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Comment by gpderetta | Hacker News Reader

I (and many other C++ programmers), on the other hand, I'm always annoyed and surprised that dictionaries on other languages do not have default insert on lookup.

It is just very convenient (hence the existence of defaultdict in python).

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Insert on lookup is convenient but encourages either double-lookup patterns (contains + get) or unintentional inserts. I very much like Python's setdefault -- except for its 10-chars name.

Well, STL encourages an iterator centric approach, so:

  if(auto i = x.find(y); i!=x.end()){ 
      // use i->{first,second}
  }

Also ideally you should be able to do this:

  for(auto [key, value]: x.equal_range(y)) { 
     
  }

But infuriatingly, x.equal_range returns a pair instead of some range view so you need some additional infrastructure (either convert the result of equal range into a range or add some begin/end overloads for pair).

My C++ is a bit rusty (no puns intended), I didn't know your first syntax was possible now (another comment states it comes from C++17), that's much nicer than the alternative that pollutes the scope indeed. Having to repeat both x and i is unfortunate, but that's par for the course with C++ iterators.

The equal_range solution looks nice from a genericity perspective (look ma I do multimaps!) but is really unintuitive with regular maps imo. Unless I'm mistaken it also does not allow an else branch for when the value is not there, which is often needed.

A for-else a-la Python would actually be generally useful!

Hopefully one day we will get proper patter matching and destructing in if statements (Herb Sutter has made significant progress on this).

For-else could be useful, but I don't think Python's semantics would actually help here? Python's for-else runs when the for loop did not break (even if nonempty), but what would be needed here would be to run when the for loop was empty, which is a different condition.

cpp2 is both interesting and ambitious, I really hope it gets somewhere.

Indeed, I misremembered the semantics of for-else! I don't think I have actually used it in python :)

Yeah for-else as implemented in Python is... not actually that useful. It's the right tool to inline std::find_if and that's basically it. I think the behavior you had in mind would actually be more intuitive and more useful.

OTOH, try...else I find myself missing at times.

You need both access patterns (otherwise you do a double-lookup to insert a default value) and C++ already provides both.

Whatever is implemented by operator[] in a given language is going to be implicitly encouraged by the language, and that's what most novices will use.

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