Why does this pointer point to nowhere?

Question

I'm trying to add functionality to the v8sharp project and I'm having some issues (I'm not very good at C++ so I'm pretty sure the issue lies in my lack of C++ abilities rather than misusing v8.)

Anyone in v8sharp we have this code

v8value.cpp:

v8sharp::V8FunctionWrapper^ V8ValueWrapper::WrapFunction(v8::Handle<v8::Value> value) {

  // Now we use the wrapper to make this safe to use
  // this works
  Console::WriteLine("IsFunction-First: {0}", value->IsFunction());
                // persistent so it doesn't get garbage collected
  v8::Persistent<v8::Value> pval(value);
                // create a function wrapper
  V8FunctionWrapper^ bla = gcnew V8FunctionWrapper(pval);
  return bla;
 }

Which should take a v8Handle<v8::Value> which contains a function (it always will because of what calls this function) and returns a nice .net wrapper so we can use it in my C# project.

The problem lies here v8functionwrapper.cpp:

#include "v8functionwrapper.h"
#include "v8value.h";


v8sharp::V8FunctionWrapper::V8FunctionWrapper(v8::Persistent<v8::Value> value)
{
    // is this wrong? 
 this->_value = &value;
     // still true
 Console::WriteLine("Constructor: {0}", (*this->_value)->IsFunction());


}

// This function is called by C# and triggers the javascript function
Object^ v8sharp::V8FunctionWrapper::Call([ParamArray]array<Object ^> ^ args)
{
 // Get a refence to the function
 Console::WriteLine("Cast 2");
    // MEMORY VIOLATION: the _value no longer points to a valid object :(
 Console::WriteLine("IsFunction: {0}", (*this->_value)->IsFunction());
 Console::ReadLine();
-- snip --

}

v8functionwrapper.h:

#pragma once
#include "v8.h"

using namespace System;
using namespace System::Reflection;

namespace v8sharp {
public ref class V8FunctionWrapper 
{
public:
 delegate bool V8FunctionCallback(array<Object ^> ^ args);
 Object^ v8sharp::V8FunctionWrapper::Call([ParamArray]array<Object ^> ^ args);
 v8::Persistent<v8::Value> Unwrap();
 V8FunctionWrapper(v8::Persistent<v8::Value> value);
 ~V8FunctionWrapper();
 !V8FunctionWrapper();

private:
 V8FunctionCallback^ _callback;
 v8::v8<Persistent::Value>* _value;

};
}

Evident from this line (debug code): Console::WriteLine("IsFunction: {0}", (*this->_value)->IsFunction()); The pointer _value is no longer valid and causes an exception. Why does my pointer invalidate? Is it because I'm pointing to an argument in the constructor and that gets deleted? If so how do I get a pointer to it that won't disappear. Keep in mind this is a .net class so I can't mix and match native types into it.

Answer 1

You need to instantiate a new v8::Persistent value as a member of your class because the one you are passing in is created on the stack and will be destroyed as soon as WrapFunction returns. Also don't forget to delete _value when your object is destroyed.

v8sharp::V8FunctionWrapper::V8FunctionWrapper(v8::Persistent<v8::Value> value)
{
    this->_value = new v8::Persistent<v8::Value>(value)
}