.. _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 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 `. 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 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 ` 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 `` .. 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") .constructor() .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 ================= JavaScript only gained support for `finalizers`_ in ECMAScript 2021, or ECMA-262 Edition 12. The new API is called `FinalizationRegistry`_ and it still does not offer any guarantees that the provided finalization callback will be called. Embind uses this 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. 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. .. 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 } } 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 ` are converted to and from JavaScript Arrays and :cpp:class:`value objects ` 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 struct ArrayInStruct { int field[2]; }; PersonRecord findPersonAtLocation(Point2f); EMSCRIPTEN_BINDINGS(my_value_example) { value_array("Point2f") .element(&Point2f::x) .element(&Point2f::y) ; value_object("PersonRecord") .field("name", &PersonRecord::name) .field("age", &PersonRecord::age) ; value_object("ArrayInStruct") .field("field", &ArrayInStruct::field) // Need to register the array type ; // Register std::array because ArrayInStruct::field is interpreted as such value_array>("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'); Advanced class concepts ======================= .. _embind-raw-pointers: Raw pointers ------------ Because raw pointers have unclear lifetime semantics, *embind* requires their use to be marked with :cpp:type:`allow_raw_pointers`. For example: .. code:: cpp class C {}; C* passThrough(C* ptr) { return ptr; } EMSCRIPTEN_BINDINGS(raw_pointers) { class_("C"); function("passThrough", &passThrough, allow_raw_pointers()); } .. note:: Currently the markup serves only 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. .. _embind-external-constructors: External constructors --------------------- There are two ways to specify constructors for a class. The :ref:`zero-argument template form ` 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") .constructor() .function("someFunction", &MyClass::someFunction) ; } The :ref:`second form of the 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") .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``. 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") .smart_ptr_constructor("C", &std::make_shared) ; } When an object of this type is constructed (e.g. using ``new Module.C()``) it returns a ``std::shared_ptr``. 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") .constructor<>() .smart_ptr>("C") ; } Using this definition, functions can return ``std::shared_ptr`` or take ``std::shared_ptr`` 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") .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 void invoke(const std::string& str) = 0; }; struct InterfaceWrapper : public wrapper { EMSCRIPTEN_WRAPPER(InterfaceWrapper); void invoke(const std::string& str) { return call("invoke", str); } }; EMSCRIPTEN_BINDINGS(interface) { class_("Interface") .function("invoke", &Interface::invoke, pure_virtual()) .allow_subclass("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 { EMSCRIPTEN_WRAPPER(BaseWrapper); void invoke(const std::string& str) { return call("invoke", str); } }; EMSCRIPTEN_BINDINGS(interface) { class_("Base") .allow_subclass("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"); class_>("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"); class_>("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. 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") .function("foo", select_overload(&HasOverloadedMethods::foo)) .function("foo_int", select_overload(&HasOverloadedMethods::foo)) .function("foo_float", select_overload(&HasOverloadedMethods::foo)) ; } .. _embind-enums: Enums ===== *Embind*'s :cpp:class:`enumeration support ` 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") .value("ONE", OLD_STYLE_ONE) .value("TWO", OLD_STYLE_TWO) ; enum_("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; .. _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: 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 #include 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 #include #include using namespace emscripten; int main() { val AudioContext = val::global("AudioContext"); if (!AudioContext.as()) { printf("No global AudioContext, trying webkitAudioContext\n"); AudioContext = val::global("webkitAudioContext"); } printf("Got an AudioContext\n"); val context = AudioContext.new_(); val oscillator = context.call("createOscillator"); printf("Configuring oscillator\n"); oscillator.set("type", val("triangle")); oscillator["frequency"].set("value", val(261.63)); // Middle C printf("Playing\n"); oscillator.call("connect", context["destination"]); oscillator.call("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`` (:cpp:func:`register_vector`), ``std::map`` (:cpp:func:`register_map`), and ``std::optional`` (:cpp:func:`register_optional`) types: .. code:: cpp EMSCRIPTEN_BINDINGS(stl_wrappers) { register_vector("VectorInt"); register_map("MapIntInt"); register_optional("Optional); } A full example is shown below: .. code:: cpp #include #include #include #include using namespace emscripten; std::vector returnVectorData () { std::vector v(10, 1); return v; } std::map returnMapData () { std::map m; m.insert(std::pair(10, "This is a string.")); return m; } std::optional returnOptionalData() { return "hello"; } EMSCRIPTEN_BINDINGS(module) { function("returnVectorData", &returnVectorData); function("returnMapData", &returnMapData); function("returnOptionalData", &returnOptionalData); // register bindings for std::vector, std::map, and // std::optional. register_vector("vector"); register_map("map"); register_optional(); } 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 value from the vector for (var i = 0; i < retVector.size(); i++) { console.log("Vector Value: ", retVector.get(i)); } // 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` // 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 ` option:: emcc -lembind quick_example.cpp --embind-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: .. 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("(message: string) => void"); } 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