Calling C++ function from JavaScript script running in a web browser control

I have embedded a web browser control in my c++ application. I want javascript running in the web browser control to be able to call a c++ function/method.

I have found mentions of three ways to do this:

Implement an ActiveX component that acts as a middle man. (Implementation details here: http://blogs.msdn.com/b/nicd/archive/2007/04/18/calling-into-your-bho-from-a-client-script.aspx)
Use window.external. (Also discussed in the link above, but no implementation provided)
Add a custom object to the window object

I want to go with the third option, but I haven't found any working examples on how to do that. Can someone please show me how to do it, or link to a working example on the net somewhere.

The closest to an example that I have found is the first reply by Igor Tandetnik in a thread in the webbrowser_ctl news group. But I'm afraid I need more help than that.

I'm embedding an IWebBrowser2 control and I am not using MFC, ATL or WTL.

EDIT:

Going by the pseudo-code given by Igor in the thread I linked earlier, and code found in the codeproject article "Creating JavaScript arrays and other objects from C++" I've produced some code.

void WebForm::AddCustomObject(IDispatch *custObj, std::string name)
{
    IHTMLDocument2 *doc = GetDoc();
    IHTMLWindow2 *win = NULL;
    doc->get_parentWindow(&win);

    if (win == NULL) {
        return;
    }

    IDispatchEx *winEx;
    win->QueryInterface(&winEx);

    if (winEx == NULL) {
        return;
    }

    int lenW = MultiByteToWideChar(CP_ACP, MB_PRECOMPOSED, name.c_str(), -1, NULL, 0);
    BSTR objName = SysAllocStringLen(0, lenW);
    MultiByteToWideChar(CP_ACP, MB_PRECOMPOSED, name.c_str(), -1, objName, lenW);

    DISPID dispid; 
    HRESULT hr = winEx->GetDispID(objName, fdexNameEnsure, &dispid);

    SysFreeString(objName);

    if (FAILED(hr)) {
        return;
    }

    DISPID namedArgs[] = {DISPID_PROPERTYPUT};
    DISPPARAMS params;
    params.rgvarg = new VARIANT[1];
    params.rgvarg[0].pdispVal = custObj;
    params.rgvarg[0].vt = VT_DISPATCH;
    params.rgdispidNamedArgs = namedArgs;
    params.cArgs = 1;
    params.cNamedArgs = 1;

    hr = winEx->InvokeEx(dispid, LOCALE_USER_DEFAULT, DISPATCH_PROPERTYPUT, &params, NULL, NULL, NULL); 

    if (FAILED(hr)) {
        return;
    }
}

The code above runs all the way through, so everything looks alright that far.

I call AddCustomObject when I receive the DISPID_NAVIGATECOMPLETE2 DWebBrowserEvents2 event passing this as *custObj:

class JSObject : public IDispatch {
private:
    long ref;

public:
    // IUnknown
    virtual HRESULT STDMETHODCALLTYPE QueryInterface(REFIID riid, void **ppv);
    virtual ULONG STDMETHODCALLTYPE AddRef();
    virtual ULONG STDMETHODCALLTYPE Release();

    // IDispatch
    virtual HRESULT STDMETHODCALLTYPE GetTypeInfoCount(UINT *pctinfo);
    virtual HRESULT STDMETHODCALLTYPE GetTypeInfo(UINT iTInfo, LCID lcid,
        ITypeInfo **ppTInfo);
    virtual HRESULT STDMETHODCALLTYPE GetIDsOfNames(REFIID riid,
        LPOLESTR *rgszNames, UINT cNames, LCID lcid, DISPID *rgDispId);
    virtual HRESULT STDMETHODCALLTYPE Invoke(DISPID dispIdMember, REFIID riid,
        LCID lcid, WORD wFlags, DISPPARAMS *Params, VARIANT *pVarResult,
        EXCEPINFO *pExcepInfo, UINT *puArgErr);
};

Noteworthy implementations might be

HRESULT STDMETHODCALLTYPE JSObject::QueryInterface(REFIID riid, void **ppv)
{
    *ppv = NULL;

    if (riid == IID_IUnknown || riid == IID_IDispatch) {
        *ppv = static_cast<IDispatch*>(this);
    }

    if (*ppv != NULL) {
        AddRef();
        return S_OK;
    }

    return E_NOINTERFACE;
}

and

HRESULT STDMETHODCALLTYPE JSObject::Invoke(DISPID dispIdMember, REFIID riid,
    LCID lcid, WORD wFlags, DISPPARAMS *Params, VARIANT *pVarResult,
    EXCEPINFO *pExcepInfo, UINT *puArgErr)
{
    MessageBox(NULL, "Invoke", "JSObject", MB_OK);
    return DISP_E_MEMBERNOTFOUND;
}

Unfortunately I never get the "Invoke" message box when I try to use the "JSObject" object from the javascript code.

JSObject.randomFunctionName();  // This should give me the c++ "Invoke" message
                                // box, but it doesn't

EDIT 2:

I implemented GetIDsOfNames like so:

HRESULT STDMETHODCALLTYPE JSObject::GetIDsOfNames(REFIID riid,
    LPOLESTR *rgszNames, UINT cNames, LCID lcid, DISPID *rgDispId)
{
    HRESULT hr = S_OK;

    for (UINT i = 0; i < cNames; i++) {
        std::map<std::wstring, DISPID>::iterator iter = idMap.find(rgszNames[i]);
        if (iter != idMap.end()) {
            rgDispId[i] = iter->second;
        } else {
            rgDispId[i] = DISPID_UNKNOWN;
            hr = DISP_E_UNKNOWNNAME;
        }
    }

    return hr;
}

and this is my constructor

JSObject::JSObject() : ref(0)
{
    idMap.insert(std::make_pair(L"execute", DISPID_USER_EXECUTE));
    idMap.insert(std::make_pair(L"writefile", DISPID_USER_WRITEFILE));
    idMap.insert(std::make_pair(L"readfile", DISPID_USER_READFILE));
}

with the DISPID_USER_* constants defined as private class members

class JSObject : public IDispatch {
private:
    static const DISPID DISPID_USER_EXECUTE = DISPID_VALUE + 1;
    static const DISPID DISPID_USER_WRITEFILE = DISPID_VALUE + 2;
    static const DISPID DISPID_USER_READFILE = DISPID_VALUE + 3;

    // ...
};

EDIT 3, 4 and 5:

Moved to a separate question

EDIT 6:

Made a separate question out of the "returning a string" edits. That way I can accept Georg's reply as that answers the original question.

EDIT 7:

I have gotten a few requests for a fully working, self contained, example implementation. Here it is: https://github.com/Tobbe/CppIEEmbed. Please fork and improve if you can :)

You need to implement GetIDsOfNames() to do something sensible as that function will be called by client code before Invoke().
If you have your interfaces in a type library see here for an example. If you want to use late-binding instead, you can use DISPIDs greater DISPID_VALUE and less than 0x80010000 (all values <= 0 and in the range 0x80010000 through 0x8001FFFF are reserved):

HRESULT GetIDsOfNames(REFIID riid, LPOLESTR *rgszNames, UINT cNames, 
                      LCID lcid, DISPID *rgDispId)
{
    HR hr = S_OK;
    for (UINT i=0; i<cNames; ++i) {
        if (validName(rgszNames)) {
            rgDispId[i] = dispIdForName(rgszNames);
        } else {
            rgDispId[i] = DISPID_UNKNOWN;
            hr = DISP_E_UNKNOWNNAME;
        }
    }
    return hr;
}

HRESULT Invoke(DISPID dispIdMember, REFIID riid, LCID lcid, WORD wFlags, 
               DISPPARAMS *Params, VARIANT *pVarResult, EXCEPINFO *pExcepInfo, 
               UINT *puArgErr)
{
    if (wFlags & DISPATCH_METHOD) {
       // handle according to DISPID ...
    }

    // ...

Note that the DISPIDs are not supposed to change suddenly, so e.g. a static map or constant values should be used.

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