The Secret to Understanding C++ (and why we teach C++ the wrong way)

A little over the year ago, I asked “How on earth do normal people learn C++?” which reflected some of my frustration as I re-engaged with the language and tried to make sense of what “modern” C++ had become. Over time, I’ve found that as I’ve become more and more familiar with the language again, things have begun to make more sense. Then a couple of days ago I went to a talk by Bjarne Stroustrup (whose name, apparently, I have no hope of ever pronouncing correctly) and the secret of understanding C++ suddenly crystallized in my mind.

I have to say, I found the talk quite interesting which was a huge accomplishment because: a) I usually don’t like sitting listening to talks under the best of circumstances, and b) he was basically covering the “whys and wherefores” of C++, which is something I’m already fairly familiar with. However, in listening to the designer of C++ talk about his language, I was struck by a realization: the secret to understanding C++ is to think like the machine, not like the programmer.

You see, the fundamental mistake that most teachers make when teaching C++ to human beings is to teach C++ like they teach other programming languages. But with the exception of C, most modern programming languages are designed around hiding as many details of how things actually happen on the machine as possible. They’re designed to allow humans to explain to the computer what they want to do in a way that’s as close to the way humans think (or, at least, how engineers thing) as possible. And since C++ superficially looks like some of those languages, teachers just apply the same methodology. Hence, when you learn about classes, teachers tend to spend most of their time on the conceptual level, talking about inheritance and encapsulation and such.

But the way Bjarne talks about C++, it’s clear that everything that C++ does is designed while thinking hard about the question how will this translate to the machine level? This may be a completely obvious point for a language whose main purpose in life is a systems programming language, but I don’t think I’d ever really groked how deeply that idea is baked in to C++. And once I really looked at things that way, things make a lot more sense. Instead of teaching classes at just the conceptual level, you really need to teach classes at the implementation level for C++ to make sense.

For example, I’ve never been in a programming class that discussed how C++ classes are actually implemented at runtime using Vtables and such. Instead, I had to learn all that on my own by implementing a programming language on the Common Language Runtime. The CLR hides a lot of the nitty-gritty of implementing inheritance from the C# and VB programmer, but the language implementer has to understand it at a fairly deep level to make sure they handle cross-language interop correctly. As such, I find myself continually falling back on my CLR experience when looking at C++ features and thinking, “How is this supposed to work?” I can’t imagine how people who haven’t had to confront these kinds of implementation-level details figure it out.

It makes me wonder if a proper C++ programming course would actually work in the opposite direction of how most classes (that I’ve seen) do it. Instead of starting at the conceptual level, start at the machine level. Here is a machine: CPU, registers, memory. Here’s how the basic C++ expression map to them. Here’s how basic C++ structures map to them. Here’s how you use those to build C++ classes and inheritance. And so on. By the time you got to move semantics vs. copy semantics, people might actually understand what you’re talking about.

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2 thoughts on “The Secret to Understanding C++ (and why we teach C++ the wrong way)

  1. Bryan St. Amour

    Personally I think that the best way of understanding what C++ is really about is to pick up one of the books by Stroustrup. For example, his new *a tour of C++* is a fairly light read, but it covers the essence of what modern C++ really is about.

    One of the biggest benefits (and drawbacks) of C++ is that it contains a wealth of abstraction mechanisms, from classes and templates, to operators that can be overloaded. Don’t forget exceptions, constexpr statements, lambdas, etc etc etc. It’s easy to become overwhelmed with such a large set of tools. In *A tour of C++*, and the 4th edition of *The C++ Programming Language*, Stroustrup really goes the whole nine yard with showing the reader how these tools are meant to be used to lay out the bits and pieces of your program. C++11 in particular actually makes writing code *easier*, even though the language itself grew quite a bit.

    I don’t always program in imperative languages. But when I do, I prefer modern C++.

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