New Category: Programming (this week: swap and concept_check)

I’m starting a new category for programming tips. I jokingly referred to something as the C++ feature of the week (for Mace development) with one of our developers, and he responded that he needed to subscribe. And it seemed like a good idea, so now I think I’ll start trying to blog about these new features I learn about.

So to start it off, there are two C++ features of the week for this past week:

  1. STL collection swap. The C++ STL Collections contain a swap() method, which takes another collection (of the same type) as a parameter. The method does what’s expected — to swap the one collection’s elements with the other. What makes this an interesting function is that it does it in constant time. It doesn’t require constructing, copying, or otherwise wasting time with the two collections. It just does pointer copies of internal collection state. To see how this is useful, consider these two cases I’ve applied this to:
    • Maps within maps. In one case, a Mace programmer had a map from an int to a vector. In this case, the int represented the number of things in the vector (admittedly, this is a bit of a simplification). So, when removing something from the vector, you would remove the vector from the map, then re-add it with the new key. (This is because for other good reasons, the key of a map entry cannot be changed). Because of the cost of this removal and re-addition, the programmer had originally implemented this using pointers, which, while correct and efficient, gave some of our other tools problems, and so we wanted to re-write it without using pointers. swap() made this possible. To do the update, use this code:

      void removeElement(IntVectorMap& ivmap, int size) {
      IntVectorMap::iterator i = ivmap.find(size);
      assert(i != ivmap.end());

      This does involve a construction of a new collection, but moves the elements of the collection quite efficiently. Since we will erase the original map entry, causing the old vector to cease to exist, the fact that it now holds no elements does not matter.
    • The second case was one where we wanted to iterate through a set, but other code might be adding things to the set at the same time. To maintain code safety, we must not lose newly added things, so we can process them later. The original design involved making a copy of the set, then clearing the original set, and iterating over the copy. Once again, swap() is the right tool here too.

      void processSet(IntSet& s) {
      IntSet t;
      for(IntSet::iterator i = t.begin(); ...) {
      if (!s.empty()) { processSet(s); }

      As an added bonus, if you need to hold a lock to touch S, you can simply acquire the lock and do the switch, a very fast operation, the release the lock.
  2. boost::concept_check. We were updating our serialization code, but found that the compiler error messages on template errors are hard-to-decipher (to be generous). These errors were caused by one of two problems in one of our cases. First, we had added a new template parameter, and inserted it before some existing ones. In code which hadn’t been updated though, if they provided the older optional template parameters, the compiler would get very confused, and report error messages which could not be deciphered, and pointed to lines of code which didn’t make any sense. In the other error case, the default template parameter might not work with other types passed. (Specifically, it was a parameter telling how to serialize a collection, and the collection elements might not have been serializable.) This message was a little easier to decipher, complaining about types which could not be serialized, but still didn’t point to the right lines of code.

    Using boost’s concept check, we were able to help both of these problems. In the first case, we wrote a base class for all valid parameters of the template, then used a concept check to make sure the template parameter was convertible to the base class. Passing in the older parameter now would generate a shorter, easier to understand message, and the concept check library makes sure that the line of code makes sense. In the second case, we had to write our own concept checker, which would essentially just write code that needed to be able to compile (in this case, instantiating the type, serializing it, and deserializing it). Again, the concept_check library would make sure the error message was pointed to in the right place.

That’s all for this edition. Watch the programming category if you want to see other programming tips.

Server Cookies, and I don't think they quite understand advertising…

I should start by explaining I regularly run my web browser with cookies disabled. The reason is that I decided websites are tracking you too closely, and especially websites which you didn’t even know you were visiting. For example, open up your cookie list. (In firefox, this is: Tools->Options (under Windows, Edit->Preferences under Linux), then Privacy->”Show Cookies”. The questions to ask yourself are:

  1. How many of the sites listed do I even recognize?
  2. Of the sites I do recognize, what do I want that site to remember about me the next time I visit?

Cookies, you see, are files that a server gives to a web browser, and asks it to present them whenever they visit a set of pages on a set of sites. Cookies have a number of legitimate uses, most notably to give the browser a “session” id. The “session” id is used so the browser user can, e.g., log in, and have the server remember keep track of information related to the login. (The other option, not using cookies, is to make the sessionid part of the URLs, which is both ugly, and more likely to be logged by third parties such as proxies and caches run by ISPs)

Then there are some arguably useful features of cookies. For example, many online retailers will set a cookie identifying you at your browser, and recognize you immediately when you visit again (not for purchasing, but for welcoming, tracking the products you look at, so to remind you of past products you’ve visited and to suggest new products based on your viewing history. I personally find that a little creepy, though I admit in some cases it can be valuable. A few years ago, there were even reports of sites using cookies to do Dynamic Pricing (story by CNN), a practice where sites change the prices based on information they keep about the customer. There were reports of users visiting Amazon from a new computer, finding an item they like, then logging in, and seeing it for a new price. In my opinion, these types of things outweigh the possible positive benefits from having a site remember me just for cause.

Next there are in my book some outright despicable practices. Advertisements placed on sites will add cookies which get reported back to these tracking sites anytime you visit any site with an advertisement from the same company. As a result, there are sites which simply compile vast amounts of information about where you go and what you do online, to use in any way they seem fit. These are commonly called “Tracking Cookies” by products such as Ad-aware and Spybot, which will remove the ones they recognize for you.

I have simply taken the approach (mostly as an experiment) that sites shall not store cookies without my express consent. To that end, I have installed CookieSafe, which makes it easier to manage cookie settings. I either give or reject cookies from specific sites. This occurs as a site preference, meaning if a site uses both kinds of cookies, and I want to use the site, I accept them both. Importantly, the third-party cookies are still rejected — I have to authorize them separately.

So my browsing works like this: I browse normally, then if a site isn’t working (and particularly if submitting a login doesn’t work), I realize it needed cookies to work. I then decide if I really want to use the site, and if I do, I enable cookies for that site only.

Now, when I view my list of cookies, I can identify most of the sites. (Some I must have authorized, but don’t quite recognize by site name, like the third party my bank uses to process online billpay.). I find this to be much more acceptable, and my browsing hasn’t been worse for the wear.

A few days ago, however, I saw something that really brought a smile to my face. On a site I visited while trying to figure out what it meant to buy fertile eggs, I saw this image, where an ad belongs:
“No Cookie” Advertisement

I just had to laugh. If a site wants to not send me ads because I reject cookies — then great! I didn’t want them anyway. But somehow I think they’ve missed the point of advertising. If I were they, I would send SOMETHING back. But all the same – I hope other sites take this approach. It could be the end to all the annoying flash ads I get, if instead I got these images everywhere!