tech page 14 - Dave's Blog

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.

Last time I wrote about how I switched from Delicious to Google Reader's shared links feature only to find out that week that Google was removing the Google Reader shared links feature in favor of Google Plus social features (I'll save my Google Plus rant for another day).

Forced to find something new again, I'm now very pleased with Tumblr. Google Reader has Tumblr in its preset list of Send To sites which makes it relatively easy to add articles. And Tumblr's UX for adding things lets me easily pick a photo or video to display from the article - something which I had put together with a less convenient UX on my bespoke blogging system. For adding things outside of Google Reader I made a Tumblr accelerator to hookup to the Tumblr Add UX.

Of course they have an RSS feed which I hooked up to my blog. The only issue I had there is that when you add a link (and not a video or photo) to Tumblr, the RSS feed entry title for that link is repeated in the entry description as a link followed by a colon and then the actual description entered into Tumblr. I want my title separate so I can apply my own markup so I did a bit of parsing of the description to remove the repeated title from the description.

Shortly after joining the Internet Explorer team I got a bug from a PM on a popular Microsoft web server product that I'll leave unnamed (from now on UWS). The bug said that IE was handling empty path segments incorrectly by not removing them before resolving dotted path segments. For example UWS would do the following:

A.1. http://example.com/a/b//../
A.2. http://example.com/a/b/../
A.3. http://example.com/a/

In step 1 they are given a URI with dotted path segment and an empty path segment. In step 2 they remove the empty path segment, and in step 3 they resolve the dotted path segment. Whereas, given the same initial URI, IE would do the following:

B.1. http://example.com/a/b//../
B.2. http://example.com/a/b/

IE simply resolves the dotted path segment against the empty path segment and removes them both. So, how did I resolve this bug? As "By Design" of course!

The URI RFC allows path segments of zero length and does not assign them any special meaning. So generic user agents that intend to work on the web must not treat an empty path segment any different from a path segment with some text in it. In the case above IE is doing the correct thing.

That's the case for generic user agents, however servers may decide that a URI with an empty path segment returns the same resource as a the same URI without that empty path segment. Essentially they can decide to ignore empty path segments. Both IIS and Apache work this way and thus return the same resource for the following URIs:

http://exmaple.com/foo//bar///baz
http://example.com/foo/bar/baz

The issue for UWS is that it removes empty path segments before resolving dotted path segments. It must follow normal URI procedure before applying its own additional rules for empty path segments. Not doing that means they end up violating URI equivalency rules: URIs (A.1) and (B.2) are equivalent but UWS will not return the same resource for them.

A bug came up the other day involving markup containing <input type="image" src="http://example.com/.... I knew that "image" was a valid input type but it wasn't until that moment that I realized I didn't know what it did. Looking it up I found that it displays the specified image and when the user clicks on the image, the form is submitted with an additional two name value pairs: the x and y positions of the point at which the user clicked the image.

Take for example the following HTML:

<form action="http://example.com/">
<input type="image" name="foo" src="http://deletethis.net/dave/images/davebefore.jpg">
</form>

If the user clicks on the image, the browser will submit the form with a URI like the following:http://example.com/?foo.x=145&foo.y=124.

This seemed like an incredibly specific feature to be built directly into the language when this could instead be done with javascript. I looked a bit further and saw that its been in HTML since at least HTML2, which of course makes much more sense. Javascript barely existed at that point and sending off the user's click location in a form may have been the only way to do something interesting with that action.

I had previously replaced my use of Delicious with Google Reader. Delicious had a number of issues during their switch over from Yahoo to the new owners and I was eventually fed up enough to remove it from daily use. I used Delicious to do the following things:

• Create a list of things to read later
• Save things to read again in the future
• Search through things I read and enjoyed (esp via tags)
• Annotate and share things on my blog

I realized that since I did most of my web browsing in Google Reader now anyway I may as well make use of its features. I star things to note I want to read it later or save to read again later. I can annotate with notes in Google Reader and I can share items to my web site by way of the shared items feed. Additionally for when I'm not in Google Reader there's a bookmarklet to add an arbitrary web site as a shared item in Google Reader.

Of course I wrote this and switched over about 1 week before Google removed the sharing feature from Google Reader. I'm irritated but in practice it forced me to find a different option which has worked out mostly better. New blog post coming soon about that...