On Sunday, March 11, 2018 08:39:54 aliak via Digitalmars-d-learn wrote: > On Saturday, 10 March 2018 at 23:00:07 UTC, Jonathan M Davis > > issue in practice. That doesn't mean that it's never a problem, > > but from what I've seen, it's very rarely a problem, and it's > > easy to work around if you run into a particular case where it > > is a problem. > > Ya, it's easy to work around but the caveat there is you need to > realize it's happening first, and add that to that it's "rarely a > problem" and well ... now it seems scary enough for this to > mentioned somewhere I'd say.
You're talking about a situation where you used a function whose parameters match that of a member function exactly enough that a member function gets called instead of a free function. That _can_ happen, but in most cases, there's going to be a mismatch, and you'll get a compiler error if the type defines a member function that matches the free function. I don't think that I have ever seen that happen or ever seen anyone complain about it. The only case I recall along those lines was someone who was trying to use a free function that they'd decided to call front instead of something else, and it had parameters beyond just the input range, so that programmer got compilation errors when they tried to use it in their range-based functions. I think that this is really a theoretical concern and not a practical one. Certainly, it's really only going to potentially be an issue in library code that gets used by a ton of folks with completely unexpected types. If it's in your own code, you're usually well aware of what types are going to be used with a generic function, and proper testing would catch the rare case where there would be a problem. If you're really worried about it, then just don't use UFCS, but for better or worse, it seems to be the case that the vast majority of D programmers use UFCS all the time and don't run into problems like this. > > The one case that I am aware of where best practice is to avoid > > UFCS is with put for output ranges, but that has nothing to > > with your concerns here. Rather, it has to do with the fact > > that std.range.primitives.put has a lot of overloads for > > handling various arguments (particularly when handling ranges > > of characters), and almost no one implements their output > > ranges with all of those overloads. So, if you use put with > > UFCS, you tend to run into problems if you do anything other > > than put a single element of the exact type at a time, whereas > > the free function handles more cases (even if they ultimately > > end up calling that member function with a single argument of > > the exact type). We probably shouldn't have had the free > > function and the member function share the same name. > > Oh, can you share a short example here maybe? Not sure I followed > completely > > Is it basically: > > // if r is output range > > r.put(a, b) // don't do this? > > put(r, a, b) // but do this? > > (Cause compilation error) Essentially yes, though you're passing too many arguments to put. There are cases where put(output, foo) will compile while output.put(foo) will not. In particular, std.range.primitives.put will accept both individual elements to be written to the output range and ranges of elements to be written, whereas typically, an output range will be written to only accept an element at a time. It's even more extreme with output ranges of characters, because the free function put will accept different string types and convert them, and even if the programmer who designed the output range added various overloads to put for completeness, it's enough extra work to deal with all of the various character types that they probably didn't. And put also works with stuff like delegates (most frequently used with a toString that accepts an output range), which don't have member functions. So, if you write your generic code to use the member function put, it's only going to work with user-defined types that define the particular overload(s) of put that you're using in your function, whereas if you use the free function, you have more variety in the types of output ranges that your code works with, and you have more ways that you can call put (e.g. passing a range of elements instead of a single element). > How about if it's not part of the overload set, but is looked up > if the function does not exist in the overload set. What would > the problems there be? > > Basically I don't see a reason why we wouldn't want the following > to work: > > struct S { void f() {} } > void f(S s, int i) {} > S().f(3); // error So, are you complaining that it's an error, or you want it to be an error? As it stands, it's an error, because as far as the compiler is concerned, you tried to call a member function with an argument that it doesn't accept. If you want that code to work, then it would have to add the free function to the overload set while somehow leaving out the overloads that matches the member function, which isn't how D deals with overloading at this point. But if it did, then you have problems as soon as the type adds another member function overload. Also, if you have a free function that matches the name of a member function but where their parameters don't match, wouldn't they be unrelated functions? At that point, if you wrote code that accidentally matched the free function instead of the member function, you end up with code hijacking. Just because you made a mistake when typing the code, you called entirely the wrong function, and it's very hard to see, because the function names match. Hopefully, testing will catch it (and there's a decent chance that it will), but essentially, the member function has been hijacked by the free function. D's overload rules were written with a strong bias towards preventing function hijacking. To an extent, that's impossible once UFCS comes into play, and Walter went with the choice that hijacked the least and was the simplest to deal with. Basically, once UFCS comes into play, you have these options: 1. Put all of the functions in the overload set. 2. The member function wins. 3. The free function wins. 4. Have a pseudo-overload set where when there's a conflict between a member function and a free function, the member function wins, but free functions that don't match can be called as well. 5. Have a pseudo-overload set where when there's a conflict between a member function and a free function, the free function wins, but member functions that don't match can be called as well. If it's ever the case that the free function wins, then you can't call the member function if the free function is available, which definitely causes problems, so #3 and #5 are out. If all of the functions are in the overload set, then you're in basically the same boat, because you can't call the member function if there's a conflict. It's just that the free function results in a compilation error as well without using an alias or the full import path or some other trick to get at the free function. So, #5 is out. That leaves #2 and #4. And as I said, aside from the fact that #4 doesn't fit with how D does overloads in general, you run the risk of the free function hijacking the member function whenever there's a mistake, and you have problems whenever the member functions are altered, making it so that which function gets called can change silently. So, that leaves #2, which is what we have. Basically, D's overload rules are designed to favor compilation errors over the risk of calling the wrong function, and while its import system provides ways to differentiate between free functions, it really doesn't provide a way to differentiate between a member function and a free function except via whether you use UFCS or not. And when those facts are taken into account, it makes the most sense for member functions to just win whenenver a free function and a member function have the same name. It also has the bonus that it reduces compilation times, because if a free function could ever trump a member function or was in any fashion included in its overload set, then the compiler would have to check all of the available functions when UFCS is used instead of looking at the member functions and then only looking at free functions if there was no member function with that name. - Jonathan M Davis