Too Busy For Words - the PaulWay Blog

Wed 1st Feb, 2012

Going from zero

A friend of mine and I were discussing cars the other day. He said that he thought the invention of the electric motor was a curse on cars, because it meant you wouldn't have a gearbox to control which gear you were in. A suitable electric motor has enough power to drive the car from zero to a comfortable top speed (110km/hr) at a reasonable acceleration using a fixed gear ratio - the car stays in (in this case third) gear and you drive it around like that. He maintained, however, that you needed to know which gear you were in, and to change gears, because otherwise you could find yourself using a gear that you hadn't chosen.

I argued that, in fact, having to select a gear meant that drivers both new and experienced would occasionally miss a gear change and put the gearbox into neutral by mistake, causing grinding of gears and possible crashes as the car was now out of control. He claimed to have heard of a clever device that would sit over your gearbox and tell you when you weren't in gear, but you couldn't use the car like that all the time because it made the car too slow. So you tested the car with this gearbox-watcher, then once you knew that the car itself wouldn't normally miss a gear you just had to blame the driver if the car blew up, crashed, or had other problems. But he was absolutely consistent in attitude towards electric motors: you lost any chance to find out that you weren't in the right gear, and therefore the whole invention could be written off as basically misguided.

Now, clever readers will have worked out that at this point my conversation was not real, and was in fact by way of an analogy (from the strain on the examples, for one). The friend was real - Rusty Russell - but instead of electric motors we were discussing the Go programming language and instead of gearboxes we were discussing the state of variables.

In Go, all variables are defined as containing zero unless initialised otherwise. In C, a variable can be declared but undefined - the language standard AFAIK does not specify the state of a variable that is declared but not initialised. From the C perspective, there are several reasons you might not want to automatically pre-initialise a variable when you define it - it's about to be set from some other structure, for example - and pre-initialising it is a waste of time. And being able to detect when a variable has been used without knowing what its stage is - using valgrind, for example - means you can detect subtle programming errors that can have hard-to-find consequences when the variable's meaning or initialisation is changed later on. If you can't know whether the programmer is using zero because that's what they really wanted or because it just happened to be the default and they didn't think about it, then how do you know which usage is correct?

From the Go perspective, in my opinion, these arguments are a kludgy way of seeing a bug as a feature. Optimising compilers can easily detect when a variable will be set twice without any intervening examination of state, and simply remove the first initialisation - so the 'waste of time' argument is a non-issue. Likewise, any self-respecting static analysis tool can determine if a variable is tested before it's explicitly defined, and I can think of a couple of heuristics for determining when this usage isn't intended.

And one of the most common errors in C is use of undefined variables; this happens to new and experienced programmers alike, and those subtle programming problems happen far more often in real-world code as it evolves over time - it is still rare for people to run valgrind over their code every time before they commit it to the project. It's far more useful to eliminate this entire category of bugs once and for all. As far as I can see, you lose nothing and you gain a lot more security.

To me, the arguments against a default value are a kind of lesser Stockholm Syndrome. C programmers learn from long experience to do things the 'right way', including making sure you initialise your variables explicitly before you use them, because of all the bugs - from brutally obvious to deviously subtle - that are caused by doing things in any other way. Tools like valgrind work around indirectly fixing this problem after the fact. People even come to love them - like the people who love being deafened by the sound of growling, blaring petrol engines and associate the feeling of power with that cacophany. They mock those new silent electric motors because they don't have the same warts and the same pain-inducing behaviour as the old petrol engine.

I'm sure C has many good things to recommend it. But I don't think lack of default initialisation is one.

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