Export integer values from Node.js C++ module

Question

I am writing a C++ module for Node.js that wraps a C library, so we can use the C library from JavaScript. One of the functions in the C library takes an enum parameter, with the enum values included in the corresponding header file. I would like to export the enum values as integers from the C++ module so the Node.js application wouldn't have to hardcode the value. Here is the basic idea:

C:

typedef enum 
{
    LOGLEVEL_ERROR = 0,
    LOGLEVEL_WARN,
    LOGLEVEL_INFO
} log_level_t;

write_to_log(log_level_t level, char* message);

C++:

Logger::Write(int level, char* message)
{
    write_to_log(level, message);
}

Node.js:

// This is what I want:
logger.write(logger.ERROR, "Oh no!  Something bad happened.");

How do I expose LOGLEVEL_ERROR from C++ so I can use logger.ERROR in JS? (I would even be OK with logger.LOGLEVEL_ERROR in JS.) I found an old Node.js native module that used a macro EXPORT_INT32 but this doesn't seem to exist anymore (I'm using Node.js 0.8.8).

JP Richardson · Accepted Answer · 2012-09-19 18:58:51Z

2

Wrap the number, like this:

HandleScope scope;

/** method code here **/

Local<Number> num = Number::New(5);
return scope.Close(num);

See my module kexec as an example: https://github.com/jprichardson/node-kexec/blob/master/src/kexec.cc

Edit: Ooops, I didn't entirely read your question clearly. I would declared those values in JS.

Edit 2: Upon further inspection, it seems that it may be a bit harry, but possible. It seems that you might want to use V8 Juice.

Here is the relevant section: Converting Enums

Hope this helps some, but the easiest route would be to just declare them in JavaScript.

edited Sep 19, 2012 at 18:58

answered Sep 19, 2012 at 18:43

JP Richardson

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1 Comment

Zik Over a year ago

Thanks. I've been using V8Convert but haven't look at V8Juice yet, I'll check that out. Although putting the values in JS probably will turn out to be the easiest...

Zik · Accepted Answer · 2013-01-03 18:52:35Z

1

This is what I ended up doing:

Local<Object> instance = constructor->NewInstance(2, argv);

Local<Object> logLevel = Object::New();
logLevel->Set(String::NewSymbol("ERROR"), cvv8::CastToJS(LOGLEVEL_ERROR), ReadOnly);
logLevel->Set(String::NewSymbol("WARN"), cvv8::CastToJS(LOGLEVEL_WARN), ReadOnly);
logLevel->Set(String::NewSymbol("INFO"), cvv8::CastToJS(LOGLEVEL_INFO), ReadOnly);

instance->Set(String::NewSymbol("Level"), logLevel, ReadOnly);

Then from JavaScript you can access logger.Level.ERROR.

answered Jan 3, 2013 at 18:52

Zik

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