works page 2 - Dave's Blog

Favorite Windows 8 Feature: Intra-Line Tab Completion

Why Did This Work?

Bug Spotting: Ctors with default parameters

The following code compiled just fine but did not at all act in the manner I expected:

BOOL CheckForThing(__in CObj *pObj, __in IFigMgr* pFigMgr, __in_opt LPCWSTR url)
{
    BOOL fCheck = FALSE;
    if (SubCheck(pObj))
    {
        ...

I’m calling SubCheck which looks like:

bool SubCheck(const CObj& obj);

Did you spot the bug? As you can see I should be passing in *pObj not pObj since the method takes a const CObj& not a CObj*. But then why does it compile?

It works because CObj has a constructor with all but one param with default values and CObj is derived from IUnknown:

CObj(__in_opt IUnknown * pUnkOuter, __in_opt LPCWSTR pszUrl = NULL);

Accordingly C++ uses this constructor as an implicit conversion operator. So instead of passing in my CObj, I end up creating a new CObj on the stack passing in the CObj I wanted as the outer object which has a number of issues.

The lesson is unless you really want this behavior, don't make constructors with all but 1 or 0 default parameters. If you need to do that consider using the 'explicit' keyword on the constructor.

More info about forcing single argument constructors to be explicit is available on stack overflow.

Bug Spotting: Smart pointers and parameter evaluation order

The following code works fine. I have a ccomptr named resolvedUri and I want to update its hostname so I do the following:

        CreateIUriBuilder(resolvedUri, 0, 0, &builder);
        builder->SetHost(host);
        builder->CreateUri(0xFFFFFFFF, 0, 0, &resolvedUri);

But the following similar looking code has a bug:

    ResolveHost(resolvedUri, &resolvedUri);

The issue is that doing &resolvedUri gets the address of the pointer but also clears out the pointer due to the definition of my smart pointer class:

    operator T**()  
    {  
        T *ptrValue = mPtrValue;
        mPtrValue->Release();
        mPtrValue = NULL;
        return &ptrValue;
    }

In C++ there’s no guarantee about the order in which parameters for a function or method are evaluated. In the case above, &resolvedUri clears out the ccomptr before evaluating resolvedUri.Get() and so ResolveHostAlias gets a nullptr.

An interesting and related thread on stack overflow on undefined behavior in C++.

Easy: Connect your RSS or Atom feed to Google Buzz

It was relatively easy, although still more difficult than I would have guessed, to hook my bespoke website's Atom feed up to Google Buzz. I already have a Google email account and associated profile so Buzz just showed up in my Gmail interface. Setting it up it offered to connect to my YouTube account or my Google Chat account but I didn't see an option to connect to an arbitrary RSS or Atom feed like I expected.

But of course hooking up an arbitrary Atom or RSS feed is documented. You hook it up in the same manner you claim a website as your own via the Google Profile (for some reason they want to ensure you own the feed connected to your Buzz account). You do this via Google's social graph API which uses XFN or FOAF. I used XFN by simply adding a link to my feed to my Google profile (And be sure to check the 'This is a profile page about me' which ensures that a rel="me" tag is added to the HTML on your profile. This is how XFN works.) And by adding a corresponding link in my feed back to my Google profile page with the following:

atom:link rel="me" href="http://www.google.com/profiles/david.risney"

So more difficult than I would have expected (more difficult than just an 'Add your feed' button and textbox) but not super difficult. And yet after reading this Buzz from DeWitt Clinton I feel better about opting-in to Google's Social API.

Photos Bahamas Anecdote

Sarah and I just got back home from a Eric and Jane's wedding / Sarah and Dave's vacation trip to the Bahamas (note the lack of activity for the past twelve days on my website). I've got plenty of photos and things to post but for now I'll just relate this humorous anecdote during the rehearsal dinner. I had said something about photos to Jim, Eric's brother and he gave me a crazy look. "Oh, I thought you meant like pho-tos" he said. It took me a moment to realize he misunderstood what I said as "faux toes". I laughed until I cried a little. Also works with digital faux toes.

View PDFs on Android

Irritatingly, my G1 won't show me PDFs so I've made the Google Docs PDF viewer which will load PDFs on the web up in Google Docs. Google Docs has the useful ability to display PDFs in web browsers without any Adobe software and works (mostly) on Android.

This was very easy to put together as an Android activity. First its necessary to register the application as handling PDFs from the web. This is done via the intent-filter declaration in the manifest:

   intent-filter
      action android:name="android.intent.action.VIEW"/
      data android:scheme="http" android:mimeType="application/pdf"/
      category android:name="android.intent.category.DEFAULT"/
      category android:name="android.intent.category.BROWSABLE"/
   /intent-filter

Second, the activity opens up the browser to Google Docs pointing to the PDF.

   Intent intent = new Intent();
   intent.setAction(getIntent().getAction());
   intent.setData(Uri.parse(
    "http://docs.google.com/gview?embedded=true&url=" + 
    percentEncodeForQuery(getIntent().getData().toString())));

   startActivity(intent);