6 ms·

Iterators: Signs of Weakness in Object-Oriented Languages

29 points by pcof 16y ago

deleted 16y ago
[deleted]
scott_s 16y ago
While C++0x will have proper lamdbas and closures (I've said this how many times in the past few weeks?), I think it's worth noting the "proper" way to emulate a closure in C++ is with a function object. Example: struct Summation { int& sum; Summation(int& s): sum(s) {} void operator()(int n) { sum += n; } }; vector<int> numbers; int sum = 0; // give numbers some interesting values for_each(numbers.begin(), numbers.end(), Summation(sum)); cout << "sum of all numbers: " << sum << endl; C++0x lambdas will generate code very similar to this. The need to explicitly define a named struct/class and function object is an obvious weakness of this technique. Although I do agree with his overall point, which is that the need to use iterators for all sequences is a code smell. It forces the programmer to work at a lower level when they often just want to say "Perform this operation on my data."
- lincolnq 16y ago
  Interesting. Your comment about C++0x closures inspired me to look it up on Wikipedia. The verbosity of the fragment above is greatly reduced. You had to write 'sum' or 's' lots of extra times ('Summation(sum)', 'sum(s)', 'int& sum', and 'int& s'), and you won't have to do that anymore -- according to wikipedia your function will change in C++0x to: for_each( numbers.begin(), numbers.end(), [&sum](int n) { sum += n; } ); Which is semantically much less to think about. It also still has the feature / annoyance (?) of having to specifically identify which variables to capture from the environment ('sum', as a reference; it seems you can capture variables by value too). Fortunately, it seems the C++ people even thought of that annoyance, and allow you to write: for_each( numbers.begin(), numbers.end(), [&](int n) { sum += n; } ); capturing references to all variables in the enclosing scope. Neat!
  - scott_s 16y ago
    Indeed. I actually use this technique in my own code, and I feel like I'm laying down pipe all over the place to make sure the captured variable gets routed to the function object correctly. Once you do it a few times, it is obviously boiler-plate code and obviously a hindrance to momentum.
  - Figs 16y ago
    Of course, C++ already has std::accumulate, which is basically the same thing as a general fold or a summation, depending on which version you use. So, there isn't really much of a reason to write sum manually at all. :p http://www.sgi.com/tech/stl/accumulate.html http://www.sgi.com/tech/stl/accumulate.html
aaronbrethorst 16y ago
Something about the writing style in the Introduction section reminds me of this classic Dilbert strip: http://tomayko.com/writings/that-dilbert-cartoon http://tomayko.com/writings/that-dilbert-cartoon
pcof 16y ago
It's worth reading the Abstract Heresies' entry that turned my attention to this paper: http://funcall.blogspot.com/2010/05/c-vs-lisp.html http://funcall.blogspot.com/2010/05/c-vs-lisp.html (After giving it a thought I posted this entry to HN too, as I probably should have done in the first place... :)) The date is interesting, 1992 - it would still take years for Java to implement it...wrong.
huhtenberg 16y ago
PDF version - http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=D0759C8E59B0FC3EF23C2E6ED17A88F3?doi=10.1.1.25.1018&rep=rep1&type=pdf http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=D07...
malbs 16y ago
Conveniently ignored Smalltalk, which at the time, would have been seeing a lot more use than it does now. I missed the date of the article at first, it wasn't until I looked at the C code and had to double take it, because it wasn't ANSI C, and my initial reaction was "hang on wait a minute" then checked the date on the article.... yeah.
ct4ul4u 16y ago
It seems these are lessons we must perpetually relearn.
util 16y ago
It seems odd that he complains about lack of local (modifiable) state while at the same time talking up the advantages of pure functional programming. It's painful to have to explicitly capture state instead of depending on a closure to do it, but you can do it, and somewhat nicely using objects in C++. Also, is it not right that iterators were introduced in C++ largely to allow generic functions across arrays and other data structures? Finally, do iterators necessarily imply statefulness? Doesn't seem like it. Taking + 1 as "successor": double sum(const double* start, const double* end) { if (start == end) { return 0.0; } else { return *start + sum(start + 1, end); } }

29 points by pcof 16y ago

deleted 16y ago

[deleted]

While C++0x will have proper lamdbas and closures (I've said this how many times in the past few weeks?), I think it's worth noting the "proper" way to emulate a closure in C++ is with a function object. Example: struct Summation { int& sum; Summation(int& s): sum(s) {} void operator()(int n) { sum += n; } }; vector<int> numbers; int sum = 0; // give numbers some interesting values for_each(numbers.begin(), numbers.end(), Summation(sum)); cout << "sum of all numbers: " << sum << endl; C++0x lambdas will generate code very similar to this. The need to explicitly define a named struct/class and function object is an obvious weakness of this technique. Although I do agree with his overall point, which is that the need to use iterators for all sequences is a code smell. It forces the programmer to work at a lower level when they often just want to say "Perform this operation on my data."

lincolnq 16y ago
Interesting. Your comment about C++0x closures inspired me to look it up on Wikipedia. The verbosity of the fragment above is greatly reduced. You had to write 'sum' or 's' lots of extra times ('Summation(sum)', 'sum(s)', 'int& sum', and 'int& s'), and you won't have to do that anymore -- according to wikipedia your function will change in C++0x to: for_each( numbers.begin(), numbers.end(), [&sum](int n) { sum += n; } ); Which is semantically much less to think about. It also still has the feature / annoyance (?) of having to specifically identify which variables to capture from the environment ('sum', as a reference; it seems you can capture variables by value too). Fortunately, it seems the C++ people even thought of that annoyance, and allow you to write: for_each( numbers.begin(), numbers.end(), [&](int n) { sum += n; } ); capturing references to all variables in the enclosing scope. Neat!
- scott_s 16y ago
  Indeed. I actually use this technique in my own code, and I feel like I'm laying down pipe all over the place to make sure the captured variable gets routed to the function object correctly. Once you do it a few times, it is obviously boiler-plate code and obviously a hindrance to momentum.
- Figs 16y ago
  Of course, C++ already has std::accumulate, which is basically the same thing as a general fold or a summation, depending on which version you use. So, there isn't really much of a reason to write sum manually at all. :p http://www.sgi.com/tech/stl/accumulate.html http://www.sgi.com/tech/stl/accumulate.html

aaronbrethorst 16y ago

Something about the writing style in the Introduction section reminds me of this classic Dilbert strip: http://tomayko.com/writings/that-dilbert-cartoon http://tomayko.com/writings/that-dilbert-cartoon

pcof 16y ago

It's worth reading the Abstract Heresies' entry that turned my attention to this paper: http://funcall.blogspot.com/2010/05/c-vs-lisp.html http://funcall.blogspot.com/2010/05/c-vs-lisp.html (After giving it a thought I posted this entry to HN too, as I probably should have done in the first place... :)) The date is interesting, 1992 - it would still take years for Java to implement it...wrong.

huhtenberg 16y ago

PDF version - http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=D0759C8E59B0FC3EF23C2E6ED17A88F3?doi=10.1.1.25.1018&rep=rep1&type=pdf http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=D07...

malbs 16y ago

Conveniently ignored Smalltalk, which at the time, would have been seeing a lot more use than it does now. I missed the date of the article at first, it wasn't until I looked at the C code and had to double take it, because it wasn't ANSI C, and my initial reaction was "hang on wait a minute" then checked the date on the article.... yeah.

ct4ul4u 16y ago

It seems these are lessons we must perpetually relearn.

util 16y ago

It seems odd that he complains about lack of local (modifiable) state while at the same time talking up the advantages of pure functional programming. It's painful to have to explicitly capture state instead of depending on a closure to do it, but you can do it, and somewhat nicely using objects in C++. Also, is it not right that iterators were introduced in C++ largely to allow generic functions across arrays and other data structures? Finally, do iterators necessarily imply statefulness? Doesn't seem like it. Taking + 1 as "successor": double sum(const double* start, const double* end) { if (start == end) { return 0.0; } else { return *start + sum(start + 1, end); } }