.. _embind:

======
Embind
======

*Embind* is used to bind C++ functions and classes to JavaScript, so
that the compiled code can be used in a natural way by "normal"
JavaScript. *Embind* also supports :ref:`calling JavaScript classes
from C++ <embind-val-guide>`.

Embind has support for binding most C++ constructs, including those
introduced in C++11 and C++14. Its only significant limitation is
that it does not currently support :ref:`raw pointers with complicated
lifetime semantics <embind-raw-pointers>`.

This article shows how to use :cpp:func:`EMSCRIPTEN_BINDINGS` blocks to
create bindings for functions, classes, value types, pointers (including
both raw and smart pointers), enums, and constants, and how to create
bindings for abstract classes that can be overridden in JavaScript. It
also briefly explains how to manage the memory of C++ object handles
passed to JavaScript.

.. tip:: In addition to the code in this article:

   - There are many other examples of how to use *Embind* in the `Test Suite`_.
   - `Connecting C++ and JavaScript on the Web with Embind`_ (slides from
     CppCon 2014) contains more examples and information about *Embind*'s
     design philosophy and implementation.

.. note:: *Embind* was inspired by `Boost.Python`_ and uses a very similar
   approach for defining bindings.


A quick example
===============

The following code uses an :cpp:func:`EMSCRIPTEN_BINDINGS` block to expose
the simple C++ ``lerp()`` :cpp:func:`function` to JavaScript.

.. code:: cpp

    // quick_example.cpp
    #include <emscripten/bind.h>

    using namespace emscripten;

    float lerp(float a, float b, float t) {
        return (1 - t) * a + t * b;
    }

    EMSCRIPTEN_BINDINGS(my_module) {
        function("lerp", &lerp);
    }

To compile the above example using *embind*, we invoke *emcc* with the
:ref:`bind <emcc-bind>` option::

   emcc -lembind -o quick_example.js quick_example.cpp

The resulting **quick_example.js** file can be loaded as a node module
or via a ``<script>`` tag:

.. code:: html

    <!doctype html>
    <html>
      <script>
        var Module = {
          onRuntimeInitialized: function() {
            console.log('lerp result: ' + Module.lerp(1, 2, 0.5));
          }
        };
      </script>
      <script src="quick_example.js"></script>
    </html>

.. note:: We use the ``onRuntimeInitialized`` callback to run code when the runtime is ready, which is an asynchronous operation (in order to compile WebAssembly).
.. note:: Open the developer tools console to see the output of ``console.log``.

The code in an :cpp:func:`EMSCRIPTEN_BINDINGS` block runs when the JavaScript
file is initially loaded (at the same time as the global constructors). The
function ``lerp()``'s parameter types and return type are automatically
inferred by *embind*.

All symbols exposed by *embind* are available on the Emscripten ``Module``
object.

.. important:: Always access objects through the :ref:`module` object, as
   shown above.

   While the objects are also available in the global namespace by default,
   there are cases where they will not be (for example, if you use the
   :term:`closure compiler` to minify code or wrap compiled code in a
   function to avoid polluting the global namespace). You can of course
   use whatever name you like for the module by assigning it to a new
   variable: ``var MyModuleName = Module;``.

Binding libraries
=================

Binding code is run as a static constructor and static constructors only get
run if the object file is included in the link, therefore when generating
bindings for library files the compiler must be explicitly instructed to include
the object file.

For example, to generate bindings for a hypothetical **library.a** compiled
with Emscripten run *emcc* with ``--whole-archive`` compiler flag::

   emcc -lembind -o library.js -Wl,--whole-archive library.a -Wl,--no-whole-archive

Classes
=======

Exposing classes to JavaScript requires a more complicated binding statement.
For example:

.. code:: cpp

   class MyClass {
   public:
     MyClass(int x, std::string y)
       : x(x)
       , y(y)
     {}

     void incrementX() {
       ++x;
     }

     int getX() const { return x; }
     void setX(int x_) { x = x_; }

     static std::string getStringFromInstance(const MyClass& instance) {
       return instance.y;
     }

   private:
     int x;
     std::string y;
   };

   // Binding code
   EMSCRIPTEN_BINDINGS(my_class_example) {
     class_<MyClass>("MyClass")
       .constructor<int, std::string>()
       .function("incrementX", &MyClass::incrementX)
       .property("x", &MyClass::getX, &MyClass::setX)
       .property("x_readonly", &MyClass::getX)
       .class_function("getStringFromInstance", &MyClass::getStringFromInstance)
       ;
   }

The binding block defines a chain of member function calls on the temporary
:cpp:class:`class_` object (this same style is used in *Boost.Python*). The
functions register the class, its :cpp:func:`~class_::constructor`, member
:cpp:func:`~class_::function`, :cpp:func:`~class_::class_function` (static)
and :cpp:func:`~class_::property`.

.. note:: This binding block binds the class and all its methods. As a rule
   you should bind only those items that are actually needed, as each binding
   increases the code size. For example, it would be rare to bind private or
   internal methods.

An instance of ``MyClass`` can then be created and used in JavaScript as
shown below:

.. code:: javascript

   var instance = new Module.MyClass(10, "hello");
   instance.incrementX();
   instance.x; // 11
   instance.x = 20; // 20
   Module.MyClass.getStringFromInstance(instance); // "hello"
   instance.delete();

.. note:: The :term:`closure compiler` is unaware of the names of symbols that
   are exposed to JavaScript via *Embind*. In order to prevent such symbols
   from being renamed by the closure compiler in your own code (provided for
   example by using the ``--pre-js`` or ``--post-js`` compiler flags) it is
   necessary to annotate the code accordingly. Without such annotations, the
   resulting JavaScript code will no longer match the symbol names used in the
   *Embind* code and runtime errors will occur as a result.

In order to prevent the closure compiler from renaming the symbols in the
above example code it needs to be rewritten as follows:

.. code:: javascript

   var instance = new Module["MyClass"](10, "hello");
   instance["incrementX"]();
   instance["x"]; // 11
   instance["x"] = 20; // 20
   Module["MyClass"]["getStringFromInstance"](instance); // "hello"
   instance.delete();

Note that this is only needed for code seen by the optimizer, for example as in
``--pre-js`` or ``--post-js`` as mentioned above, or on ``EM_ASM`` or ``EM_JS``.
For other code, that is not optimized by closure compiler, you do not need to
make such changes. You also do not need it if you build without ``--closure 1``
to enable the closure compiler.

Memory management
=================

The :js:func:`delete()` JavaScript method is provided to manually signal that
a C++ object is no longer needed and can be deleted:

.. code:: javascript

    var x = new Module.MyClass;
    x.method();
    x.delete();

    var y = Module.myFunctionThatReturnsClassInstance();
    y.method();
    y.delete();

.. note:: Both C++ objects constructed from the JavaScript side as well as
    those returned from C++ methods must be explicitly deleted, unless a
    ``reference`` return value policy is used (see below).


.. tip:: The ``try`` … ``finally`` JavaScript construct can be used to guarantee
    C++ object handles are deleted for all code paths, regardless of early
    returns or errors thrown.

.. code:: javascript

    function myFunction() {
        const x = new Module.MyClass;
        try {
            if (someCondition) {
                return; // !
            }
            someFunctionThatMightThrow(); // oops
            x.method();
        } finally {
            x.delete(); // will be called no matter what
        }
    }

Automatic memory management
---------------------------

Embind integrates with the `Explicit Resource Management`_ proposal.

It allows to automatically delete short-lived C++ objects at the end of the
scope when they're declared with a `using` keyword:

.. code:: javascript

    using x = new Module.MyClass;
    x.method();

At the moment of writing, this proposal is natively supported in
Chromium-based browsers as well as Babel and TypeScript via transpilation.

Embind also supports `finalizers`_, which were added in ECMAScript 2021 under a
`FinalizationRegistry`_ API. Unlike the `using` keyword, finalizers are not
guaranteed to be called, and even if they are, there are no guarantees about
their timing or order of execution, which makes them unsuitable for general
RAII-style resource management.

Embind uses it for cleanup if available, but only for smart pointers, and only
as a last resort.

.. warning:: It is strongly recommended that JavaScript code explicitly deletes
    any C++ object handles it has received.


Cloning and Reference Counting
------------------------------

There are situations in which multiple long-lived portions of the
JavaScript codebase need to hold on to the same C++ object for different
amounts of time.

To accommodate that use case, Emscripten provides a `reference counting`_
mechanism in which multiple handles can be produced for the same underlying
C++ object. Only when all handles have been deleted does the object get
destroyed.

The :js:func:`clone()` JavaScript method returns a new handle. It must
eventually also be disposed with :js:func:`delete()`:

.. code:: javascript

    async function myLongRunningProcess(x, milliseconds) {
        // sleep for the specified number of milliseconds
        await new Promise(resolve => setTimeout(resolve, milliseconds));
        x.method();
        x.delete();
    }

    const y = new Module.MyClass;          // refCount = 1
    myLongRunningProcess(y.clone(), 5000); // refCount = 2
    myLongRunningProcess(y.clone(), 3000); // refCount = 3
    y.delete();                            // refCount = 2

    // (after 3000ms) refCount = 1
    // (after 5000ms) refCount = 0 -> object is deleted

Value types
===========

Manual memory management for basic types is onerous, so *embind* provides
support for value types. :cpp:class:`Value arrays <value_array>` are
converted to and from JavaScript Arrays and :cpp:class:`value objects
<value_object>` are converted to and from JavaScript Objects.

Consider the example below:

.. code:: cpp

    struct Point2f {
        float x;
        float y;
    };

    struct PersonRecord {
        std::string name;
        int age;
    };

    // Array fields are treated as if they were std::array<type,size>
    struct ArrayInStruct {
        int field[2];
    };

    PersonRecord findPersonAtLocation(Point2f);

    EMSCRIPTEN_BINDINGS(my_value_example) {
        value_array<Point2f>("Point2f")
            .element(&Point2f::x)
            .element(&Point2f::y)
            ;

        value_object<PersonRecord>("PersonRecord")
            .field("name", &PersonRecord::name)
            .field("age", &PersonRecord::age)
            ;

	value_object<ArrayInStruct>("ArrayInStruct")
            .field("field", &ArrayInStruct::field) // Need to register the array type
            ;

	// Register std::array<int, 2> because ArrayInStruct::field is interpreted as such
	value_array<std::array<int, 2>>("array_int_2")
            .element(index<0>())
            .element(index<1>())
            ;

        function("findPersonAtLocation", &findPersonAtLocation);
    }

The JavaScript code does not need to worry about lifetime management.

.. code:: javascript

    var person = Module.findPersonAtLocation([10.2, 156.5]);
    console.log('Found someone! Their name is ' + person.name + ' and they are ' + person.age + ' years old');

.. note::
    It is not recommended to use fields that correspond to regular C++ binding
    (such as ``class_<T>``) since those properties will obey the normal lifetime
    rules of their bound type and may require explicit cleanup in JavaScript.
    See :ref:`Object Ownership <embind-object-ownership>` for more details.

Advanced class concepts
=======================

.. _embind-object-ownership:

Object Ownership
----------------

JavaScript and C++ have very different memory models which can lead to it being
unclear which language owns and is responsible for deleting an object when it
moves between languages. To make object ownership more explicit, *embind*
supports smart pointers and return value policies. Return value
policies dictate what happens to a C++ object when it is returned to JavaScript.

To use a return value policy, pass the desired policy into function, method, or
property bindings. For example:

.. code:: cpp

    EMSCRIPTEN_BINDINGS(module) {
      function("createData", &createData, return_value_policy::take_ownership());
    }

Embind supports three return value policies that behave differently depending
on the return type of the function. The policies work as follows:

* *default (no argument)* - For return by value and reference a new object will be allocated using the
  object's copy constructor. JS then owns the object and is responsible for deleting it. Returning a
  pointer is not allowed by default (use an explicit policy below).
* :cpp:type:`return_value_policy::take_ownership` - Ownership is transferred to JS.
* :cpp:type:`return_value_policy::reference` - Reference an existing object but do not take
  ownership. Care must be taken to not delete the object while it is still in use in JS.

More details below:

+--------------------+-------------+---------------------------------------------------------------+
| Return Type        | Constructor | Cleanup                                                       |
+====================+=============+===============================================================+
| **default**                                                                                      |
+--------------------+-------------+---------------------------------------------------------------+
| Value (``T``)      | copy        | JS must delete the copied object.                             |
+--------------------+-------------+---------------------------------------------------------------+
| Reference (``T&``) | copy        | JS must delete the copied object.                             |
+--------------------+-------------+---------------------------------------------------------------+
| Pointer (``T*``)   | n/a         | Pointers must explicitly use a return policy.                 |
+--------------------+-------------+---------------------------------------------------------------+
| **take_ownership**                                                                               |
+--------------------+-------------+---------------------------------------------------------------+
| Value (``T``)      | move        | JS must delete the moved object.                              |
+--------------------+-------------+---------------------------------------------------------------+
| Reference (``T&``) | move        | JS must delete the moved object.                              |
+--------------------+-------------+---------------------------------------------------------------+
| Pointer (``T*``)   | none        | JS must delete the object.                                    |
+--------------------+-------------+---------------------------------------------------------------+
| **reference**                                                                                    |
+--------------------+-------------+---------------------------------------------------------------+
| Value (``T``)      | n/a         | Reference to a value is not allowed.                          |
+--------------------+-------------+---------------------------------------------------------------+
| Reference (``T&``) | none        | C++ must delete the object.                                   |
+--------------------+-------------+---------------------------------------------------------------+
| Pointer (``T*``)   | none        | C++ must delete the object.                                   |
+--------------------+-------------+---------------------------------------------------------------+

.. _embind-raw-pointers:

Raw pointers
------------

Because raw pointers have unclear lifetime semantics, *embind* requires
their use to be marked with either :cpp:type:`allow_raw_pointers` or with a
:cpp:type:`return_value_policy`. If the function returns a pointer it is
recommended to use a :cpp:type:`return_value_policy` instead of the general
:cpp:type:`allow_raw_pointers`.

For example:

.. code:: cpp

    class C {};
    C* passThrough(C* ptr) { return ptr; }
    C* createC() { return new C(); }
    EMSCRIPTEN_BINDINGS(raw_pointers) {
        class_<C>("C");
        function("passThrough", &passThrough, allow_raw_pointers());
        function("createC", &createC, return_value_policy::take_ownership());
    }

.. note::

   Currently allow_raw_pointers for pointer arguments only serves to allow raw
   pointer use, and show that you've thought about the use of the raw pointers.
   Eventually we hope to implement `Boost.Python-like raw pointer policies`_ for
   managing object ownership of arguments as well.

.. _embind-external-constructors:

External constructors
---------------------

There are two ways to specify constructors for a class.

The :ref:`zero-argument template form <embind-class-zero-argument-constructor>`
invokes the natural constructor with the arguments specified in the template.
For example:

.. code:: cpp

   class MyClass {
   public:
     MyClass(int, float);
     void someFunction();
   };

   EMSCRIPTEN_BINDINGS(external_constructors) {
     class_<MyClass>("MyClass")
       .constructor<int, float>()
       .function("someFunction", &MyClass::someFunction)
       ;
   }


The :ref:`second form of the constructor <embind-class-function-pointer-constructor>`
takes a function pointer argument, and is used for classes that construct
themselves using a factory function. For example:

.. code:: cpp

   class MyClass {
     virtual void someFunction() = 0;
   };
   MyClass* makeMyClass(int, float); //Factory function.

   EMSCRIPTEN_BINDINGS(external_constructors) {
     class_<MyClass>("MyClass")
       .constructor(&makeMyClass, allow_raw_pointers())
       .function("someFunction", &MyClass::someFunction)
       ;
   }

The two constructors present *exactly the same interface* for constructing
the object in JavaScript. Continuing the example above:

.. code-block:: cpp

   var instance = new MyClass(10, 15.5);
   // instance is backed by a raw pointer to a MyClass in the Emscripten heap


Smart pointers
--------------

To manage object lifetime with smart pointers, *embind* must be told about
the smart pointer type.

For example, consider managing a class ``C``'s lifetime with
``std::shared_ptr<C>``. The best way to do this is to use
:cpp:func:`~class_::smart_ptr_constructor` to register the
smart pointer type:

.. code:: cpp

    EMSCRIPTEN_BINDINGS(better_smart_pointers) {
        class_<C>("C")
            .smart_ptr_constructor("C", &std::make_shared<C>)
            ;
    }

When an object of this type is constructed (e.g. using ``new Module.C()``)
it returns a ``std::shared_ptr<C>``.

An alternative is to use :cpp:func:`~class_::smart_ptr` in the
:cpp:func:`EMSCRIPTEN_BINDINGS` block:

.. code:: cpp

    EMSCRIPTEN_BINDINGS(smart_pointers) {
        class_<C>("C")
            .constructor<>()
            .smart_ptr<std::shared_ptr<C>>("C")
            ;
    }

Using this definition, functions can return ``std::shared_ptr<C>`` or take
``std::shared_ptr<C>`` as arguments, but ``new Module.C()`` would still
return a raw pointer.


unique_ptr
++++++++++

*embind* has built-in support for return values of type ``std::unique_ptr``.

Custom smart pointers
+++++++++++++++++++++

To teach *embind* about custom smart pointer templates, you must specialize
the :cpp:type:`smart_ptr_trait` template.



Non-member-functions on the JavaScript prototype
------------------------------------------------

Methods on the JavaScript class prototype can be non-member functions, as
long as the instance handle can be converted to the first argument of the
non-member function. The classic example is when the function exposed to
JavaScript does not exactly match the behavior of a C++ method.

.. code:: cpp

    struct Array10 {
        int& get(size_t index) {
            return data[index];
        }
        int data[10];
    };

    val Array10_get(Array10& arr, size_t index) {
        if (index < 10) {
            return val(arr.get(index));
        } else {
            return val::undefined();
        }
    }

    EMSCRIPTEN_BINDINGS(non_member_functions) {
        class_<Array10>("Array10")
            .function("get", &Array10_get)
            ;
    }

If JavaScript calls ``Array10.prototype.get`` with an invalid index, it will
return ``undefined``.

Deriving from C++ classes in JavaScript
---------------------------------------

If C++ classes have virtual or abstract member functions, it's possible to
override them in JavaScript. Because JavaScript has no knowledge of the C++
vtable, *embind* needs a bit of glue code to convert C++ virtual function
calls into JavaScript calls.

Abstract methods
++++++++++++++++

Let's begin with a simple case: pure virtual functions that must be
implemented in JavaScript.

.. code:: cpp

    struct Interface {
        virtual ~Interface() {}
        virtual void invoke(const std::string& str) = 0;
    };

    struct InterfaceWrapper : public wrapper<Interface> {
        EMSCRIPTEN_WRAPPER(InterfaceWrapper);
        void invoke(const std::string& str) {
            return call<void>("invoke", str);
        }
    };

    EMSCRIPTEN_BINDINGS(interface) {
        class_<Interface>("Interface")
            .function("invoke", &Interface::invoke, pure_virtual())
            .allow_subclass<InterfaceWrapper>("InterfaceWrapper")
            ;
    }

:cpp:func:`~class_::allow_subclass` adds two special methods to the
Interface binding: ``extend`` and ``implement``. ``extend`` allows
JavaScript to subclass in the style exemplified by `Backbone.js`_.
``implement`` is used when you have a JavaScript object, perhaps
provided by the browser or some other library, and you want to
use it to implement a C++ interface.

.. note:: The :cpp:type:`pure_virtual` annotation on the function binding
   allows JavaScript to throw a helpful error if the JavaScript class
   does not override ``invoke()``. Otherwise, you may run into confusing
   errors.


``extend`` example
+++++++++++++++++++

.. code:: javascript

    var DerivedClass = Module.Interface.extend("Interface", {
        // __construct and __destruct are optional.  They are included
        // in this example for illustration purposes.
        // If you override __construct or __destruct, don't forget to
        // call the parent implementation!
        __construct: function() {
            this.__parent.__construct.call(this);
        },
        __destruct: function() {
            this.__parent.__destruct.call(this);
        },
        invoke: function() {
            // your code goes here
        },
    });

    var instance = new DerivedClass;

``implement`` example
+++++++++++++++++++++

.. code:: javascript

    var x = {
        invoke: function(str) {
            console.log('invoking with: ' + str);
        }
    };
    var interfaceObject = Module.Interface.implement(x);

Now ``interfaceObject`` can be passed to any function that takes an
``Interface`` pointer or reference.

Non-abstract virtual methods
++++++++++++++++++++++++++++

If a C++ class has a non-pure virtual function, it can be overridden — but
does not have to be. This requires a slightly different wrapper
implementation:

.. code:: cpp

    struct Base {
        virtual void invoke(const std::string& str) {
            // default implementation
        }
    };

    struct BaseWrapper : public wrapper<Base> {
        EMSCRIPTEN_WRAPPER(BaseWrapper);
        void invoke(const std::string& str) {
            return call<void>("invoke", str);
        }
    };

    EMSCRIPTEN_BINDINGS(interface) {
        class_<Base>("Base")
            .allow_subclass<BaseWrapper>("BaseWrapper")
            .function("invoke", optional_override([](Base& self, const std::string& str) {
                return self.Base::invoke(str);
            }))
            ;
    }

When implementing ``Base`` with a JavaScript object, overriding ``invoke`` is
optional. The special lambda binding for invoke is necessary to avoid infinite
mutual recursion between the wrapper and JavaScript.

Base classes
------------

Base class bindings are defined as shown:

.. code:: cpp

    EMSCRIPTEN_BINDINGS(base_example) {
        class_<BaseClass>("BaseClass");
        class_<DerivedClass, base<BaseClass>>("DerivedClass");
    }

Any member functions defined on ``BaseClass`` are then accessible to
instances of ``DerivedClass``. In addition, any function that accepts
an instance of ``BaseClass`` can be given an instance of ``DerivedClass``.

Automatic downcasting
+++++++++++++++++++++

If a C++ class is polymorphic (that is, it has a virtual method), then
*embind* supports automatic downcasting of function return values.

.. code:: cpp

    class Base { virtual ~Base() {} }; // the virtual makes Base and Derived polymorphic
    class Derived : public Base {};
    Base* getDerivedInstance() {
        return new Derived;
    }
    EMSCRIPTEN_BINDINGS(automatic_downcasting) {
        class_<Base>("Base");
        class_<Derived, base<Base>>("Derived");
        function("getDerivedInstance", &getDerivedInstance, allow_raw_pointers());
    }

Calling ``Module.getDerivedInstance`` from JavaScript will return a
``Derived`` instance handle from which all of ``Derived``'s methods
are available.

.. note:: *Embind* must understand the fully-derived type for automatic
   downcasting to work.

.. note:: *Embind* does not support this unless RTTI is enabled.


Overloaded functions
====================

Constructors and functions can be overloaded on the number of arguments,
but *embind* does not support overloading based on type. When specifying
an overload, use the :cpp:func:`select_overload` helper function to select
the appropriate signature.

.. code:: cpp

    struct HasOverloadedMethods {
        void foo();
        void foo(int i);
        void foo(float f) const;
    };

    EMSCRIPTEN_BINDING(overloads) {
        class_<HasOverloadedMethods>("HasOverloadedMethods")
            .function("foo", select_overload<void()>(&HasOverloadedMethods::foo))
            .function("foo_int", select_overload<void(int)>(&HasOverloadedMethods::foo))
            .function("foo_float", select_overload<void(float)const>(&HasOverloadedMethods::foo))
            ;
    }

.. _embind-enums:

Enums
=====

*Embind*'s :cpp:class:`enumeration support <enum_>` works with both C++98
enums and C++11 "enum classes".

.. code:: cpp

    enum OldStyle {
        OLD_STYLE_ONE,
        OLD_STYLE_TWO
    };

    enum class NewStyle {
        ONE,
        TWO
    };

    EMSCRIPTEN_BINDINGS(my_enum_example) {
        enum_<OldStyle>("OldStyle")
            .value("ONE", OLD_STYLE_ONE)
            .value("TWO", OLD_STYLE_TWO)
            ;
        enum_<NewStyle>("NewStyle")
            .value("ONE", NewStyle::ONE)
            .value("TWO", NewStyle::TWO)
            ;
    }

In both cases, JavaScript accesses enumeration values as properties of the
type.

.. code:: javascript

    Module.OldStyle.ONE;
    Module.NewStyle.TWO;


You can control how C++ enums are exposed to JavaScript by specifying
``enum_value_type`` when registering the enum.

By default, enums use ``enum_value_type::object``. Enum values are bound
as JavaScript objects with a ``value`` property containing the underlying
C++ integer.

.. code:: cpp

    enum class Enum { ONE, TWO };

    EMSCRIPTEN_BINDINGS(my_enum_example) {
        enum_<Enum>("ObjectEnum", enum_value_type::object)
            .value("ONE", Enum::ONE)
            .value("TWO", Enum::TWO);
    }

.. code:: javascript

    Module.ObjectEnum.ONE.value === 0;
    Module.ObjectEnum.TWO.value === 1;

Alternatively, you can use:

- ``enum_value_type::number``: Enum values are bound directly as JavaScript numbers matching their C++
  integer values.

.. code:: cpp

    EMSCRIPTEN_BINDINGS(my_enum_example) {
        enum_<Enum>("NumberEnum", enum_value_type::number)
            .value("ONE", Enum::ONE)
            .value("TWO", Enum::TWO);
    }

.. code:: javascript

    Module.NumberEnum.ONE === 0;
    Module.NumberEnum.TWO === 1;

- ``enum_value_type::string``: Enum values are bound as JavaScript strings containing their name.

.. code:: cpp

    EMSCRIPTEN_BINDINGS(my_enum_example) {
        enum_<Enum>("StringEnum", enum_value_type::string)
            .value("ONE", Enum::ONE)
            .value("TWO", Enum::TWO);
    }

.. code:: javascript

    Module.StringEnum.ONE === "ONE";
    Module.StringEnum.TWO === "TWO";

Regardless of the ``enum_value_type`` used, enum values can always be used as
arguments to functions expecting the enum type.

.. code:: cpp

    void takesEnum(Enum e);

    EMSCRIPTEN_BINDINGS(my_enum_example) {
        function("takesEnum", &takesEnum);
    }

.. code:: javascript

    // enum_value_type::object
    Module.takesEnum(Module.ObjectEnum.ONE);

    // enum_value_type::number
    Module.takesEnum(Module.NumberEnum.ONE);
    // OR
    Module.takesEnum(0);

    // enum_value_type::string
    Module.takesEnum(Module.StringEnum.ONE);
    // OR
    Module.takesEnum("ONE");

.. _embind-constants:

Constants
=========

To expose a C++ :cpp:func:`constant` to JavaScript, simply write:

.. code:: cpp

    EMSCRIPTEN_BINDINGS(my_constant_example) {
        constant("SOME_CONSTANT", SOME_CONSTANT);
    }

``SOME_CONSTANT`` can have any type known to *embind*.


.. _embind-memory-view:

Class Properties
================

.. warning:: By default ``property()`` bindings to objects use
    ``return_value_policy::copy`` which can very easily lead to memory leaks
    since each access to the property will create a new object that must be
    deleted. Alternatively, use ``return_value_policy::reference``, so a new
    object is not allocated and changes to the object will be reflected in the
    original object.

Class properties can be defined several ways as seen below.

.. code:: cpp

    struct Point {
        float x;
        float y;
    };

    struct Person {
        Point location;
        Point getLocation() const { // Note: const is required on getters
            return location;
        }
        void setLocation(Point p) {
            location = p;
        }
    };

    EMSCRIPTEN_BINDINGS(xxx) {
        class_<Person>("Person")
            .constructor<>()
            // Bind directly to a class member with automatically generated getters/setters using a
            // reference return policy so the object does not need to be deleted from JS.
            .property("location", &Person::location, return_value_policy::reference())
            // Same as above, but this will return a copy and the object must be deleted or it will
            // leak!
            .property("locationCopy", &Person::location)
            // Bind using a only getter method for read only access.
            .property("readOnlyLocation", &Person::getLocation, return_value_policy::reference())
            // Bind using a getter and setter method.
            .property("getterAndSetterLocation", &Person::getLocation, &Person::setLocation,
                      return_value_policy::reference());
        class_<Point>("Point")
            .property("x", &Point::x)
            .property("y", &Point::y);
    }

    int main() {
        EM_ASM(
            let person = new Module.Person();
            person.location.x = 42;
            console.log(person.location.x); // 42
            let locationCopy = person.locationCopy;
            // This is a copy so the original person's location will not be updated.
            locationCopy.x = 99;
            console.log(locationCopy.x); // 99
            // Important: delete any copies!
            locationCopy.delete();
            console.log(person.readOnlyLocation.x); // 42
            console.log(person.getterAndSetterLocation.x); // 42
            person.delete();
        );
    }

Memory views
============

In some cases it is valuable to expose raw binary data directly to
JavaScript code as a typed array, allowing it to be used without copying.
This is useful for instance for uploading large WebGL textures directly
from the heap.

Memory views should be treated like raw pointers; lifetime and validity
are not managed by the runtime and it's easy to corrupt data if the
underlying object is modified or deallocated.

.. code:: cpp

    #include <emscripten/bind.h>
    #include <emscripten/val.h>

    using namespace emscripten;

    unsigned char *byteBuffer = /* ... */;
    size_t bufferLength = /* ... */;

    val getBytes() {
        return val(typed_memory_view(bufferLength, byteBuffer));
    }

    EMSCRIPTEN_BINDINGS(memory_view_example) {
        function("getBytes", &getBytes);
    }

The calling JavaScript code will receive a typed array view into the emscripten heap:

.. code:: js

   var myUint8Array = Module.getBytes()
   var xhr = new XMLHttpRequest();
   xhr.open('POST', /* ... */);
   xhr.send(myUint8Array);

The typed array view will be of the appropriate matching type, such as Uint8Array
for an ``unsigned char`` array or pointer.


.. _embind-val-guide:

Using ``val`` to transliterate JavaScript to C++
================================================

*Embind* provides a C++ class, :cpp:class:`emscripten::val`, which you can
use to transliterate JavaScript code to C++. Using ``val`` you can call
JavaScript objects from your C++, read and write their properties, or
coerce them to C++ values like a ``bool``, ``int``, or ``std::string``.

.. _Using-Web-Audio-API-from-Cpp-with-the-Embind-val-class:

The example below shows how you can use ``val`` to call the JavaScript
`Web Audio API`_ from C++:

.. note:: This example is based on the excellent Web Audio tutorial:
   `Making sine, square, sawtooth and triangle waves`_ (stuartmemo.com).
   There is an even simpler example in the :cpp:class:`emscripten::val`
   documentation.

First consider the JavaScript below, which shows how to use the API:


.. code-block:: javascript

   // Get web audio api context
   var AudioContext = window.AudioContext || window.webkitAudioContext;

   // Got an AudioContext: Create context and OscillatorNode
   var context = new AudioContext();
   var oscillator = context.createOscillator();

   // Configuring oscillator: set OscillatorNode type and frequency
   oscillator.type = 'triangle';
   oscillator.frequency.value = 261.63; // value in hertz - middle C

   // Playing
   oscillator.connect(context.destination);
   oscillator.start();

   // All done!

The code can be transliterated to C++ using ``val``, as shown below:

.. code-block:: cpp

   #include <emscripten/val.h>
   #include <stdio.h>
   #include <math.h>

   using namespace emscripten;

   int main() {
     val AudioContext = val::global("AudioContext");
     if (!AudioContext.as<bool>()) {
       printf("No global AudioContext, trying webkitAudioContext\n");
       AudioContext = val::global("webkitAudioContext");
     }

     printf("Got an AudioContext\n");
     val context = AudioContext.new_();
     val oscillator = context.call<val>("createOscillator");

     printf("Configuring oscillator\n");
     oscillator.set("type", val("triangle"));
     oscillator["frequency"].set("value", val(261.63)); // Middle C

     printf("Playing\n");
     oscillator.call<void>("connect", context["destination"]);
     oscillator.call<void>("start", 0);

     printf("All done!\n");
   }

First we use :cpp:func:`~emscripten::val::global` to get the symbol for
the global ``AudioContext`` object (or ``webkitAudioContext`` if that
does not exist). We then use :cpp:func:`~emscripten::val::new_` to create
the context, and from this context we can create an ``oscillator``,
:cpp:func:`~emscripten::val::set` its properties (again using ``val``)
and then play the tone.

The example can be compiled on the Linux/macOS terminal with::

   emcc -O2 -Wall -Werror -lembind -o oscillator.html oscillator.cpp


Built-in type conversions
=========================

Out of the box, *embind* provides converters for many standard C++ types:

+---------------------+--------------------------------------------------------------------+
| C++ type            | JavaScript type                                                    |
+=====================+====================================================================+
| ``void``            | undefined                                                          |
+---------------------+--------------------------------------------------------------------+
| ``bool``            | true or false                                                      |
+---------------------+--------------------------------------------------------------------+
| ``char``            | Number                                                             |
+---------------------+--------------------------------------------------------------------+
| ``signed char``     | Number                                                             |
+---------------------+--------------------------------------------------------------------+
| ``unsigned char``   | Number                                                             |
+---------------------+--------------------------------------------------------------------+
| ``short``           | Number                                                             |
+---------------------+--------------------------------------------------------------------+
| ``unsigned short``  | Number                                                             |
+---------------------+--------------------------------------------------------------------+
| ``int``             | Number                                                             |
+---------------------+--------------------------------------------------------------------+
| ``unsigned int``    | Number                                                             |
+---------------------+--------------------------------------------------------------------+
| ``long``            | Number, or BigInt*                                                 |
+---------------------+--------------------------------------------------------------------+
| ``unsigned long``   | Number, or BigInt*                                                 |
+---------------------+--------------------------------------------------------------------+
| ``float``           | Number                                                             |
+---------------------+--------------------------------------------------------------------+
| ``double``          | Number                                                             |
+---------------------+--------------------------------------------------------------------+
| ``int64_t``         | BigInt**                                                           |
+---------------------+--------------------------------------------------------------------+
| ``uint64_t``        | BigInt**                                                           |
+---------------------+--------------------------------------------------------------------+
| ``std::string``     | ArrayBuffer, Uint8Array, Uint8ClampedArray, Int8Array, or String   |
+---------------------+--------------------------------------------------------------------+
| ``std::wstring``    | String (UTF-16 code units)                                         |
+---------------------+--------------------------------------------------------------------+
| ``emscripten::val`` | anything                                                           |
+---------------------+--------------------------------------------------------------------+

\*BigInt when MEMORY64 is used, Number otherwise.

\*\*Requires BigInt support to be enabled with the `-sWASM_BIGINT` flag.

For convenience, *embind* provides factory functions to register
``std::vector<T, class Allocator=std::allocator<T>>`` (:cpp:func:`register_vector`), ``std::map<K, V, class Compare=std::less<K>, class Allocator=std::allocator<std::pair<const K, V>>>``
(:cpp:func:`register_map`), and ``std::optional<T>`` (:cpp:func:`register_optional`) types:

.. code:: cpp

    EMSCRIPTEN_BINDINGS(stl_wrappers) {
        register_vector<int>("VectorInt");
        register_map<int,int>("MapIntInt");
        register_optional<std::string>();
    }

A full example is shown below:

.. code:: cpp

    #include <emscripten/bind.h>
    #include <string>
    #include <vector>
    #include <optional>

    using namespace emscripten;

    std::vector<int> returnVectorData () {
      std::vector<int> v(10, 1);
      return v;
    }

    std::map<int, std::string> returnMapData () {
      std::map<int, std::string> m;
      m.insert(std::pair<int, std::string>(10, "This is a string."));
      return m;
    }

    std::optional<std::string> returnOptionalData() {
      return "hello";
    }

    EMSCRIPTEN_BINDINGS(module) {
      function("returnVectorData", &returnVectorData);
      function("returnMapData", &returnMapData);
      function("returnOptionalData", &returnOptionalData);

      // register bindings for std::vector<int>, std::map<int, std::string>, and
      // std::optional<std::string>.
      register_vector<int>("vector<int>");
      register_map<int, std::string>("map<int, string>");
      register_optional<std::string>();
    }


The following JavaScript can be used to interact with the above C++.

.. code:: js

    var retVector = Module['returnVectorData']();

    // vector size
    var vectorSize = retVector.size();

    // reset vector value
    retVector.set(vectorSize - 1, 11);

    // push value into vector
    retVector.push_back(12);

    // retrieve a value from the vector
    console.log("Vector Value at index 0: ", retVector.get(0));

    // iterate over vector
    for (var value of retVector) {
        console.log("Vector Value: ", value);
    }

    // expand vector size
    retVector.resize(20, 1);

    var retMap = Module['returnMapData']();

    // map size
    var mapSize = retMap.size();

    // retrieve value from map
    console.log("Map Value: ", retMap.get(10));

    // figure out which map keys are available
    // NB! You must call `register_vector<key_type>`
    // to make vectors available
    var mapKeys = retMap.keys();
    for (var i = 0; i < mapKeys.size(); i++) {
        var key = mapKeys.get(i);
        console.log("Map key/value: ", key, retMap.get(key));
    }

    // reset the value at the given index position
    retMap.set(10, "OtherValue");

    // Optional values will return undefined if there is no value.
    var optional = Module['returnOptionalData']();
    if (optional !== undefined) {
        console.log(optional);
    }


TypeScript Definitions
======================

Generating
----------

Embind supports generating TypeScript definition files from :cpp:func:`EMSCRIPTEN_BINDINGS`
blocks. To generate **.d.ts** files invoke *emcc* with the
:ref:`embind-emit-tsd <emcc-emit-tsd>` option::

   emcc -lembind quick_example.cpp --emit-tsd interface.d.ts

Running this command will build the program with an instrumented version of embind
that is then run in *node* to generate the definition files.
Not all of embind's features are currently supported, but many of the commonly used
ones are.  Examples of input and output can be seen in `embind_tsgen.cpp`_ and
`embind_tsgen.d.ts`_.

Custom ``val`` Definitions
--------------------------

:cpp:class:`emscripten::val` types are mapped to TypeScript's `any` type by default,
which does not provide much useful information for API's that consume or
produce `val` types. To give better type information, custom `val` types can be
registered using :cpp:func:`EMSCRIPTEN_DECLARE_VAL_TYPE` in combination with
:cpp:class:`emscripten::register_type`. An example below:

Two registration forms are supported:

* Single parameter: ``register_type<T>(definition)`` — the provided string is inlined
    everywhere the type appears.
* Two parameters: ``register_type<T>(name, definition)`` — creates a named TypeScript
    type alias (``type name = definition;``) and uses ``name`` at call sites.

.. code:: cpp

    EMSCRIPTEN_DECLARE_VAL_TYPE(CallbackType);

    int function_with_callback_param(CallbackType ct) {
        ct(val("hello"));
        return 0;
    }

    EMSCRIPTEN_BINDINGS(custom_val) {
        function("function_with_callback_param", &function_with_callback_param);
        register_type<CallbackType>("(message: string) => void");

        // Named alias form (emits: type Callback = (message: string) => void;)
        register_type<CallbackType>("Callback", "(message: string) => void");
    }


``nonnull`` Pointers
--------------------

C++ functions that return pointers generate TS definitions with ``<SomeClass> |
null`` to allow ``nullptr`` by default. If the C++ function is guaranteed to
return a valid object, then a policy parameter of ``nonnull<ret_val>()`` can be
added to the function binding to omit ``| null`` from TS. This avoids having to
handle the ``null`` case in TS.

Performance
===========

At time of writing there has been no *comprehensive* *embind* performance
testing, either against standard benchmarks, or relative to
:ref:`WebIDL-Binder`.

The call overhead for simple functions has been measured at about 200 ns.
While there is room for further optimisation, so far its performance in
real-world applications has proved to be more than acceptable.

.. _Test Suite: https://github.com/emscripten-core/emscripten/tree/main/test/embind
.. _Connecting C++ and JavaScript on the Web with Embind: http://chadaustin.me/2014/09/connecting-c-and-javascript-on-the-web-with-embind/
.. _Boost.Python: http://www.boost.org/doc/libs/1_56_0/libs/python/doc/
.. _finalizers: http://en.wikipedia.org/wiki/Finalizer
.. _FinalizationRegistry: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/FinalizationRegistry
.. _Reference Counting: https://en.wikipedia.org/wiki/Reference_counting
.. _Boost.Python-like raw pointer policies: https://wiki.python.org/moin/boost.python/CallPolicy
.. _Backbone.js: http://backbonejs.org/#Model-extend
.. _Web Audio API: https://developer.mozilla.org/en-US/docs/Web/API/Web_Audio_API
.. _Making sine, square, sawtooth and triangle waves: http://stuartmemo.com/making-sine-square-sawtooth-and-triangle-waves/
.. _embind_tsgen.cpp: https://github.com/emscripten-core/emscripten/blob/main/test/other/embind_tsgen.cpp
.. _embind_tsgen.d.ts: https://github.com/emscripten-core/emscripten/blob/main/test/other/embind_tsgen.d.ts
.. _Explicit Resource Management: https://tc39.es/proposal-explicit-resource-management/
