cut down example of making a C++ 'object' for V8

v8CXXObjectExample.cpp

// Example Class instantiation
// by Kent Williams chaircrusher@gmail.com
//
#include <iostream>
#include <cstring>
#include <memory>
#include <v8.h>
#include <libplatform/libplatform.h>

// template for TestClass
v8::Eternal<v8::ObjectTemplate> TestClassGlobal;
long long TestClassIdentifier(0);

// a sort-of destructor in v8 land
void TestClassMakeWeak(const v8::WeakCallbackInfo<char> &data) {
   // the private object is the doc root handle, which doesn't need
   // deleting.
   //
   delete [] data.GetParameter();
}

// SetTestClassData -- set the private data in the class methods
static void SetTestClassData(v8::Isolate *isolate,v8::Local<v8::Object> obj, char *s) {
   v8::HandleScope handle_scope(isolate);
#if 0
   // I think anything wrapped with v8::External needs to be
   // explicitly freed.
   // get the saved field
   v8::Local<v8::Value> oldValue = obj->GetInternalField(0);
   v8::Local<v8::External> Extern = v8::Local<v8::External>::Cast(oldValue);
   // get the raw pointer
   void *oldval = Extern->Value();
   // delete it
   if(oldval)
      delete [] static_cast<char *>(oldval);
#endif
   obj->SetInternalField(0,v8::External::New(isolate,s));
   obj->SetAlignedPointerInInternalField(1,&TestClassIdentifier);
   v8::Persistent<v8::Object,v8::CopyablePersistentTraits<v8::Object>> pobj(isolate,obj);
   pobj.SetWeak<char>(s,TestClassMakeWeak,v8::WeakCallbackType::kParameter);
}

// true if this object is an instance of the TestClass
bool IsTestClassObject(v8::Local<v8::Object> obj) {
   int fieldcount = obj->InternalFieldCount();
   const void *id = nullptr;
   if(fieldcount >= 2)
      id = obj->GetAlignedPointerFromInternalField(1);
   return  id == static_cast<void *>(&TestClassIdentifier);
}

//
// Constructor function to call from C++ to get a new instance.
// the internal field is set to null
v8::Local<v8::Object> NewTestClass(v8::Isolate *isolate,v8::Local<v8::Context> context) {
   v8::EscapableHandleScope handle_scope(isolate);
   v8::Local<v8::Object> retobj = TestClassGlobal.Get(isolate)->NewInstance(context).ToLocalChecked();
   // does this do the right thing?
   retobj->SetAlignedPointerInInternalField(0,nullptr);
   retobj->SetAlignedPointerInInternalField(1,static_cast<void *>(&TestClassIdentifier));
   return handle_scope.Escape(retobj);
}

//
// given a string argument, set it as the internal value
void SetPrivateString(const v8::FunctionCallbackInfo<v8::Value> &args) {
   v8::Isolate *isolate(args.GetIsolate());
   v8::HandleScope handle_scope(isolate);
   args.GetReturnValue().Set(v8::Null(isolate));
   if(!IsTestClassObject(args.This()))
      return;
   if(args.Length() < 1 || !args[0]->IsString())
      return;
   v8::String::Utf8Value tmp(args[0]->ToString());
   // allocate string
   const char *val = *tmp ? *tmp : "";
   char *setval = new char [strlen(val)+1];
   strcpy(setval,val);
   // set the internal value
   SetTestClassData(isolate,args.This(),setval);
   args.GetReturnValue().Set(v8::True(isolate));
}

// retrieve string and return it.
void GetPrivateString(const v8::FunctionCallbackInfo<v8::Value> &args) {
   v8::Isolate *isolate(args.GetIsolate());
   v8::HandleScope handle_scope(isolate);
   args.GetReturnValue().Set(v8::Null(isolate));
   if(!IsTestClassObject(args.This()))
      return;
   v8::Local<v8::External> Extern =
      v8::Local<v8::External>::Cast(args.This()->GetInternalField(0));
   char *rval = static_cast<char *>(Extern->Value());
   v8::Local<v8::String> s = v8::String::NewFromUtf8(isolate,rval);
   args.GetReturnValue().Set(s);
}

// to list object methods
struct v8FunctionDescriptor {
   const char *name;
   v8::FunctionCallback func;
};
// list of object methods
static v8FunctionDescriptor TestClassMethods[] = {
   // list of name/functions
   { "SetPrivateString", SetPrivateString },
   { "GetPrivateString", GetPrivateString },
   { nullptr, nullptr },
};

// constructor called from JS to make 
static void TestClassConstructor(const v8::FunctionCallbackInfo<v8::Value> &args) {
   v8::Isolate *isolate(args.GetIsolate());
   v8::HandleScope handle_scope(isolate);
   v8::Local<v8::Context> context = isolate->GetCurrentContext();

   //
   // get upset if this isn't a constructor call
   if(!args.IsConstructCall()) {
      isolate->ThrowException(v8::String::NewFromUtf8(isolate,
                                                      "Cannot call constructor as function"));
   }

   v8::Local<v8::Object> holder = args.Holder();
   v8::Local<v8::Object> This = args.This();
   v8::Local<v8::Value> proto = holder->GetPrototype();
   if(proto->IsObject())
      This->SetPrototype(context,proto);
   //
   // private data
   char *privateString = new char[512];
   std::strcpy(privateString,"Hello From TestClass");
   SetTestClassData(isolate,This,privateString);
   args.GetReturnValue().Set(This);
}

void init_TestClass(v8::Isolate *isolate,
                    v8::Local<v8::Context> &context) {
   v8::HandleScope handle_scope(isolate);
   // name of 'class'
   v8::Local<v8::String> name =
      v8::String::NewFromUtf8(isolate, "TestClass",
                              v8::NewStringType::kNormal).ToLocalChecked();
   // object template for the class
   v8::Local<v8::ObjectTemplate> instance_template = v8::ObjectTemplate::New(isolate);
   // tell it to allocate 2 internal fields
   instance_template->SetInternalFieldCount(2);
   // give it the constructor to call
   instance_template->SetCallAsFunctionHandler(TestClassConstructor);
   // save Eternal pointer in order to call NewInstance from C++
   TestClassGlobal.Set(isolate,instance_template);

   // add functions to object template
   for(unsigned i = 0; TestClassMethods[i].name != nullptr; ++i) {
      v8::Local<v8::FunctionTemplate> ft =
         v8::FunctionTemplate::New(isolate, TestClassMethods[i].func);
      ft->SetClassName(name);
      v8::Local<v8::String> curName =
         v8::String::NewFromUtf8(isolate, TestClassMethods[i].name,
                                 v8::NewStringType::kNormal).ToLocalChecked();
      // testClass_PrototypeTemplate->Set(curName, ft->GetFunction(context).ToLocalChecked());
      instance_template->Set(curName, ft);
   }
   v8::Local<v8::Object> instance = instance_template->NewInstance(context).ToLocalChecked();
   context->Global()->Set(context,name, instance).FromJust();
}

// Extracts a C string from a V8 Utf8Value.
const char* ToCString(const v8::String::Utf8Value& value) {
  return *value ? *value : "<string conversion failed>";
}

void Print(const v8::FunctionCallbackInfo<v8::Value>& args);
void ReportException(v8::Isolate* isolate, v8::TryCatch* try_catch);
v8::Local<v8::Context> CreateShellContext(v8::Isolate* isolate);

const char TestScript[] =
   "print('1. this is a test');\n"
   "var x = new TestClass;\n"
   "x.SetPrivateString('This Is A test');\n"
   "print('2. ' + x.GetPrivateString());\n"
   "x.SetPrivateString('This Is A test');\n"
   "print('3. ' + x.GetPrivateString());\n";

int main(int argc, char **argv) {
   // Initialize V8.
   v8::V8::InitializeICUDefaultLocation(argv[0]);
   v8::V8::InitializeExternalStartupData(argv[0]);

   v8::Platform* platform = v8::platform::CreateDefaultPlatform();
   v8::V8::InitializePlatform(platform);
   v8::V8::Initialize();

   // Create a new Isolate and make it the current one.
   v8::Isolate::CreateParams create_params;
   create_params.array_buffer_allocator =
      v8::ArrayBuffer::Allocator::NewDefaultAllocator();
   v8::Isolate* isolate = v8::Isolate::New(create_params);

   {
      v8::Isolate::Scope isolate_scope(isolate);

      // Create a stack-allocated handle scope.
      v8::HandleScope handle_scope(isolate);

      // create context with global template
      v8::Local<v8::Context> context = CreateShellContext(isolate);

      if (context.IsEmpty()) {
         fprintf(stderr, "Error creating context\n");
         return 1;
      }

      // Enter the context for compiling and running the hello world script.
      v8::Context::Scope context_scope(context);


      // Create a string containing the JavaScript source code.
      v8::Local<v8::String> source =
         v8::String::NewFromUtf8(isolate, TestScript,
                                 v8::NewStringType::kNormal).ToLocalChecked();

      v8::TryCatch try_catch(isolate);
      v8::Local<v8::Script> script;
      // Compile the source code.
      if(!v8::Script::Compile(context, source).ToLocal(&script)) {
         ReportException(isolate, &try_catch);
      }

      // Run the script to get the result.
      v8::MaybeLocal<v8::Value> result = script->Run(context);
      if(!result.IsEmpty()) {
         // Convert the result to an UTF8 string and print it.
         v8::String::Utf8Value utf8(result.ToLocalChecked());
         printf("%s\n", *utf8);
      } else {
         ReportException(isolate, &try_catch);
      }
   }

   // Dispose the isolate and tear down V8.
   isolate->Dispose();
   v8::V8::Dispose();
   v8::V8::ShutdownPlatform();
   delete platform;
   delete create_params.array_buffer_allocator;
   return 0;

}

// The callback that is invoked by v8 whenever the JavaScript 'print'
// function is called.  Prints its arguments on stdout separated by
// spaces and ending with a newline.
void Print(const v8::FunctionCallbackInfo<v8::Value>& args) {
   v8::Isolate *isolate(args.GetIsolate());
   bool first = true;
   for (int i = 0; i < args.Length(); i++) {
      v8::HandleScope handle_scope(isolate);
      if (first) {
         first = false;
      } else {
         printf(" ");
      }
      v8::String::Utf8Value str(args[i]);
      const char* cstr = ToCString(str);
      printf("%s", cstr);
   }
   printf("\n");
   fflush(stdout);
   args.GetReturnValue().Set(v8::True(isolate));
}

v8::Local<v8::Context> CreateShellContext(v8::Isolate* isolate) {
  // Create a template for the global object.
  v8::Local<v8::ObjectTemplate> global = v8::ObjectTemplate::New(isolate);
  // Bind the global 'print' function to the C++ Print callback.
  global->Set(
      v8::String::NewFromUtf8(isolate, "print", v8::NewStringType::kNormal)
          .ToLocalChecked(),
      v8::FunctionTemplate::New(isolate, Print));
  v8::Local<v8::Context> rval = v8::Context::New(isolate, NULL, global);
  // add our test class
  init_TestClass(isolate,rval);
  return rval;
}

void ReportException(v8::Isolate* isolate, v8::TryCatch* try_catch) {
  v8::HandleScope handle_scope(isolate);
  v8::String::Utf8Value exception(try_catch->Exception());
  const char* exception_string = ToCString(exception);
  v8::Local<v8::Message> message = try_catch->Message();
  if (message.IsEmpty()) {
    // V8 didn't provide any extra information about this error; just
    // print the exception.
    fprintf(stderr, "%s\n", exception_string);
  } else {
    // Print (filename):(line number): (message).
    v8::String::Utf8Value filename(message->GetScriptOrigin().ResourceName());
    v8::Local<v8::Context> context(isolate->GetCurrentContext());
    const char* filename_string = ToCString(filename);
    int linenum = message->GetLineNumber(context).FromJust();
    fprintf(stderr, "%s:%i: %s\n", filename_string, linenum, exception_string);
    // Print line of source code.
    v8::String::Utf8Value sourceline(
        message->GetSourceLine(context).ToLocalChecked());
    const char* sourceline_string = ToCString(sourceline);
    fprintf(stderr, "%s\n", sourceline_string);
    // Print wavy underline (GetUnderline is deprecated).
    int start = message->GetStartColumn(context).FromJust();
    for (int i = 0; i < start; i++) {
      fprintf(stderr, " ");
    }
    int end = message->GetEndColumn(context).FromJust();
    for (int i = start; i < end; i++) {
      fprintf(stderr, "^");
    }
    fprintf(stderr, "\n");
    v8::Local<v8::Value> stack_trace_string;
    if (try_catch->StackTrace(context).ToLocal(&stack_trace_string) &&
        stack_trace_string->IsString() &&
        v8::Local<v8::String>::Cast(stack_trace_string)->Length() > 0) {
      v8::String::Utf8Value stack_trace(stack_trace_string);
      const char* stack_trace_string = ToCString(stack_trace);
      fprintf(stderr, "%s\n", stack_trace_string);
    }
  }
}