13 Jun 2006
It’s high time for the next installment in my irregular series of C++ rants. This time I’m going to talk about expressiveness (or rather, C++’s lack thereof)
Expressiveness indicates a capability to accurately and briefly express something. A human being is considered to be expressive if they are able to speak in such a way that someone listening to them can quickly understand the person’s message. A computer language’s expressiveness is a measure of how quickly a programmer can write code that is unambiguous and a correct reflection of the design in the programmer’s mind.
Let’s illustrate this with an example. Imagine we’ve got a class with an instance variable containing an array (or list, or vector, or what have you) of some other type. We want to give our class a method that takes another array, iterates through all the objects in it, and adds them to its own array. Obviously this is a contrived example, since nearly all container classes in all languages must include functionality for adding the contents of one array to another, but the point here is to demonstrate how a simple iteration is done.
Let’s start off with the C++ version, something like this:
class ValueContainer
{
// a vector containing Value objects
std::vector<boost::shared_ptr<Value> > m_values;
void addValues(std::vector<boost::shared_ptr<Value> >values)
{
// an iterator pointing at the first value
std::vector<boost::shared_ptr<Value> >::const_iterator it = values.begin();
// an iterator pointing at the special “end” value indicating we’ve reached the end
std::vector<boost::shared_ptr<Value> >::const_iterator endIt = values.end();
// step through the entire array
for (; it != endIt; ++it)
{
// add the value to our array
m_values.push_back(*it);
}
}
};
Some of you may be unfamiliar with the
boost::shared_ptr stuff in there. The boost project is a sort of proving ground for classes that are probably destined for the standard c++ library in the future, and shared pointers are a very valuable concept that make c++ development a little more do-able, by helping you manage object ownership.
Anyway, that’s quite a pile of code, isn’t it? All we’re doing is walking through a list of things!
Some may balk at this, saying that the boost shared pointers are overkill here. OK, fine, let’s take those away, and take away the comments, and stuff all we can into the
for, getting this down as tight as we can:
class ValueContainer
{
std::vector<Value> m_values;
void addValues(std::vector<Value> values)
{
for (std::vector<Value >::const_iterator it = values.begin(); it != values.end(); ++it)
{
m_values.push_back(*it);
}
}
};
Now let’s see how a similar class might look in python:
class ValueContainer:
def __init__(self):
self.values = []
def addValues(self, values):
for v in values:
self.values.append(v)
Unlike C++, Python has no free-standing declarations of instance variables—they’re defined only as they’re referenced inside of methods, and the best practice is typically to provide an initial definition inside the initilializer method
__init__ as I’ve done here.
There are several nice things about Python that this highlights:
- the syntactic indicator that we’re dealing with an instance variable by always prepending
self.
- the more sensible method names in Python’s array class compared to C++’s std::vector class (
append vs push_back)
- the
values parameter isn’t statically typed, so it doesn’t need to actually be a standard python array, but can be any class that implements the methods required for iteration
but the big win I want to point out here is really the difference in the for constructs.
Look at the mass of ugliness in the C++
for construct. To start off with, you’re forced to declare a variable with a bizarre
iterator type. Why should I have to care about that? I’m not using it in any way; that type is completely irrelevant to my code. Later, to iterate I have to use
it++, which may not be pointer arithmetic, but at least
looks like pointer arithmetic, and that is just
so 1985.
Then, take a look at the simplicity of the Python
for construct. The designers of Python built iteration right into the language! You simply give
for the name of the variable that should be used to hold a value from the array, and the array itself, and off it goes. Behind the scenes, iteration occurs through well-defined methods, and if you’re making your own collection class you can define your own iteration scheme, but all of that is completely invisible to a user of the class.
IMHO, this sort of thing is really one of the biggest wins for languages like Python. You get to leave out all sorts of meaningless busy-work, since the language includes useful mechanisms that C-based languages don’t, and you are left with the job of programming the actual functionality you’re after.
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02 May 2006
This has already been linked-to all over the place, but anyone who already hasn’t should go check out Steven Colbert’s speech (especially parts one and two; I thought part three kind of fell flat) from the White House Corresponts’ Dinner held last weekend. It takes some serious balls to stand there, just a few feet away from ol’ monkey-boy, and lower him a notch or ten like that.
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11 Apr 2006
I’m working on this C++ project, and suddenly realize how much we’re using the visitor pattern. The point of the visitor pattern is to extract non-core functionality out of data-bearing classes, instead gathering that functionality into a visitor class that knows how to operate on all the data-bearing classes. You do this by making each “visitable” class implement a method that accepts a visitor object, which in turn calls a method in the visitor to do its work, along these lines:
void MyClass::accept(Visitor &v)
{
v.visitMyClass(this);
}
That way, new visitors can be created to implement new functionality involving visitable classes, without the visitable classes needing to be changed or even recompiled. Here’s where the fun kicks in. For each new “visitable” class I add, I need to add a member function for each existing visitor, which means editing dozens of .h and .cpp files, adding a small “visitXXX” member function to each.
The kicker is that most of the functionality we’re implementing with visitors consists of either trivial one-liners or empty implementations for most of the visitable types. So we have visitor classes containing dozens of identical member functions. Nothing quite like the joy of editing dozens of files to insert meaningless boilerplate code!
At first I wondered what my co-workers were thinking when they set out on this path, but eventually I came to realize, much to my chagrin, that this is an inescapable reality of C++ programming in many situtations, thanks to the compile-time binding that C++ does. The compile-time binding actually precludes you from doing certain things without this sort of indirection.
To see why this is, first consider an example that demonstrates how things
can work in a more full-featured OO language, such as python. Consider the following python program:
class A:
def emit(self):
print "Class A"
class B (A):
def emit(self):
print “Class B”
a = A()
b = B()
a.emit()
b.emit()
print “-—”
objects = [a, b]
objects.emit()
objects.emit()
This code snippet defines and manipulates a simple class hierarchy, with B inheriting from A. Common sense tells you what the output of this program may be:
Class A
Class B
-—
Class A
Class B
And your common sense would be right. Whe calling the
emit method on the
a and
b objects, the correct method is triggered and the corresponding class names are output.
Now then, considering the following C++ program:
#include <iostream>
class A
{
public:
virtual void emit() {
std::cout << "Class A\n";
}
};
class B : public A
{
public:
virtual void emit() {
std::cout << "Class B\n";
}
};
int main (int argc, char * const argv[]) {
A a;
B b;
a.emit();
b.emit();
std::cout << "-----\n";
A objects[] = {a, b};
objects[0].emit();
objects[1].emit();
return 0;
}
Does your common sense tell you this program should have the same output as the earlier python version? Think again:
Class A
Class B
-—
Class A
Class A
Say what? “But, but, but, I put an instance of
B in that array, wtf?” Well, here’s where C++’s compile-time binding came and shot you in the foot. Note that C++ doesn’t offer any sort of generic array of objects. Whether using a C array as I’ve done, or a std::vector object, you must indicate the type of the object, in this case the root of our hierarchy,
A. C++ then makes use of this type information when your program is compiled to match up the call to
emit with the implementation in the named class, rather than the implementation in the derived class you’re actually instantiating! Doh!
This has dire consequences for any situation where you’ve got a collection of objects of varying types. Say you’ve got a bunch of model classes, and you’d like each class to emit an xml string containing its values. In Objective-C, Python, Ruby, Smalltalk, and probably even Perl and Javascript, this is pretty straightforward; You just give each class a method that emits a string (and, assuming you’ve got an acyclic directed graph, calls the emitting method in each of its “child” objects). Then you just call the emitting method in the root object, and away you go. For example, once again consider some python:
class RootElement:
def emitXml(self):
print "<root>"
for child in self.children:
child.emitXml()
print "</root>"
class SomethingElement:
def emitXml(self):
print '<something value="', self.value, '"/>'
class SomeoneElement:
def emitXml(self):
print '<someone value="', self.value, '"/>'
r = RootElement()
c1 = SomethingElement()
c2 = SomeoneElement()
c1.value = “first child”
c2.value = “second child”
r.children = [c1, c2]
r.emitXml()
(Note that this code breaks encapsulation quite a bit in setting attributes on the objects, and is not what I’d do in production code. But python lets you do it, and it makes examples short and sweet, so there you have it.)
Now then, that code outputs about what you might expect:
<root>
<something value=" first child "/>
<someone value=" second child "/>
Simple, right? Well, if you try to do something similar in C++ (or Java for that matter), you’ll run into trouble. You can start by giving your element classes a common parent declaring
emitXml (don’t need to bother with such a thing in python, where method calls aren’t bound to implementations until runtime), but as soon as you have something like RootElement maintaining a list of child elements, you’ll hit the same snag in our earlier example; the relevant implementations of
emitXml will be passed over in favor of the generic implementation in the parent class.
Which is where the visitor pattern comes in. It’s basically a kludge to work around the compile-time binding limitation of C++. Even when there are times you’d
want to use it in order to extract responsibilities out of a data-bearing class, you still end up with this ridiculous double-dispatch syntax (compare with Objective-C, where you can actually add methods to an existing class by using a category, so you can accomplish the same design feat of grouping functionally-related methods together, yet outside the classes they operate on, without the artifices of the visitor pattern).
The moral of this story is that the visitor pattern is a necessary evil for statically-bound languages like C++. And that if you want to avoid this evil thing, start by avoiding C++. Really, pick up python or ruby or Objective-C or whatever. Really!
[update] fixed some typos, thanks weinholt
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02 Apr 2006
I’ve been thinking about this for a while, been meaning to blog about it, and some recent conversations have brought it to the forefront of my mind. Here goes.
Surely no one has missed the fact that the music industry has gone through some changes in the past few years. Things like the iTunes music store have dragged the record labels, kicking and screaming, into the 21st century. Most of them are onboard by now, but there’s still an undertone of terror visible in their actions; More and more of the CDs they sell have been whammied so they sometimes
won’t work on a computer at all. Or, if they do work on a computer, they may install
deadly evil mojo on your machine in the process.
The reason for these paranoid practices on the part of the record labels is pretty easy to suss out: For decades, they’ve built their businesses around selling bits of plastic or vinyl. They’ve taken for granted that they’ll be able to continue doing this forever, and that this will always be their most important source of revenue.
So naturally they’re worried about mp3 and similar technologies robbing them of some of their profits. I think it may be instructive to consider what happened around the birth of the recording industry. At that time, many live performers became concerned for their livelihoods, worrying that if people could hear music on demand at any time, they’d no longer bother going to performances, and these live performers would be out of work. Of course, this turned out not to be the case; If anything, recorded music has served to propel any number of performers to a level of fame and fortune that would have been impossible without recordings. I believe that the music industry’s concerns about losing revenues due to illegal digital distribution are similarly unfounded.
For one thing, there is a notion that the people who download lots of music illegally are probably the people who also buy some of the largest amounts of legitimate copies of music. Downloading doesn’t replace their music-purchasing, but rather enhances it by letting them sample music from unfamiliar artists (potentially leading to future purchases) or downloading unavailable-for-purchase bootleg recordings of familiar artists (which increases the “bonding” between a listener and a favorite artist even more).
Perhaps more importantly, the music industry is sitting on the keys to unlocking its own future potential. For instance: Propelling public awareness of artists through cross-promotion. Let’s say you’re a big Nine Inch Nails fan, having heard their music on the radio, maybe gone to a show or two, etc. At some point you point your browser at
http://www.nin.com, look around a bit at some lyrics and whatnot, and then you end up backtracking and surfing on somewhere else; Your interest in
NIN didn’t lead to any info about anything non-
NIN.
How could this be improved? Well,
NIN is currently on Interscope. What if Interscope stipulated that all of its artists needed to provide linkage (in the form of a side-bar, or a dismissable slide-over graphic) to other, similar Interscope artists? Then you might find out about
Helmet or
Audioslave or, perhaps, some up-and-coming band that you’ve really never heard of. Some would complain that Interscope infringing on their artists websites would be a terrible, terrible thing, but considering all the
other shit they pull on the artists in their stables, this would be pretty minor.
A bigger problem is that relationships between artists come and go. As of this writing, for instance, the above-mentioned
Helmet is still listed on Interscopes
Artists page, despite the fact that one of the latest entries on Helmet’s page mentions that they’ve left Interscope. Oops! So Interscope’s (hypothetical) forced linkage from
NIN to Helmet now isn’t helping Interscope any more. But… what if it wasn’t just Interscope? What if the industry as a whole were presenting interesting ways for listeners to find similar artists? That way Interscope would still benefit from a
NIN<->Helmet linkage since in the future, visitors to Helmet’s site would still get back to
NIN.
Apart from just a plain old list of links between similar bands, things could be user-tailored so that sites would bring to the user’s attention news about upcoming local concerts with relevant artists, eventually including (gasp) unsigned artists as well (which the industry as a whole benefits from promoting since one day, presumably, any decent unsigned artist will be under contract with one of the labels).
Now all someone has to do is convince the music industry that I’m right, and implement a solution for them, giving them an offer that’s so good they can’t say no.
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20 Mar 2006
Since back in the 80’s, this funky Welsh dude (complete with an unhealthy fascination with farmyard animals) named Jeff Minter (aka yak) has produced videogames and software toys for the platform du jour, starting with the old Commodore Vic-20 and continuing right up to the current Xbox 360. The hallmark of all his games has been extremely trippy visuals, moreso over time…
Now, one of his latest works, gridrunner++, has been
ported to J2ME, which means that you (yes,
you!) can run it on your cellphone! OK, not just
any cellphone, but most of the middling-or-better European-market phones from the past couple years (and probably some of their American counterparts). I bought a copy
here, and you should do the same. Even on the tiny screen, this thing is a dazzling display of exploding, twisting, pixel-shattery goodness. You owe it to yourself, I’m sure.
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