I’d like to address something that I’ll call the “I’ll tell you when you’re older” phenomenon in computer science education. This mostly has to do with programming, but there are also situations in theory education where this happens as well. Specifically here I’m going to discuss how it can affect the choice of a teaching language.
As is often the case with something so complex and with such a storied history, programming languages have quirks, gotchas, and many layers of operation. Therefore, it turns out to be extremely tempting when introducing an aspiring programmer to his or her first computer language to try as much as possible to avoid confusing the student by eliding material that isn’t relevant to the lesson at hand. Essentially, this means deflecting tangential questions, or anticipated questions, with the educational equivalent of the commonly experienced childhood situation where your parent promises to explain to you what some mature or risque term means “when you’re older.” In this sense, the instructor is requiring the student to wait until a later point in the course before explaining the meaning of a particular concept or construct.
In the abstract, this seems to be an advisable thing to do, but (though I certainly haven’t done formal study to back up this assertion) I suspect that it encourages the dreaded practice known as cargo-cult programming. Cargo-cult programming is the software engineering manifestation of an appeal to authority. It entails the use of patterns or code fragments when one does not really understand what they do or why they are being used, simply because one has been instructed to use them, or has seen an instructor silently do so.
It is my belief that a programmer, be he a student or a professional, should never, ever write a line of code without understanding what it does or why they wrote it. This maxim is fundamentally incompatible with the “I’ll tell you when you’re older” educational tactic because that tactic necessarily entails that the student use something without having understood it. So this tactic should be avoided whenever possible, and it should absolutely be avoided in a student’s very first lesson.
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One problem that programmers new to C++ often run into when they begin to write non-trivial programs is that of slicing.
In object-oriented programming, a subclass often holds more information than its superclass. Thus, if we assign an instance of the subclass to a variable of type superclass, there is not enough space to store the extra information, and it is sliced off.
The simple way to avoid slicing is to avoid making copies. Simply pass polymorphic objects around by reference (or by pointer) rather than by value, and one never has to worry about slicing. But sometimes the need to copy is unavoidable. In particular, when the need arises to make a copy of a derived object and all you have is a base pointer, things get hairy. How do you know which derived class the pointer’s target really belongs to, in order to make a proper, deep, non-sliced copy?
Fortunately, this problem has a well-known solution pattern called the virtual copy constructor idiom. One implements a virtual clone() method that makes a proper deep, non-sliced copy. There’s nothing wrong with this idiom, but it does entail the inelegance of having to repeat near-identical code for the clone() method in each derived class.
C++ programmers encountering this pattern for the first time often get the bright idea that this code can be centralized through the use of templates. Doing this while maintaining all the advantages of a non-templatized virtual copy constructor turns out to be much trickier than it first appears. This post will explain the problem, and how to do something that I have not yet found described on the Internet: implement the virtual copy constructor idiom using templates without sacrificing covariance.
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In Monday’s post, “Out-of-Band Error Reporting Without Exceptions”, I described a method for elegantly obtaining three out of five major advantages of C++ exceptions without actually using them. Now I’ll tell you how to go about grabbing a fourth advantage: exception type hierarchies.
C++ exceptions are hierarchical, and their hierarchy is a type hierarchy such that a more-derived exception can be treated, if desired, as one of its base exception types. For example, a FileNotFoundException might be derived from FileIOException. Some code may choose to handle FileNotFoundExceptions distinctly from other FileIOExceptions. Other code might delegate all FileIOExceptions to a common handler. In C++ this is done with implicit typecasting in the catch statements in a try block.
Wouldn’t it be useful if we could do something similar with the ErrorReport and CheckedValue classes that were defined in Monday’s post, instead of just having an error/not-an-error indicator and a human-readable message? Well, we can. It’s easy to use but a little tricky to set up.
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The newest article in my series on the Lottery Problem describes the design decisions I made when making my first attempt at an implementation of an efficient wheel generator based on the simple greedy algorithm for set cover. The design decisions described were influenced by three key insights I had when considering how one might attempt to generate a close-to-optimal wheel for a 6-from-49 lottery in a reasonable amount of time.
- Once the relationships between tickets have been completely expressed, the actual numbers that constitute the tickets are of no significance and can be discarded.
- It is only necessary to keep track of how many uncovered tickets an unselected ticket could potentially cover if selected. It is unnecessary to know exactly which tickets those are.
- Every ticket initially covers the same number of other tickets, and this number can be computed through combinatorics alone.
You can read the full article here. It primarily addresses the challenges involved in expressing the problem within a reasonable amount of memory, and gives an overview of how the computation will be approached. The computation itself will be described in detail in the next article.
Let’s say, for the sake of argument, that you’re working in C++ and you happen to be in an environment where exceptions don’t work. In my case, it was an embedded environment where the miniaturized C++ standard library didn’t support them, but in your case it may be that you can’t use them for performance reasons, or they’re against policy or something else.
As opposed to the old C-style return codes, C++ exceptions offer five major advantages:
- They are out of band (that is, a function returning an integer need not assign a special value, e.g. -1, to mean “error”).
- They can carry information about the specific error that occurred, as opposed to simply a code with a general meaning.
- They exist in a type hierarchy. A
FileNotFoundException may be derived from a FileIOException, and a handler for the latter can handle the former.
- If you ignore them, your program blows up. This is a good thing – an undetected error is far worse than a fatal error.
- They automatically unwind the stack to find a handler.
And these are all reasons why, unless you have a good reason not to, you should prefer using exceptions. But in the case when you can’t, it turns out that point 5 is really the only advantage that you have to do without. There’s a relatively elegant method that you can use to achive the first four advantages without using exceptions at all.
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