如何使用Emscripten将对象从Javascript传递到C++

How do I pass objects from Javascript to C++ using Emscripten

本文关键字：C++ Javascript 何使用 Emscripten 对象      更新时间：2023-09-26

我想把一个Javascript对象从JS传递回C++类/方法。我有以下不起作用的简单代码片段：

// Access JS objects in C++ like C++ objects
#include "emscripten/val.h"   //Causing errors !
using namespace emscripten;
int main() {
    val Math = val::global("Math");
    return Math.call("abs", -10); // returns 10
}

我使用以下代码在windows上使用emscripten进行编译：

cmd>emcc Access_Objects.cpp

我得到大量错误日志：

In file included from Access_Objects.cpp:2:
In file included from C:'Program Files'Emscripten'emscripten'1.16.0'system'inclu
de'emscripten/val.h:4:
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:2
4:5: error:
      unknown type name 'constexpr'
    constexpr bool has_unbound_type_names = true;
    ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:2
4:15: error:
      expected unqualified-id
    constexpr bool has_unbound_type_names = true;
              ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:5
6:21: error:
      use of undeclared identifier 'has_unbound_type_names'
                if (has_unbound_type_names || std::is_polymorphic<C>::value) {
                    ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:6
7:17: error:
      use of undeclared identifier 'has_unbound_type_names'
            if (has_unbound_type_names || std::is_polymorphic<C>::value) {
                ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:8
2:40: error:
      a space is required between consecutive right angle brackets (use '> >')
        struct TypeID<std::unique_ptr<T>> {
                                       ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:9
8:42: error:
      a space is required between consecutive right angle brackets (use '> >')
        struct TypeID<AllowedRawPointer<T>> {
                                         ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
82:9: error:
      unknown type name 'constexpr'
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(char);
        ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:13: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
82:9: error:
      expected member name or ';' after declaration specifiers
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(char);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:23: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ~~~~~~~~~ ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
83:9: error:
      unknown type name 'constexpr'
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(signed char);
        ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:13: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
83:9: error:
      expected member name or ';' after declaration specifiers
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(signed char);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:23: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ~~~~~~~~~ ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
84:9: error:
      unknown type name 'constexpr'
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(unsigned char);
        ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:13: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
84:9: error:
      expected member name or ';' after declaration specifiers
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(unsigned char);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:23: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ~~~~~~~~~ ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
85:9: error:
      unknown type name 'constexpr'
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(signed short);
        ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:13: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
85:9: error:
      expected member name or ';' after declaration specifiers
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(signed short);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:23: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ~~~~~~~~~ ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
86:9: error:
      unknown type name 'constexpr'
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(unsigned short);
        ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:13: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
86:9: error:
      expected member name or ';' after declaration specifiers
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(unsigned short);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:23: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ~~~~~~~~~ ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
87:9: error:
      unknown type name 'constexpr'
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(signed int);
        ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:13: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
87:9: error:
      expected member name or ';' after declaration specifiers
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(signed int);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:23: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ~~~~~~~~~ ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
88:9: error:
      unknown type name 'constexpr'
        EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE(unsigned int);
        ^
C:'Program Files'Emscripten'emscripten'1.16.0'system'include'emscripten/wire.h:1
74:13: note:
      expanded from macro 'EMSCRIPTEN_DEFINE_NATIVE_BINDING_TYPE'
            constexpr static WireType toWireType(const type& v) {   '
            ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.
ERROR    root: compiler frontend failed to generate LLVM bitcode, halting

错误是由'emscripten/val.h'中包含的'wire.h'引起的。

如何解决这些错误？或有没有其他方法可以将对象从JS传递到C++？

JS和C++之间还有其他交互方式。我发现的最稳定的方法是通过包装JS函数来构建自己的库，这样您就可以从C++中调用它们。

如果你想从C++中调用Javascript的Math.abs((，你可以编辑"src/library.JS"。在那里你会发现C++中可用的函数名的巨大映射。如果您想将新函数称为"JS_Math_abs"，您可以添加：

JS_Math_abs: function(value) {
  return Math.abs(value);
},

然后，您将修改C++代码以指定输入和输出类型。添加：

extern "C" {
  extern int JS_Math_abs(int);
}

不要误解我的意思，这种方法比val应该的更烦人(如果它工作正常的话(，但它最终也应该更高效。一旦您有了所需的主库，您就会发现它非常稳定。然而，在大多数情况下，使用像abs((这样的C++版本的函数是很容易的。

您可以在这里找到更多关于创建自己的库的信息：http://kripken.github.io/emscripten-site/docs/porting/connecting_cpp_and_javascript/Interacting-with-code.html#interacting-使用来自本机的代码调用javascript

如上所述，您需要将-std=c++11传递给编译器，因为它使用c++11

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