内容简介:For those who only read above the fold: II originally wrote about this specific piece of codeIf this kind of thing appeals to you, I recommend with the highest possible recommendation that you get yourself a copy of Kernighan and Plauger’s
For those who only read above the fold: I don’t
say that all object-orientation is bad!
OOP, especially classical polymorphic OOP, has a well-deserved place in real code.
I’m going to talk about a very specific antipattern that I see with some frequency:
the use of class
for things that should be simple free functions.
I originally wrote about this specific piece of code on Code Review StackExchange (February 2018). I gave an unrecorded lightning talk based on the same material in April 2018.
If this kind of thing appeals to you, I recommend with the highest possible recommendation that you get yourself a copy of Kernighan and Plauger’s The Elements of Programming Style (1974, reissued 1978).
The original code
Students who’ve learned C++ in a particular kind of academic setting often approach
it with the “everything is an object” mindset. Suppose they’re given a task to compute
some value; then their first step is to create a ValueComputer
object with a
member function vc.computeResult()
.
For example: A student is asked to use dynamic programming to count the number of distinct domino tilings of a
rectangle. The student writes:
int main() { DominoTilingCounter tc(4, 7); // in a 4x7 rectangle std::cout << tc.count() << '\n'; }
Having framed the problem, they go on to implement the DominoTilingCounter
class.
The clever student even adds memoization, so that the count()
member function won’t be
so slow the second time it’s called:
class DominoTilingCounter { int height, width; bool done = false; int tilingCount; int computeCount(int h, int w, std::string_view prevRow, int rowIdx) { [...recursive solution omitted...] } public: explicit DominoTilingCounter(int h, int w) : height(h), width(w) { if (h == 0 || w == 0 || (w*h) % 2 != 0) { tilingCount = 0; done = true; } } int count() { if (!done) { tilingCount = computeCount(height, width, "", 0); done = true; } return tilingCount; } };
(Experienced programmers may be cringing, here. That’s the point.)
Unfortunately, this code fails theconst-correctness test:
the count()
member function sounds like it should be non-modifying, but in fact
it needs to update member data and thus cannot be const.
std::cout << DominoTilingCounter(4, 7).count() << '\n'; // Fails to compile!
We can fix this issue incidentally, just by applying a little more logic.
The logical leap
Look: when you construct a DominoTilingCounter
object tc
,
it is specifically for the purpose
of computing tc.count()
, right?
There’s no other purpose that tc
could serve?
Again: When you construct
a DominoTilingCounter
object, it is specifically
for the purpose of computing
tc.count()
.
So put the computation in the constructor!
class DominoTilingCounter { int height, width; int tilingCount; int computeCount(int h, int w, std::string_view prevRow, int rowIdx) { [...recursive solution omitted...] } public: explicit DominoTilingCounter(int h, int w) : height(h), width(w) { if (h == 0 || w == 0 || (w*h) % 2 != 0) { tilingCount = 0; } else { tilingCount = computeCount(height, width, "", 0); } } int count() const { return tilingCount; } };
This refactoring does several things:
-
It permits
count()
to be a const member function. -
It eliminates the “empty, uncomputed” state from your program.
-
It eliminates the data member
done
, whose whole purpose was to track that empty state. (In C++17 we might use astd::optional
for that purpose; but look, now we don’t have to!)
In fact, the private data members height
and width
are now unused as well. It turns out
that we were using them only to pass data from the constructor to the computation in
the count()
method; and now that the computation is taking place in the constructor, we
don’t need those data members anymore. Our code shrinks drastically.
The final leap
In the original code, the student’s computeCount
member function happened to take w
and h
as function parameters, rather than reading them from the height
and width
data members.
That was a fortunate accident: computeCount
doesn’t use any of the
data members of the DominoTilingCounter
object, and so we can mark it static
.
Our code is now something like this:
class DominoTilingCounter { int tilingCount; static int computeCount(int h, int w, std::string_view prevRow, int rowIdx) { if (h == 0 || w == 0 || (w*h) % 2 != 0) { return 0; } [...recursive solution omitted...] } public: explicit DominoTilingCounter(int h, int w) { tilingCount = computeCount(h, w, "", 0); } int count() const { return tilingCount; } };
The final leap is to observe that this entire class does nothing but wrap an assignment to an int
!
Avoid classes that are tantamount to int
.
What we should
write is a simple free function countDominoTilings(w, h)
:
int countDominoTilingsImpl(int h, int w, std::string_view prevRow, int rowIdx) { if (h == 0 || w == 0 || (w*h) % 2 != 0) { return 0; } [...recursive solution omitted...] } int countDominoTilings(int h, int w) { return countDominoTilingsImpl(h, w, "", 0); } int main() { int tc = countDominoTilings(4, 7); // in a 4x7 rectangle std::cout << tc << '\n'; }
No more class
, no more worrying about const, no more worrying about memoization (it becomes the
caller’s problem, for better or worse). Our original DominoTilingCounter
object wasn’t thread-safe,
but now we don’t have to worry about that, either. And our code is about a dozen lines shorter.
Again, this is not to say that all classes are bad! In fact, the antipattern discussed here is very close to the Builder pattern , and there’s nothing wrong with the Builder pattern — when it’s needed, that is. All I’m saying is:
When you have to compute a value, don’t write a ValueComputer
class.
Write a compute_value
function instead.
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