JsRT: Wrappers for C++

One unfortunate side-effect of the fact that the JsRT API is a flat Win32-style API is that it can be a little… painful to use in C++. Some of the conventions, particularly the use of error code returns everywhere and the use of return parameters, mean that you have to write a lot of boilerplate code. And marshalling values between C++ and JavaScript can be very cumbersome. So with that in mind, I’ve written some sample C++ wrappers that make the process of working with the JsRT APIs a lot nicer in C++.

You can get them from my GitHub account here. For more information, here’s part of the readme, which covers some of the highlights of the wrappers:

A Tour Through the Helpers

Most of the wrappers simply provide a nice object-oriented interface over the JsRT API. For example, creating a runtime and a context works like this:

jsrt::runtime runtime = jsrt::runtime::create();
jsrt::context context = runtime.create_context();

There are, however, a number of special features in the wrappers.

Context scopes

A context scope (jsrt::context::scope) automatically sets the current context when it comes into scope and clears the current context when it goes out of scope. This greatly simplifies working with scopes. For example:

jsrt::runtime runtime = jsrt::runtime::create();
jsrt::context context = runtime.create_context();
{
    jsrt::context::scope scope(context);
    ... work with context ...
}
// Context has automatically been set back to original value.
runtime.dispose();

Pinned references

To keep a reference to a Chakra object alive, it must be visible to the Chakra garbage collector (i.e. on the stack or stored in the garbage collected heap) or it has to be pinned using JsAddRef. The pinned<T> class functions as a smart-pointer template that keeps a reference alive using JsAddRef and JsRelease automatically. For example:

{
    jsrt::pinned<jsrt::object> pinned_obj = jsrt::object::create();
    // Object reference does not have to stay on the stack or in the GC heap
}
// Reference can now be garbage collected

Value translation

Value getter and setter functions can be strongly-typed, allowing automatic marshalling of JavaScript values to C++ values and vice versa. For example:

jsrt::object obj = jsrt::object::create();
obj.set_property(jsrt::property_id::create(L"boolProperty"), true);
bool bool_value = obj.get_property<bool>(jsrt::property_id::create(L"boolProperty"));
obj.set_property(jsrt::property_id::create(L"stringProperty"), L"foo");

The wrappers marshal types in the following way:

  • number values are marshalled to/from double

  • string values are marshalled to/from std::wstring

  • Boolean values are marshalled to/from bool

Strongly-typed arrays

Similarly, arrays can be strongly typed and accessed using numeric indexes:

jsrt::array<double> darray = jsrt::array<double>::create(1);
darray[0] = 10;
darray[1] = 20;

Functions

Perhaps the most useful aspect of the wrappers is the way that JavaScript functions can be wrapped and their arguments and return values marshalled to C++ types, and vice versa. For example:

auto f = (jsrt::function<double, double, double>)jsrt::context::evaluate(
    L"function f(a, b) { return a + b; }; f;");
double a = f(jsrt::context::undefined(), 1, 2);

Native functions can also be wrapped:

double add(const jsrt::call_info &info, double a, double b) { return a + b; }
auto nf = jsrt::function<double, double, double>::create(add);
jsrt::context::global().set_property(jsrt::property_id::create(L"add"), nf);
jsrt::context::run(L"add(1, 2)");

When calling a function wrapper, the first argument to the function call is the this value for the call. Function wrappers can also be bound to a particular value of this. For example:

auto bf = jsrt::bound_function<jsrt::value, double, double, double>::create(
    jsrt::context::undefined(),
    (jsrt::function<double, double, double>)jsrt::context::evaluate(
        L"function f(a, b) { return a + b; }; f;"));
double ba = bf(1, 2);

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