Tuple Madness and STL in C++ 52
I have already complained about Tuple Madness with reference to Python and Ruby, and UncleBob has seen the same problem and noted it on the old blog. I have come to realize that the same problems appear in modern C++ code.
I have always looked at the STL as a way of implementing a class, but never as a replacement for “real” classes with named member values. In some ways, I think that templates have made our C++ code worse.
For instance, I need a two-member structure. Say I am going to store Name and Address. Now, those with C background would immediately think along these lines:
struct name_and_address { char name[34]; struct address address; };
struct name_and_address get_name_and_address(void);
// blah blah blah
struct name_and_address x = get_name_and_address();
printf("%s", x.name );
printf("%s", x.address.line1);
// ... and so on...
printf("%05d", x.address.zip);
Got to love those old C programmers. There is a nice, primitive struct, maybe with a nice, primitive struct embedded in it. Look at the clear names of those variables. It is even pretty efficient to copy these things. With a Plain Old Data Object, the compiler knows what you’re doing and can do block copies instead of memberwise copies. That can be very efficient, enough that you can often pass structures around by value instead of by reference. That’s kinda cool. That same efficiency also applies to plain old data classes in C++, but a lot of people don’t know it.
Sadly, a modern C++ developer does what? Yeah, he returns a pair. This is annoying because when I call my function, I have to know that the name is now called “first” and the address is called “second”. But that is only for this pair-returning function. In another pair-returning function, “first” means birthday and “second” is account number. Of course the prototype is not always in view, so I have to have more tiles in my window to check my work as I code.
Pair is named well enough for an arbitrary pair of primitive values, but frankly it makes no sense for x,y coordinates or names and address, or for the rolls of two twenty-sided dice. Wait. For the dice, it works just fine. After all, what would you call them? Dice1 and Dice2? But I digress. The point is that for most two-member structures, a two-member structure with named members is probably a better choice.
Now pair is not alone (oh look, a pun). Even when we are dealing with collections, the interface to a vector, map, or set is unlikely to be the right set of names and semantics for our problem domain objects. Maybe our property should be listed, and not push_back()-ed. Maybe we care about a policy, that:
policy.covers(theCar)
Maybe we’re not so interested in whether list x will:
x.find(z.first) != x.end()
The fascination with exposing STL containers (combined with unfortunate naming, as given here for effect) is hurting readability. It also is a very unfortunate form of primitive obsession and a violation of the Dependency Inversion Principle.
The people who work on your code with you should not have to reverse engineer your work in order to add new features or correct bugs. The code should be written so clearly that mistaken assumptions will be more obvious. They should not have to chase down typedefs and keep notes on paper to keep track of your structures. They should especially not have to look for data in obliquely named pairs of pairs (x.second.first means “price”? Or was that x.first.second? Or…).
If you write your code with a pair programming partner, you should get plenty of feedback about the readability of your code. Even if you do not work in pairs, you should read your tests when you get back from lunch or a meeting, and you should see if primitive containers, primitive member functions, and template syntax are obscuring their purpose. If your tests don’t speak clearly, then none of the other users of your class can speak clearly. Get the syntax out of their way by giving them reasonable wrapper functions.
You might think that wrapping your containers will cost you performance by adding dispatch here and there, but you might be surprised to find out that not wrapping it may cost you far more by concealing bugs and obscuring functionality than it ever save you in performance. A better policy is to keep your code obvious and simple, and try a little profiling and optimization where it really matters.
Of course, if one was worried about the cost of one dispatch for a named method v. an exposed container, there is always the option to build an inline member function in the header.
I’m mindful that this is not the time for me to rant about C++ programmers who never profile, and who manage performance by rule of thumb and old wives’ tales. There are more of them than us, my dear reader, and I fear their wrath. Maybe some day later, when I’m feeling brave.
My roots run pretty deep into C++, and I appreciate how hard it is to produce a system. I just want the guys who are now running with the ball right now to realize that it is far more valuable to be clear and obvious than they appreciate, and it is far less expensive to do so than they fear.
I guess the points are:
- Don’t be afraid to write small classes and structs.
- Don’t be afraid to wrap your containers.
- Don’t be afraid to use abstraction.
- Everything does not have to be written in templates.
- Be afraid to write test code that doesn’t tell a story.
- Be afraid to give too much access to your data by exposing containers.
Adjust your values accordingly.
Tuple Madness is probably the biggest reason I haven’t jumped onto the Ruby bandwagon yet.
Much of the Ruby code that I’ve come across (mostly through contracted projects that I later have to maintain) is riddled with very non-expressive code.
While this is annoying in statically typed languages, the language, being statically type, still gives me information about the object.
Although recently, this contracting company was ‘forced’ to write some Java code for us and got around the whole statically typed thing by simply using vectors, hashtables and lists for all of their objects. Ugh.
> Sadly, a modern C++ developer does what? Yeah, he returns a pair.
Some programmers have always found ways to churn out poorly named, leaky abstractions, even without templates.
On the other hand, exposing an STL-like interface (even if only begin & end) can be useful for container-like types, because it is a lingua franca and follows the really powerful part of the STL, the concepts behind the generic components.
With language support for concepts coming in the next C++ standard you’ll be able to provide a (compile-time) mapping for struct {string name; Address address;} so that it can be used in a template expecting a pair<string,Address> with members first and second. That will allow you to use generic templates without having to conform to specific names, so there’ll be even less excuse for tuple madness.
The thing is, there are advantages to std::pair (and tr1::tuple) over structs/classes; namely, the comparison operators are already defined for it (making it easy to store as keys in an ordered container, for example).
boost::tuple also has a streaming operator already defined for it. Along those lines, a particular environment may have other extended functionality (via free functions) already defined for these generic types. It isn’t hard to make a pair or a tuple generically serializable, for instance.
So we get readable names at the cost of more user code to maintain. Is that worth it? It isn’t always a clear cut ‘yes’, at least to me.
it’s funny cuz I had also that debate with my coworkers today. They gathered with my boss to oppose me because i was using pair instead of struct. sayin it is less clear blah blah one even said that he thought of second as time (seconds)
I don’t undestand you people why polluting your code with uncountable structs for the sake of readability… I mean you can put some comments to explain what actually first and second mean. is it so hard typedef string Name; typedef string FirstName; std::pair std::pair
...
I thought it was pretty clear that the issue is that primitive obsession, especially paired with poor naming, means that the next developer to come along has to reverse-engineer your code to find out what it is returning.
The whole point of having function names and variable names is we should not have to reverse-engineer functions to find out what they’re returning and why. It should be obvious.
The pair bit is bad enough, but it’s not as bad as the tuple madness I’ve seen in python and ruby. When you get a list of n-tuples (where the extent n is only apparent in the code you’re calling), you end up having to reverse-engineer a lot of code before you have any idea what you’re working with—or why these transformations are taking place.
Clearly “x1” or “x.first” is far less obvious than “address.city”. That other programmers can look it up in a comment is not enough. That other programmers can study the function to learn what you put in the pair is not enough.
The goal here is to keep code devilishly simple. Sometimes that means having more classes and shorter functions, and better names. Hence, I am with your coworkers on this. Clear is a kind of “good” that matters.
Typedef is better than using unadorned primitives, but quite often a structure or class is better yet.
The arguments that you lose built-in behavior when making a custom struct has some merit. Why not use inheritance and getters:
struct NameAndAddress :: pair<string, Address> { string& name() { return first; } Address& address() { return second; } };The correct answer is for the language to have some compile-time reflection capability so that you can “mix-in” the functionality you don’t want to rewrite, such as outputting to a stream, lexicographic comparison, etc.
I agree completely with this article; although it is unpleasant to write boiler-plate code, it is more clear.
The advantage of tuples if obviously the ability to create generic code, and for meta-programming… despite some hangups (e.g. index names) the resulting decrease in code size will actually increase readability.
It is also quite common to use enums to name the indexes into the tuple, giving the best of both worlds.
E.g.
tuple row; enum { column, data }; row[data] = “blah”;
etc.
The advantage of tuples if obviously the ability to create generic code, and for meta-programming… despite some hangups (e.g. index names) the resulting decrease in code size will actually increase readability.
It is also quite common to use enums to name the indexes into the tuple, giving the best of both worlds.
E.g.
tuple row; enum { column, data }; row[data] = “blah”;
etc.
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How about something like: void get_name_and_address (std::string &name, address &addr); No need for structs or pairs at all. =)
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I know there are several GUI libraries out there, with Qt being one of the better, but I’m just very reluctant to having a company owning the API.
My idea is that with all years of experience, plus C++0x, a new GUI API could be very clean and intuitive. It’s also important to emphasize that Boost does not need to provide any implementation; just the API. Then several different vendors could profit on selling full implementations, as long as I can swap out one implementation for another just changing a library file. Apartments for rent Bucharest
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I think pairs and tuples can be usefull some times.
Today, I’ve wrote a function with the following signature: std::pair<Vector, Traffic> generate_requests(const Graph& graph);
This function used to return just a Vector with the weights of the vertices after the generated requests passed through it. But now, I need the information about the generated traffic too. Thus, the function have to return two different things.
It will not make sense to group both informations in a struct. What name will I give to this struct? They are not two parts of the same thing, they are two different things.
In the documentation of the function, I’ve wrote something like “This function return two diffent things: a Vector with the weights of the vertices and the information about the generated traffic”. And in the calling code I’ve put:
Vector weights;
Traffic requests;
boost::tie(weights, requests) = generate_requests(graph);
I agree with the article in most parts. But, I think we cannot stipulate a rule like “tuples are never usefull, never”.
The goal here is to keep code devilishly simple. Sometimes that means having more classes and shorter functions, and better names. Hence, I am with your coworkers on this. Clear is a kind of “good” that matters.
The goal here is to keep code devilishly simple. Sometimes that means having more classes and shorter functions, and better names. Hence, I am with your coworkers on this. Clear is a kind of “good” that matters.
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