Embedding a scripting engine in C++

Question

I'm researching how to best extend a C++ application with scripting capability, and I am looking at either Python or JavaScript. User-defined scripts will need the ability to access the application's data model.

Have any of you had experiences with embedding these scripting engines? What are some potential pitfalls?

Answer 1

It's sure easy to embed Python by using the Boost::Python library (ok, ok, sarcasm.) Nothing is "easy" when it comes to cross-language functionality. Boost has done a great deal to aid such development. One of the developers I've worked with swears on the Boost->Python interface. His code can be programmed by a user in Python, with a REPL built right into the UI. Amazing.

However, my experience has been better observed using SWIG and other languages such as Java. I'm currently working with SWIG to wrap C++ with Python. There's all sorts of gotchas with exceptions, threading, cross-language polymorphism and the like.

I'd look at these two places first. As I said, nothing will be "easy" but both these make life more livable.

Answer 2

Boost::Python, as in wheaties answer, is a very mature solution.

Lua has a reputation for being easy to embed but I have not tried this myself.

As a user of R, I am more interested in embedding R which is possible using the RInside package. A simple example is

#include <RInside.h>                // for the embedded R via RInside

int main(int argc, char *argv[]) {

    RInside R(argc, argv);          // create an embedded R instance 

    R["txt"] = "Hello, world!\n";   // assign a char* (string) to 'txt'

    R.parseEvalQ("cat(txt)");       // eval the init string, ignoring any returns

    exit(0);
}

and there are a couple more examples in the package. RInside essentially provides you a nice wrapper around the R engine using some of the Rcpp interface package.

Answer 3

Lua is also a great candidate for embedding in programs. Its very self contained, and even the native cross-language call system isn't bad.

For JavaScript, your best bet right now is to look at V8 (from Google), which is easy enough to work with.

Answer 4

Unless you're really set on Python or Javascript, I'd give some consideration to using Lua. Since it's designed entirely as an embedded scripting engine, it eliminates quite a bit of overlap with what C and C++ already do well. It's also pretty easy to embed as long as you only interface between your code and the Lua engine in terms of C callable functions.

If you want to use a C++ level interface, you might want to take a look at LuaBind, which allows things like a Lua class deriving from (the proxy it generates for) a C++ class you wrote.