[webkit-dev] Optional parameter in IDL and undefined JS value

Jian Li jianli at chromium.org
Wed Apr 20 18:16:06 PDT 2011


I've just found a problem in our generated code for handling optional
parameters. Suppose we define a method with optional parameter in numeric
type, like the following in IDL:
         Foo bar(in [Optional] long long start, in [Optional] long long

And we declare our C++ method as the following. Note that the default value
of the 2nd parameter is not 0.
         PassRefPtr<Foo> bar(long long start = 0, long long end =
std::numeric_limits<long long>::max());

If we call the JS method with only 1 parameter, everything works as
expected. However, if we call the JS method with 2 parameters and pass
'undefined' as the 2nd parameter, we trigger the problem.

By looking into the generated JSC code below, I found out that we are
converting undefined JS value to 0 and pass it to the function. As the
result, the default parameter value in the declaration is not respected.

EncodedJSValue JSC_HOST_CALL jsFooPrototypeFunctionFoo(ExecState* exec)

    JSValue thisValue = exec->hostThisValue();
    if (!thisValue.inherits(&JSFoo::s_info))
        return throwVMTypeError(exec);
    JSFoo* castedThis = static_cast<JSFoo*>(asObject(thisValue));
    Foo* imp = static_cast<Foo*>(castedThis->impl());

    int argsCount = exec->argumentCount();
    if (argsCount <= 0) {
        JSC::JSValue result = toJS(exec, castedThis->globalObject(),
        return JSValue::encode(result);

    long long start(static_cast<long
    if (exec->hadException())
        return JSValue::encode(jsUndefined());
    if (argsCount <= 1) {
        JSC::JSValue result = toJS(exec, castedThis->globalObject(),
        return JSValue::encode(result);

    long long end(static_cast<long
    if (exec->hadException())
        return JSValue::encode(jsUndefined());

    JSC::JSValue result = toJS(exec, castedThis->globalObject(),
WTF::getPtr(imp->bar(start, end)));
    return JSValue::encode(result);

One solution is to add the default value support in IDL. For example, we can
change the above definition of bar to something like:
         Foo bar(in [Optional, DefaultValue=0] long long start, in
[Optional, DefaultValue=2147483647] long long end);

Or the other way is to add a bool parameter for each optional parameter in
the class method declaration, that is used to indicate if the passing
parameter is defined or not. This would involve the change to both code
generator scripts and the existing implementations.

How do you think? Personally I like the 1st approach since it is simpler.


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.webkit.org/pipermail/webkit-dev/attachments/20110420/c33a3618/attachment.html>

More information about the webkit-dev mailing list