v8::FunctionTemplate referencing a non-global variable

Question

Google's v8 documentation describes how to add a global function to a JavaScript context. We can implement a printf-like function quite easily using the new lambda feature from C++11:

Handle<ObjectTemplate> global = ObjectTemplate::New();
global->Set(String::New("print"), FunctionTemplate::New(
[](const v8::Arguments &args) -> v8::Handle<v8::Value>
{
  v8::String::AsciiValue ascii(args[0]);
  std::cout << *ascii << "\n";
} ));
Persistent<Context> context = Context::New(NULL, global);

This works well for any global JavaScript function that is either stateless or references a global C++ variable (i.e. std::cout). But what if we want our global JavaScript function to reference a non-global C++ variable? For example, suppose we are creating several different JavaScript contexts each with its own global print function that uses a different C++ std::ostream? If v8 function templates used std::function objects instead of function pointers, the we would do something like this:

Persistent<Context> create_context(std::ostream &out)
{
  Handle<ObjectTemplate> global = ObjectTemplate::New();
  global->Set(String::New("print"), FunctionTemplate::New(
  [&out](const v8::Arguments &args) -> v8::Handle<v8::Value>
  {
    v8::String::AsciiValue ascii(args[0]);
    out << *ascii << "\n";
  } ));
  return Context::New(NULL, global);
}

Unfortunately, v8 does not seem to support this. I assume (hope?) that v8 has a way of doing something functionally equivalent, but I find myself mystified by the Doxygen for v8::FunctionTemplate. Would anyone who has attempted something similar be willing to distill the process down into something more understandable? I would also like to learn how to create a global instance of a JavaScript object that is bound to an existing, non-global instance of a C++ object.

score 6 · Answer 1 · answered Dec 22 '12 at 21:40

In answer to my own question... the key is to realize that v8::Arguments is not simply an array of arguments. It also contains the exceedingly useful Callee() and Data() methods. If the function is a method of a JavaScript object then Callee() can, I think, be used to get ahold of whatever instance of that object the method was called on. Useful state information could then be stored in the object instance. You can also supply a data handle, which may point to any C++ object through void*, when adding a function template to an object. This function-specific data handle may then be accessed through the Data() method.

Below is a reasonably complete example of what I was trying to do in the question using v8::Arguments::Data(). Hopefully this will be useful to anyone who wants to do something similar. If you have an alternative strategy you like (and I am certain there is more than one way of doing this), please feel free to add it in another answer!

#include <iostream>
#include <ostream>
#include <v8.h>

// add print() function to an object template
void add_print(v8::Handle<v8::ObjectTemplate>& ot, std::ostream* out)
{
  // add function template to ot
  ot->Set(v8::String::New("print"), v8::FunctionTemplate::New(
    // parameter 1 is the function callback (implemented here as a lambda)
    [](const v8::Arguments& args)->v8::Handle<v8::Value>
    {
      // recover our pointer to an std::ostream from the
      // function template's data handle
      v8::Handle<v8::External> data = v8::Handle<v8::External>::Cast(args.Data());
      std::ostream* out = static_cast<std::ostream*>(data->Value());

      // verify that we have the correct number of function arguments
      if ( args.Length() != 1 )
        return v8::ThrowException(v8::String::New("Too many arguments to print()."));

      // print the ascii representation of the argument to the output stream
      v8::String::AsciiValue ascii(args[0]);
      *out << *ascii << "\n";

      // like 'return void;' only in JavaScript
      return v8::Undefined();
    },

    // parameter 2 is the data handle with the pointer to an std::ostream
    v8::External::New(out)
  ));
}

int main()
{
  // create a stack-allocated handle scope
  v8::HandleScope handle_scope;

  // create a global template
  v8::Local<v8::ObjectTemplate> global = v8::ObjectTemplate::New();

  // add a print() function using std::cout to the global template
  add_print(global, &std::cout);

  // create a context
  v8::Persistent<v8::Context> context = v8::Context::New(nullptr, global);

  // enter the created context
  v8::Context::Scope context_scope(context);

  // create a string containing the JavaScript source code
  v8::Local<v8::String> source = v8::String::New("print('1 + 1 = ' + (1 + 1));");

  // compile the source code
  v8::Local<v8::Script> script = v8::Script::Compile(source);

  // run the script
  script->Run();

  // dispose of the persistent context
  context.Dispose();

  return 0;
}

v8::FunctionTemplate referencing a non-global variable

1 Answers1