwnedReceiver`**](https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/self_owned_receiver.h)
function is used to create such a receiver.
.

``` cpp
class LoggerImpl : public sample::mojom::Logger {
 public:
  LoggerImpl() {}
  ~LoggerImpl() override {}

  // sample::mojom::Logger:
  void Log(const std::string& message) override {
    LOG(ERROR) << "[Logger] " << message;
  }

 private:
  // NOTE: This doesn't own any Receiver object!
};

mojo::Remote<db::mojom::Logger> logger;
mojo::MakeSelfOwnedReceiver(std::make_unique<LoggerImpl>(),
                        logger.BindNewPipeAndPassReceiver());

logger->Log("NOM NOM NOM MESSAGES");
```

Now as long as `logger` remains open somewhere in the system, the bound
`LoggerImpl` on the other end will remain alive.

### Receiver Sets

Sometimes it's useful to share a single implementation instance with multiple
clients. [**`ReceiverSet`**](https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/receiver_set.h)
makes this easy. Consider the Mojom:

``` cpp
module system.mojom;

interface Logger {
  Log(string message);
};

interface LoggerProvider {
  GetLogger(Logger& logger);
};
```

We can use `ReceiverSet` to bind multiple `Logger` pending receivers to a single
implementation instance:

``` cpp
class LogManager : public system::mojom::LoggerProvider,
                   public system::mojom::Logger {
 public:
  explicit LogManager(mojo::PendingReceiver<system::mojom::LoggerProvider> receiver)
      : provider_receiver_(this, std::move(receiver)) {}
  ~LogManager() {}

  // system::mojom::LoggerProvider:
  void GetLogger(mojo::PendingReceiver<Logger> receiver) override {
    logger_receivers_.Add(this, std::move(receiver));
  }

  // system::mojom::Logger:
  void Log(const std::string& message) override {
    LOG(ERROR) << "[Logger] " << message;
  }

 private:
  mojo::Receiver<system::mojom::LoggerProvider> provider_receiver_;
  mojo::ReceiverSet<system::mojom::Logger> logger_receivers_;
};

```


### Remote Sets

Similar to the `ReceiverSet` above, sometimes it's useful to maintain a set of
`Remote`s for *e.g.* a set of clients observing some event.
[**`RemoteSet`**](https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/remote_set.h)
is here to help. Take the Mojom:

``` cpp
module db.mojom;

interface TableListener {
  OnRowAdded(int32 key, string data);
};

interface Table {
  AddRow(int32 key, string data);
  AddListener(pending_remote<TableListener> listener);
};
```

An implementation of `Table` might look something like like this:

``` cpp
class TableImpl : public db::mojom::Table {
 public:
  TableImpl() {}
  ~TableImpl() override {}

  // db::mojom::Table:
  void AddRow(int32_t key, const std::string& data) override {
    rows_.insert({key, data});
    listeners_.ForEach([key, &data](db::mojom::TableListener* listener) {
      listener->OnRowAdded(key, data);
    });
  }

  void AddListener(mojo::PendingRemote<db::mojom::TableListener> listener) {
    listeners_.Add(std::move(listener));
  }

 private:
  mojo::RemoteSet<db::mojom::Table> listeners_;
  std::map<int32_t, std::string> rows_;
};
```

## Associated Interfaces

Associated interfaces are interfaces which:

* enable running multiple interfaces over a single message pipe while
  preserving message ordering.
* make it possible for the receiver to access a single message pipe from
  multiple sequences.

### Mojom

New types `pending_associated_remote` and `pending_associated_receiver` are
introduced for remote/receiver fields. For example:

``` cpp
interface Bar {};

struct Qux {
  pending_associated_remote<Bar> bar;
};

interface Foo {
  // Uses associated remote.
  PassBarRemote(pending_associated_remote<Bar> bar);
  // Uses associated receiver.
  PassBarReceiver(pending_associated_receiver<Bar> bar);
  // Passes a struct with associated interface pointer.
  PassQux(Qux qux);
  // Uses associated interface pointer in callback.
  AsyncGetBar() => (pending_associated_remote<Bar> bar);
};
```

In each case the interface impl/client will communicate using the same message
pipe over which the associated remote/receiver is passed.

### Using associated interfaces in C++

When generating C++ bindings, the pending_associated_remote of `Bar` is
mapped to `mojo::PendingAssociatedRemote<Bar>`; pending_associated_receiver to
`mojo::PendingAssociatedReceiver<Bar>`.

``` cpp
// In mojom:
interface Foo {
  ...
  PassBarRemote(pending_associated_remote<Bar> bar);
  PassBarReceiver(pending_associated_receiver<Bar> bar);
  ...
};

// In C++:
class Foo {
  ...
  virtual void PassBarRemote(mojo::PendingAssociatedRemote<Bar> bar) = 0;
  virtual void PassBarReceiver(mojo::PendingAssociatedReceiver<Bar> bar) = 0;
  ...
};
```

#### Passing pending associated receivers

Assume you already have a `Remote<Foo> foo`, and you would like to call
`PassBarReceiver()` on it. You can do:

``` cpp
mojo::PendingAssociatedRemote<Bar> pending_bar;
mojo::PendingAssociatedReceiver<Bar> bar_receiver = pending_bar.InitWithNewEndpointAndPassReceiver();
foo->PassBarReceiver(std::move(bar_receiver));

mojo::AssociatedRemote<Bar> bar;
bar.Bind(std::move(pending_bar));
bar->DoSomething();
```

First, the code creates an associated interface of type `Bar`. It looks very
similar to what you would do to setup a non-associated interface. An
important difference is that one of the two associated endpoints (either
`bar_receiver` or `pending_bar`) must be sent over another interface. That is
how the interface is associated with an existing message pipe.

It should be noted that you cannot call `bar->DoSomething()` before passing
`bar_receiver`. This is required by the FIFO-ness guarantee: at the receiver
side, when the message of `DoSomething` call arrives, we want to dispatch it to
the corresponding `AssociatedReceiver<Bar>` before processing any subsequent
messages. If `bar_receiver` is in a subsequent message, message dispatching gets
into a deadlock. On the other hand, as soon as `bar_receiver` is sent, `bar`
is usable. There is no need to wait until `bar_receiver` is bound to an
implementation at the remote side.

`AssociatedRemote` provides a `BindNewEndpointAndPassReceiver` method
to make the code a little shorter. The following code achieves the same purpose:

``` cpp
mojo::AssociatedRemote<Bar> bar;
foo->PassBarReceiver(bar.BindNewEndpointAndPassReceiver());
bar->DoSomething();
```

The implementation of `Foo` looks like this:

``` cpp
class FooImpl : public Foo {
  ...
  void PassBarReceiver(mojo::AssociatedReceiver<Bar> bar) override {
    bar_receiver_.Bind(std::move(bar));
    ...
  }
  ...

  Receiver<Foo> foo_receiver_;
  AssociatedReceiver<Bar> bar_receiver_;
};
```

In this example, `bar_receiver_`'s lifespan is tied to that of `FooImpl`. But you
don't have to do that. You can, for example, pass `bar2` to another sequence to
bind to an `AssociatedReceiver<Bar>` there.

When the underlying message pipe is disconnected (e.g., `foo` or
`foo_receiver_` is destroyed), all associated interface endpoints (e.g.,
`bar` and `bar_receiver_`) will receive a disconnect error.

#### Passing associated remotes

Similarly, assume you have already got an `Remote<Foo> foo`, and you
would like to call `PassBarRemote()` on it. You can do:

``` cpp
mojo::AssociatedReceiver<Bar> bar_receiver(some_bar_impl);
mojo::PendingAssociatedRemote<Bar> bar;
mojo::PendingAssociatedReceiver<Bar> bar_pending_receiver = bar.InitWithNewEndpointAndPassReceiver();
foo->PassBarRemote(std::move(bar));
bar_receiver.Bind(std::move(bar_pending_receiver));
```

The following code achieves the same purpose:

``` cpp
mojo::AssociatedReceiver<Bar> bar_receiver(some_bar_impl);
mojo::PendingAssociatedRemote<Bar> bar;
bar_receiver.Bind(bar.InitWithNewPipeAndPassReceiver());
foo->PassBarRemote(std::move(bar));
```

### Performance considerations

When using associated interfaces on different sequences than the primary
sequence (where the primary interface lives):

* Sending messages: send happens directly on the calling sequence. So there
  isn't sequence hopping.
* Receiving messages: associated interfaces bound on a different sequence from
  the primary interface incur an extra sequence hop during dispatch.

Therefore, performance-wise associated interfaces are better suited for
scenarios where message receiving happens on the primary sequence.

### Testing

Associated interfaces need to be associated with a primary interface before
they can be used. This means one end of the associated interface must be sent
over one end of the primary interface, or over one end of another associated
interface which itself already has a primary interface.

If you want to test an associated interface endpoint without first
associating it, you can use `AssociatedRemote::BindNewEndpointAndPassDedicatedReceiver`.
This will create working associated interface endpoints which are not actually
associated with anything else.

### Read more

* [Design: Mojo Associated Interfaces](https://docs.google.com/document/d/1nq3J_HbS-gvVfIoEhcVyxm1uY-9G_7lhD-4Kyxb1WIY/edit)

## Synchronous Calls

### Think carefully before you decide to use sync calls

Although sync calls are convenient, you should avoid them whenever they
are not absolutely necessary:

* Sync calls hurt parallelism and therefore hurt performance.
* Re-entrancy changes message order and produces call stacks that you
probably never think about while you are coding. It has always been a
huge pain.
* Sync calls may lead to deadlocks.

### Mojom changes

A new attribute `[Sync]` (or `[Sync=true]`) is introduced for methods.
For example:

``` cpp
interface Foo {
  [Sync]
  SomeSyncCall() => (Bar result);
};
```

It indicates that when `SomeSyncCall()` is called, the control flow of
the calling thread is blocked until the response is received.

It is not allowed to use this attribute with functions that don’t have
responses. If you just need to wait until the service side finishes
processing the call, you can use an empty response parameter list:

``` cpp
[Sync]
SomeSyncCallWithNoResult() => ();
```

### Generated bindings (C++)

The generated C++ interface of the Foo interface above is:

``` cpp
class Foo {
 public:
  // The service side implements this signature. The client side can
  // also use this signature if it wants to call the method asynchronously.
  virtual void SomeSyncCall(SomeSyncCallCallback callback) = 0;

  // The client side uses this signature to call the method synchronously.
  virtual bool SomeSyncCall(BarPtr* result);
};
```

As you can see, the client side and the service side use different
signatures. At the client side, response is mapped to output parameters
and the boolean return value indicates whether the operation is
successful. (Returning false usually means a connection error has
occurred.)

At the service side, a signature with callback is used. The reason is
that in some cases the implementation may need to do some asynchronous
work which the sync method’s result depends on.

*** note
**NOTE:** you can also use the signature with callback at the client side to
call the method asynchronously.
***

### Re-entrancy

What happens on the calling thread while waiting for the response of a
sync method call? It continues to process incoming sync request messages
(i.e., sync method calls); block other messages, including async
messages and sync response messages that don’t match the ongoing sync
call.

![Diagram illustrating sync call flow](/docs/images/mojo_sync_call_flow.png)

Please note that sync response messages that don’t match the ongoing
sync call cannot re-enter. That is because they correspond to sync calls
down in the call stack. Therefore, they need to be queued and processed
while the stack unwinds.

### Avoid deadlocks

Please note that the re-entrancy behavior doesn’t prevent deadlocks
involving async calls. You need to avoid call sequences such as:

![Diagram illustrating a sync call deadlock](/docs/images/mojo_sync_call_deadlock.png)

### Read more

* [Design Proposal: Mojo Sync Methods](
https://docs.google.com/document/d/1dixzFzZQW8e3ldjdM8Adbo8klXDDE4pVekwo5aLgUsE)

## Type Mapping

In many instances you might prefer that your generated C++ bindings use a more
natural type to represent certain Mojom types in your interface methods. For one
example consider a Mojom struct such as the `Rect` below:

``` cpp
module gfx.mojom;

struct Rect {
  int32 x;
  int32 y;
  int32 width;
  int32 height;
};

interface Canvas {
  void FillRect(Rect rect);
};
```

The `Canvas` Mojom interface would normally generate a C++ interface like:

``` cpp
class Canvas {
 public:
  virtual void FillRect(RectPtr rect) = 0;
};
```

However, the Chromium tree already defines a native
[`gfx::Rect`](https://cs.chromium.org/chromium/src/ui/gfx/geometry/rect.h) which
is equivalent in meaning but which also has useful helper methods. Instead of
manually converting between a `gfx::Rect` and the Mojom-generated `RectPtr` at
every message boundary, wouldn't it be nice if the Mojom bindings generator
could instead generate:

``` cpp
class Canvas {
 public:
  virtual void FillRect(const gfx::Rect& rect) = 0;
}
```

The correct answer is, "Yes! That would be nice!" And fortunately, it can!

### Defining `StructTraits`

In order to teach generated bindings code how to serialize an arbitrary native
type `T` as an arbitrary Mojom type `mojom::U`, we need to define an appropriate
specialization of the
[`mojo::StructTraits`](https://cs.chromium.org/chromium/src/mojo/public/cpp/bindings/struct_traits.h)
template.

A valid specialization of `StructTraits` MUST define the following static
methods:

* A single static accessor for every field of the Mojom struct, with the exact
  same name as the struct field. These accessors must all take a (preferably
  const) ref to an object of the native type, and must return a value compatible
  with the Mojom struct field's type. This is used to safely and consistently
  extract data from the native type during message serialization without
  incurring extra copying costs.

* A single static `Read` method which initializes an instance of the the native
  type given a serialized representation of the Mojom struct. The `Read` method
  must return a `bool` to indicate whether the incoming data is accepted
  (`true`) or rejected (`false`).

In order to define the mapping for `gfx::Rect`, we want the following
`StructTraits` specialization, which we'll define in
`//ui/gfx/geometry/mojo/geometry_mojom_traits.h`:

``` cpp
#include "mojo/public/cpp/bindings/mojom_traits.h"
#include "ui/gfx/geometry/rect.h"
#include "ui/gfx/geometry/mojo/geometry.mojom.h"

namespace mojo {

template <>
class StructTraits<gfx::mojom::RectDataView, gfx::Rect> {
 public:
  static int32_t x(const gfx::Rect& r) { return r.x(); }
  static int32_t y(const gfx::Rect& r) { return r.y(); }
  static int32_t width(const gfx::Rect& r) { return r.width(); }
  static int32_t height(const gfx::Rect& r) { return r.height(); }

  static bool Read(gfx::mojom::RectDataView data, gfx::Rect* out_rect);
};

}  // namespace mojo
```

And in `//ui/gfx/geometry/mojo/geometry_mojom_traits.cc`:

``` cpp
#include "ui/gfx/geometry/mojo/geometry_mojom_traits.h"

namespace mojo {

// static
bool StructTraits<gfx::mojom::RectDataView, gfx::Rect>::Read(
    gfx::mojom::RectDataView data,
  gfx::Rect* out_rect) {
  if (data.width() < 0 || data.height() < 0)
    return false;

  out_rect->SetRect(data.x(), data.y(), data.width(), data.height());
  return true;
};

}  // namespace mojo
```

Note that the `Read()` method returns `false` if either the incoming `width` or
`height` fields are negative. This acts as a validation step during
deserialization: if a client sends a `gfx::Rect` with a negative width or
height, its message will be rejected and the pipe will be closed. In this way,
type mapping can serve to enable custom validation logic in addition to making
callsites and interface implemention more convenient.

When the struct fields have non-primitive types, e.g. string or array,
returning a read-only view of the data in the accessor is recommended to
avoid copying. It is safe because the input object is guaranteed to
outlive the usage of the result returned by the accessor method.

The following example uses `StringPiece` to return a view of the GURL's
data (`//url/mojom/url_gurl_mojom_traits.h`):

``` cpp
#include "base/strings/string_piece.h"
#include "url/gurl.h"
#include "url/mojom/url.mojom.h"
#include "url/url_constants.h"

namespace mojo {

template <>
struct StructTraits<url::mojom::UrlDataView, GURL> {
  static base::StringPiece url(const GURL& r) {
    if (r.possibly_invalid_spec().length() > url::kMaxURLChars ||
        !r.is_valid()) {
      return base::StringPiece();
    }
    return base::StringPiece(r.possibly_invalid_spec().c_str(),
                             r.possibly_invalid_spec().length());
  }
}  // namespace mojo
```

### Enabling a New Type Mapping

We've defined the `StructTraits` necessary, but we still need to teach the
bindings generator (and hence the build system) about the mapping. To do this we
must add some more information to our `mojom` target in GN:

```
# Without a typemap
mojom("mojom") {
  sources = [
    "rect.mojom",
  ]
}

# With a typemap.
mojom("mojom") {
  sources = [
    "rect.mojom",
  ]

  cpp_typemaps = [
    {
      # NOTE: A single typemap entry can list multiple individual type mappings.
      # Each mapping assumes the same values for |traits_headers| etc below.
      #
      # To typemap a type with separate |traits_headers| etc, add a separate
      # entry to |cpp_typemaps|.
      types = [
        {
          mojom = "gfx.mojom.Rect"
          cpp = "::gfx::Rect"
        },
      ]
      traits_headers = [ "//ui/gfx/geometry/mojo/geometry_mojom_traits.h" ]
      traits_sources = [ "//ui/gfx/geometry/mojo/geometry_mojom_traits.cc" ]
      traits_public_deps = [ "//ui/gfx/geometry" ]
    },
  ]
}
```

See typemap documentation in
[mojom.gni](https://cs.chromium.org/chromium/src/mojo/public/tools/bindings/mojom.gni)
for details on the above definition and other supported parameters.

With this extra configuration present, any mojom references to `gfx.mojom.Rect`
(e.g. for method parameters or struct fields) will be `gfx::Rect` references in
generated C++ code.

For the Blink variant of bindings, add to the `blink_cpp_typemaps` list instead.

### Type Mapping Without `traits_sources`

Using `traits_sources` in a typemap configuration means that the listed sources
will be baked directly into the corresponding `mojom` target's own sources. This
can be problematic if you want to use the same typemap for both Blink and
non-Blink bindings.

For such cases, it is recommended that you define a separate `component` target
for your typemap traits, and reference this in the `traits_public_deps` of the
typemap:

```
mojom("mojom") {
  sources = [
    "rect.mojom",
  ]

  cpp_typemaps = [
    {
      types = [
        {
          mojom = "gfx.mojom.Rect"
          cpp = "::gfx::Rect"
        },
      ]
      traits_headers = [ "//ui/gfx/geometry/mojo/geometry_mojom_traits.h" ]
      traits_public_deps = [ ":geometry_mojom_traits" ]
    },
  ]
}

component("geometry_mojom_traits") {
  sources = [
    "//ui/gfx/geometry/mojo/geometry_mojom_traits.cc",
    "//ui/gfx/geometry/mojo/geometry_mojom_traits.h",
  ]

  # The header of course needs corresponding COMPONENT_EXPORT() tags.
  defines = [ "IS_GEOMETRY_MOJOM_TRAITS_IMPL" ]
}
```

### StructTraits Reference

Each of a `StructTraits` specialization's static getter methods -- one per
struct field -- must return a type which can be used as a data source for the
field during serialization. This is a quick reference mapping Mojom field type
to valid getter return types:

| Mojom Field Type             | C++ Getter Return Type |
|------------------------------|------------------------|
| `bool`                       | `bool`
| `int8`                       | `int8_t`
| `uint8`                      | `uint8_t`
| `int16`                      | `int16_t`
| `uint16`                     | `uint16_t`
| `int32`                      | `int32_t`
| `uint32`                     | `uint32_t`
| `int64`                      | `int64_t`
| `uint64`                     | `uint64_t`
| `float`                      | `float`
| `double`                     | `double`
| `handle`                     | `mojo::ScopedHandle`
| `handle<message_pipe>`       | `mojo::ScopedMessagePipeHandle`
| `handle<data_pipe_consumer>` | `mojo::ScopedDataPipeConsumerHandle`
| `handle<data_pipe_producer>` | `mojo::ScopedDataPipeProducerHandle`
| `handle<platform>`           | `mojo::PlatformHandle`
| `handle<shared_buffer>`      | `mojo::ScopedSharedBufferHandle`
| `pending_remote<Foo>`        | `mojo::PendingRemote<Foo>`
| `pending_receiver<Foo>`      | `mojo::PendingReceiver<Foo>`
| `pending_associated_remote<Foo>`    | `mojo::PendingAssociatedRemote<Foo>`
| `pending_associated_receiver<Foo>`    | `mojo::PendingAssociatedReceiver<Foo>`
| `string`                     | Value or reference to any type `T` that has a `mojo::StringTraits` specialization defined. By default this includes `std::string`, `base::StringPiece`, and `WTF::String` (Blink).
| `array<T>`                   | Value or reference to any type `T` that has a `mojo::ArrayTraits` specialization defined. By default this includes `std::vector<T>`, `mojo::CArray<T>`, and `WTF::Vector<T>` (Blink).
| `map<K, V>`                  | Value or reference to any type `T` that has a `mojo::MapTraits` specialization defined. By default this includes `std::map<T>`, `mojo::unordered_map<T>`, and `WTF::HashMap<T>` (Blink).
| `FooEnum`                    | Value of any type that has an appropriate `EnumTraits` specialization defined. By default this inlcudes only the generated `FooEnum` type.
| `FooStruct`                  | Value or reference to any type that has an appropriate `StructTraits` specialization defined. By default this includes only the generated `FooStructPtr` type.
| `FooUnion`                   | Value of reference to any type that has an appropriate `UnionTraits` specialization defined. By default this includes only the generated `FooUnionPtr` type.
| `Foo?`                       | `absl::optional<CppType>`, where `CppType` is the value type defined by the appropriate traits class specialization (e.g. `StructTraits`, `mojo::MapTraits`, etc.). This may be customized by the [typemapping](#Enabling-a-New-Type-Mapping).

### Using Generated DataView Types

Static `Read` methods on `StructTraits` specializations get a generated
`FooDataView` argument (such as the `RectDataView` in the example above) which
exposes a direct view of the serialized Mojom structure within an incoming
message's contents. In order to make this as easy to work with as possible, the
generated `FooDataView` types have a generated method corresponding to every
struct field:

* For POD field types (*e.g.* bools, floats, integers) these are simple accessor
  methods with names identical to the field name. Hence in the `Rect` example we
  can access things like `data.x()` and `data.width()`. The return types
  correspond exactly to the mappings listed in the table above, under
  [StructTraits Reference](#StructTraits-Reference).

* For handle and interface types (*e.g* `handle` or `pending_remote<Foo>`) these
  are named `TakeFieldName` (for a field named `field_name`) and they return an
  appropriate move-only handle type by value. The return types correspond
  exactly to the mappings listed in the table above, under
  [StructTraits Reference](#StructTraits-Reference).

* For all other field types (*e.g.*, enums, strings, arrays, maps, structs)
  these are named `ReadFieldName` (for a field named `field_name`) and they
  return a `bool` (to indicate success or failure in reading). On success they
  fill their output argument with the deserialized field value. The output
  argument may be a pointer to any type with an appropriate `StructTraits`
  specialization defined, as mentioned in the table above, under
  [StructTraits Reference](#StructTraits-Reference).

An example would be useful here. Suppose we introduced a new Mojom struct:

``` cpp
struct RectPair {
  Rect left;
  Rect right;
};
```

and a corresponding C++ type:

``` cpp
class RectPair {
 public:
  RectPair() {}

  const gfx::Rect& left() const { return left_; }
  const gfx::Rect& right() const { return right_; }

  void Set(const gfx::Rect& left, const gfx::Rect& right) {
    left_ = left;
    right_ = right;
  }

  // ... some other stuff

 private:
  gfx::Rect left_;
  gfx::Rect right_;
};
```

Our traits to map `gfx::mojom::RectPair` to `gfx::RectPair` might look like
this:

``` cpp
namespace mojo {

template <>
class StructTraits
 public:
  static const gfx::Rect& left(const gfx::RectPair& pair) {
    return pair.left();
  }

  static const gfx::Rect& right(const gfx::RectPair& pair) {
    return pair.right();
  }

  static bool Read(gfx::mojom::RectPairDataView data, gfx::RectPair* out_pair) {
    gfx::Rect left, right;
    if (!data.ReadLeft(&left) || !data.ReadRight(&right))
      return false;
    out_pair->Set(left, right);
    return true;
  }
}  // namespace mojo
```

Generated `ReadFoo` methods always convert `multi_word_field_name` fields to
`ReadMultiWordFieldName` methods.

<a name="Blink-Type-Mapping"></a>
### Variants

By now you may have noticed that additional C++ sources are generated when a
Mojom is processed. These exist due to type mapping, and the source files we
refer to throughout this docuemnt (namely `foo.mojom.cc` and `foo.mojom.h`) are
really only one **variant** (the *default* or *chromium* variant) of the C++
bindings for a given Mojom file.

The only other variant currently defined in the tree is the *blink* variant,
which produces a few additional files:

```
out/gen/sample/db.mojom-blink.cc
out/gen/sample/db.mojom-blink.h
```

These files mirror the definitions in the default variant but with different
C++ types in place of certain builtin field and parameter types. For example,
Mojom strings are represented by `WTF::String` instead of `std::string`. To
avoid symbol collisions, the variant's symbols are nested in an extra inner
namespace, so Blink consumer of the interface might write something like:

```
#include "sample/db.mojom-blink.h"

class TableImpl : public db::mojom::blink::Table {
 public:
  void AddRow(int32_t key, const WTF::String& data) override {
    // ...
  }
};
```

In addition to using different C++ types for builtin strings, arrays, and maps,
the custom typemaps applied to Blink bindings are managed separately from
regular bindings.

`mojom` targets support a `blink_cpp_typemaps` parameter in addition to the
regular `cpp_typemaps`. This lists the typemaps to apply to Blink bindings.

To depend specifically on generated Blink bindings, reference
`${target_name}_blink`. So for example, with the definition:

```
# In //foo/mojom
mojom("mojom") {
  sources = [
    "db.mojom",
  ]
}
```

C++ sources can depend on the Blink bindings by depending on
`"//foo/mojom:mojom_blink"`.

Finally note that both bindings variants share some common definitions which are
unaffected by differences in the type-mapping configuration (like enums, and
structures describing serialized object formats). These definitions are
generated in *shared* sources:

```
out/gen/sample/db.mojom-shared.cc
out/gen/sample/db.mojom-shared.h
out/gen/sample/db.mojom-shared-internal.h
```

Including either variant's header (`db.mojom.h` or `db.mojom-blink.h`)
implicitly includes the shared header, but may wish to include *only* the shared
header in some instances.

C++ sources can depend on shared sources only, by referencing the
`"${target_name}_shared"` target, e.g. `"//foo/mojom:mojom_shared"` in the
example above.

For converting between Blink and non-Blink variants, please see
`//third_party/blink/public/platform/cross_variant_mojo_util.h`.

## Versioning Considerations

For general documentation of versioning in the Mojom IDL see
[Versioning](/mojo/public/tools/bindings/README.md#Versioning).

This section briefly discusses some C++-specific considerations relevant to
versioned Mojom types.

### Querying Interface Versions

`Remote` defines the following methods to query or assert remote interface
version:

```cpp
void QueryVersion(base::OnceCallback<void(uint32_t)> callback);
```

This queries the remote endpoint for the version number of its binding. When a
response is received `callback` is invoked with the remote version number. Note
that this value is cached by the `Remote` instance to avoid redundant
queries.

```cpp
void RequireVersion(uint32_t version);
```

Informs the remote endpoint that a minimum version of `version` is required by
the client. If the remote endpoint cannot support that version, it will close
its end of the pipe immediately, preventing any other requests from being
received.

### Versioned Enums

All extensible enums should have one enumerator value designated as the default
using the `[Default]` attribute. When Mojo deserializes an enum value that is
not defined in the current version of the enum definition, that value will be
transparently mapped to the `[Default]` enumerator value. Implementations can
use the presence of this enumerator value to correctly handle version skew.

```cpp
[Extensible]
enum Department {
  [Default] kUnknown,
  kSales,
  kDev,
  kResearch,
};
```

### Using Mojo Bindings in Chrome

See [Converting Legacy Chrome IPC To Mojo](/docs/mojo_ipc_conversion.md).

### Additional Documentation

[Calling Mojo From Blink](/docs/mojo_ipc_conversion.md#Blink_Specific-Advice):
A brief overview of what it looks like to use Mojom C++ bindings from
within Blink code.