design by contract

Why we need try…finally, not just RAII

The claim has been made that because C++ supports RAII (resource acquisition is initialization), it doesn’t need try…finally. I think this is wrong. There is at least one use case for try…finally that’s very awkward to model with RAII, and one that’s so ugly it ought to be outlawed.

The awkward use case

What if what you want to do during a …finally block has nothing to do with freeing resources? For example, suppose you’re writing the next version of Guitar Hero, and you want to guarantee that when your avatar leaves the stage, the last thing she does is take a bow–even if the player interrupts the performance or an error occurs.

…finally, take a bow. Photo credit: gavinzac (Flickr)

Of course you can ensure this behavior with an RAII pattern, but it’s silly and artificial. Which of the following two snippets is cleaner and better expresses intent? Continue reading →

On bread recipes, maps, and intentions

[I’ve been quiet for the past three weeks–not because I have less that I want to talk about, but because I have more. Major wheels turning in my head. I’m having a hard time getting from the “intuited ideas” mode to the “crisp enough to put it in writing” mode, though. Consider this a down payment on some future discussions…]

One of my mother’s talents is bread-making. She’s been kneading and baking and pulling beautiful loaves out of the oven for as long as I can remember. Bread is one of the ways she says “I love you” to family and friends.

photo credit: treehouse1977 (Flickr)

A few years back, she created a cookbook full of family recipes, and gave one to each adult child for Christmas. I was struck by how she began the bread section. Instead of launching right into the recipes, she included a couple of pages of “bread theory”, if you will. The section about water is typical:

“Water — Just about any edible liquid could be used as the base for bread. Some that come to mind are vegetable cooking water, potato water, milk, and so on. There is no problem with substituting any of these for liquid called for in a recipe, but you should keep in mind that if the liquid is salty, the salt should be adjusted; if the liquid is sweet, the sugar should be adjusted… Fresh milk can be a problem because of enzymes that would prevent yeast action. For this reason, most old recipes that call for milk specify that the milk be scalded first. This isn’t necessary if you are using water and powdered milk, but remember that the mechanics of the recipe probably depend on at least warm milk (so use warm or even hot water).”

If you’re wondering why I am writing about bread recipes in this blog that focuses on software craftsmanship, consider how much that paragraph resembles a really high-value comment in source code.

It has to do with principles and intentions.

Software is all about recipes, right?

Recipes are a lot like software algorithms (especially in imperative programming styles): First, do this; next, do that; wait 25 minutes; return new Loaf()… We even talk about “recipes” and “cookbooks” when we make catalogs of software techniques.

How is this metaphor instructive… or worrisome?

Continue reading →

Headers, babies, and bathwater

I claim that by eliminating the C/C++-style dichotomy between headers and implementation, most modern programming languages have thrown out the baby with the bathwater.

Don't throw out the baby with the bathwater! Photo credit: StubbyFingers (Flickr)

Don’t throw out the baby with the bathwater! Photo credit: StubbyFingers (Flickr)

If that sounds crazy, just hang with me for a minute.

I know my claim runs counter to popular wisdom; have a look at this thread on stackoverflow.com. Designers of languages like python and go and D and ruby and java consider it a feature that developers don’t have two redundant pictures of the same functionality. This comment from the C# 5.0 specification is typical:

“Because an assembly is a self-describing unit of functionality containing both code and metadata, there is no need for #include directives and header files in C#. The public types and members contained in a particular assembly are made available in a C# program simply by referencing that assembly when compiling the program” (p 3).

I agree.

Sort of…

Bad headers are a royal pain

It can be onerous to maintain the parallelism between a .h and a .cpp. And most C/C++ headers are managed so poorly that the benefits you might claim for them are theoretical rather than real. Three common antipatterns that I particularly detest: Continue reading →