42 ms·

C++14 lambda tutorial

63 points by AlexeyBrin 12y ago

cengizkrbck 12y ago
In python, lambda usage extremly beautiful. Also, 'lambda' keyword solves readibility problem. I expected a keyword. Is there any challenge to add keyword?
- VMG 12y ago
  Backwards compatability. Old valid code should remain valid. That includes code which calculates wavelengths.
  - thelinked 12y ago
    An interesting example of C++ not being able to introduce new keywords is the co-opting of auto in C++11 from a storage class to type deduction.
    - ninkendo 12y ago
      But they had a decent justification: the auto keyword literally did nothing in C++. The only place it was valid, it was also the default.
      - danielweber 12y ago
        IIRC they audited a bunch of projects to see if anyone was using auto somewhere besides test code explicitly for the auto-keyword, and found nothing.
  - deleted 12y ago
    [deleted]
- TillE 12y ago
  Not shown in this article are all the things you can do within the [] (typically just [&] or [=]), which turns a simple lambda into a closure. The syntax is a bit arcane, but it does what it needs to. They could've stuck a "lambda" keyword in front of it all, but it's not necessary, and making a new keyword always has the potential to break old code.
  - guidopallemans 12y ago
    is it possible to do #define lambda [] ?
    - Kronopath 12y ago
      I think that's possible, but I would strangle anyone who actually did that. At that point, not only are you pointlessly redefining the language just so it can feel slightly more familiar to people who are more comfortable with Python and the like, but you also completely lose the ability to enclose variables in the square brackets to make a closure. If you're going to write C++, then write C++. Don't redefine language constructs to fit the patterns of your other favourite language.
    - leni536 12y ago
      I think you can #define begin { #define end } too. I would not suggest it though.
    - kps 12y ago
      That way lies http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/sh/mac.h http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/...
- leni536 12y ago
  I don't like that in python you are enforced to only use a single expression in lambda. Sure, writing long lambda functions is ugly but I don't think python should enforce a given coding style in the language level.
  - crazypyro 12y ago
    Isn't that literally a core principal of Python? They enforce all kind of coding styles within the language, not the mention all the PEPs.
    - leni536 12y ago
      But PEPs are not enforced in the language level. Also there are cases where there could fit two simple expression in a lambda. Also the language still doesn't stop you from abusing lambdas, you can nest them. It could be stated in one of the PEPs that "no overly complicated lambdas" and that's it, there is no need for the language to enforce this.
      - crazypyro 12y ago
        PEPs are literally the language, in my opinion. It may not be enforced on an implementation level, but some have been. You can't mix spaces and tabs now. That seems like a style choice being enforced on every level.
      - crazypyro 12y ago
        PEPs are literally the language, in my opinion. It may not be enforced on an implementation level, but some have been. You can't mix spaces and tabs now. That seems like a style choice being enforced on every level.
  - tel 12y ago
    That also dramatically reduces the value of lambdas. Without that limitation then they work as anonymous blocks of any complexity.
  - rbonvall 12y ago
michaelx386 12y ago
As someone currently learning C++ the biggest surprise was finding out about C++11/14. I'd assumed that the language hadn't changed for decades so it's great to see it still being updated.
- skriticos2 12y ago
  Yes. C++ is in an active standard update process. Here is a short history of notable versions: Year Version Year-Delta 1983; 1.0; 0 1989; 2.0; +6 years 1998; C++98; +9 years (first ISO standard) 2003; C++03; +5 years (minor update) 2011; C++11; +8 years (major update) 2014; C++14; +3 years (minor update) The C++14 version was the first that arrived on schedule. The next major version is planned for 2017. The future looks quite promising for C++.
tdsamardzhiev 12y ago
I work as a C++ developer, but damn, C++ has the ugliest lambda syntax ever.
- dllthomas 12y ago
  Not as ugly as having to create a new class, IMO.
- claudius 12y ago
  Do you have an alternative suggestion? It seems reasonably straightforward to me and I don’t find it all that impossible. Not having to muck around with a specific lambda() factory-like function with horrible inputs (as is needed in LPC where I sort-of learnt to code) is a plus :)
  - repsilat 12y ago
    I don't mind C++'s lambdas, but in some other languages they use "arrows", like square = (x) -> x * x C++ won't want to give up the different kinds of capturing, though, so it's hard to see anything much better than what's available now. In imaginary, never-to-be-implemented language daydreams I toy with the idea of using curly braces and implicit positional arguments to have lambdas look like square = { _1 * _1 } and factorial = { (_1 < 2) ? 1 : _0(_1 - 1) } and with explicit captures and arguments, partial_sort = [&lt_vec, &ge_vec, pivot](item){ if (item < pivot) lt_vec.add(item) else ge_vec.add(item) } I suspect, though, those ideas are just horrible shortcuts to a Perl-like hell :-)
    - claudius 12y ago
      Hrm, I don’t quite like the arrows. C++ lambdas can contain arbitrary code and these arrows always remind me of “pure” functions (in a sense), or at least functions whose main purpose it is to map input arguments to output arguments. For example, consider something like this relatively common block to do some complex operation on the elements of a vector using some (naive) threading (it might be nicer to first create some lambdas, push them into a queue and then spawn threads to work on that queue, but the syntax is essentially the same): for(std::size_t i(0); i != somevec.size(); ++i) { workers.push_back(std::thread([&somevec, i]() mutable { calc(somevec, i); })); } That is, we want an anonymous lambda which captures some elements of its environment (by both reference and value, also it’s essentially a closure by now) and call some function on these elements. That lambda doesn’t have any return value and doesn’t take any input arguments. Sure one could throw in an extra lambda somewhere or put an arrow between mutable and {, but I have to admit I don’t see the value.
jason_slack 12y ago
I've never been a good at Templates. After reading this it seems that a lot of Template uses can be replaced with the generic lambdas using auto as the types going in. Am I wrong or misunderstanding? I'm not saying completely but I feel like when I have used Templates I can now use this.
- dllthomas 12y ago
  My understanding is that auto can serve some of the role of templates, but that has no particular tie to lambdas that I can see.
- Iftheshoefits 12y ago
  Templates are the C++ implementation of "generics". Their utility extends far beyond lambdas-as-callables. The entire STL library is built using the Template feature of C++. Templates are far more important than the ability to do "true" object-oriented programming using other features, in my opinion. For a more concrete example that doesn't rely on the "authority" of an externally developed template library, consider that templates are also useful for static/compile-time polymorphism (see: Curiously Recurring Template Pattern).

63 points by AlexeyBrin 12y ago

In python, lambda usage extremly beautiful. Also, 'lambda' keyword solves readibility problem. I expected a keyword. Is there any challenge to add keyword?

VMG 12y ago
Backwards compatability. Old valid code should remain valid. That includes code which calculates wavelengths.
- thelinked 12y ago
  An interesting example of C++ not being able to introduce new keywords is the co-opting of auto in C++11 from a storage class to type deduction.
  - ninkendo 12y ago
    But they had a decent justification: the auto keyword literally did nothing in C++. The only place it was valid, it was also the default.
    - danielweber 12y ago
      IIRC they audited a bunch of projects to see if anyone was using auto somewhere besides test code explicitly for the auto-keyword, and found nothing.
- deleted 12y ago
  [deleted]
TillE 12y ago
Not shown in this article are all the things you can do within the [] (typically just [&] or [=]), which turns a simple lambda into a closure. The syntax is a bit arcane, but it does what it needs to. They could've stuck a "lambda" keyword in front of it all, but it's not necessary, and making a new keyword always has the potential to break old code.
- guidopallemans 12y ago
  is it possible to do #define lambda [] ?
  - Kronopath 12y ago
    I think that's possible, but I would strangle anyone who actually did that. At that point, not only are you pointlessly redefining the language just so it can feel slightly more familiar to people who are more comfortable with Python and the like, but you also completely lose the ability to enclose variables in the square brackets to make a closure. If you're going to write C++, then write C++. Don't redefine language constructs to fit the patterns of your other favourite language.
  - leni536 12y ago
    I think you can #define begin { #define end } too. I would not suggest it though.
  - kps 12y ago
    That way lies http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/sh/mac.h http://minnie.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/...
leni536 12y ago
I don't like that in python you are enforced to only use a single expression in lambda. Sure, writing long lambda functions is ugly but I don't think python should enforce a given coding style in the language level.
- crazypyro 12y ago
  Isn't that literally a core principal of Python? They enforce all kind of coding styles within the language, not the mention all the PEPs.
  - leni536 12y ago
    But PEPs are not enforced in the language level. Also there are cases where there could fit two simple expression in a lambda. Also the language still doesn't stop you from abusing lambdas, you can nest them. It could be stated in one of the PEPs that "no overly complicated lambdas" and that's it, there is no need for the language to enforce this.
    - crazypyro 12y ago
      PEPs are literally the language, in my opinion. It may not be enforced on an implementation level, but some have been. You can't mix spaces and tabs now. That seems like a style choice being enforced on every level.
    - crazypyro 12y ago
      PEPs are literally the language, in my opinion. It may not be enforced on an implementation level, but some have been. You can't mix spaces and tabs now. That seems like a style choice being enforced on every level.
- tel 12y ago
  That also dramatically reduces the value of lambdas. Without that limitation then they work as anonymous blocks of any complexity.
- rbonvall 12y ago

michaelx386 12y ago

As someone currently learning C++ the biggest surprise was finding out about C++11/14. I'd assumed that the language hadn't changed for decades so it's great to see it still being updated.

skriticos2 12y ago
Yes. C++ is in an active standard update process. Here is a short history of notable versions: Year Version Year-Delta 1983; 1.0; 0 1989; 2.0; +6 years 1998; C++98; +9 years (first ISO standard) 2003; C++03; +5 years (minor update) 2011; C++11; +8 years (major update) 2014; C++14; +3 years (minor update) The C++14 version was the first that arrived on schedule. The next major version is planned for 2017. The future looks quite promising for C++.

tdsamardzhiev 12y ago

I work as a C++ developer, but damn, C++ has the ugliest lambda syntax ever.

dllthomas 12y ago
Not as ugly as having to create a new class, IMO.
claudius 12y ago
Do you have an alternative suggestion? It seems reasonably straightforward to me and I don’t find it all that impossible. Not having to muck around with a specific lambda() factory-like function with horrible inputs (as is needed in LPC where I sort-of learnt to code) is a plus :)
- repsilat 12y ago
  I don't mind C++'s lambdas, but in some other languages they use "arrows", like square = (x) -> x * x C++ won't want to give up the different kinds of capturing, though, so it's hard to see anything much better than what's available now. In imaginary, never-to-be-implemented language daydreams I toy with the idea of using curly braces and implicit positional arguments to have lambdas look like square = { _1 * _1 } and factorial = { (_1 < 2) ? 1 : _0(_1 - 1) } and with explicit captures and arguments, partial_sort = [&lt_vec, &ge_vec, pivot](item){ if (item < pivot) lt_vec.add(item) else ge_vec.add(item) } I suspect, though, those ideas are just horrible shortcuts to a Perl-like hell :-)
  - claudius 12y ago
    Hrm, I don’t quite like the arrows. C++ lambdas can contain arbitrary code and these arrows always remind me of “pure” functions (in a sense), or at least functions whose main purpose it is to map input arguments to output arguments. For example, consider something like this relatively common block to do some complex operation on the elements of a vector using some (naive) threading (it might be nicer to first create some lambdas, push them into a queue and then spawn threads to work on that queue, but the syntax is essentially the same): for(std::size_t i(0); i != somevec.size(); ++i) { workers.push_back(std::thread([&somevec, i]() mutable { calc(somevec, i); })); } That is, we want an anonymous lambda which captures some elements of its environment (by both reference and value, also it’s essentially a closure by now) and call some function on these elements. That lambda doesn’t have any return value and doesn’t take any input arguments. Sure one could throw in an extra lambda somewhere or put an arrow between mutable and {, but I have to admit I don’t see the value.

jason_slack 12y ago

I've never been a good at Templates. After reading this it seems that a lot of Template uses can be replaced with the generic lambdas using auto as the types going in. Am I wrong or misunderstanding? I'm not saying completely but I feel like when I have used Templates I can now use this.

dllthomas 12y ago
My understanding is that auto can serve some of the role of templates, but that has no particular tie to lambdas that I can see.
Iftheshoefits 12y ago
Templates are the C++ implementation of "generics". Their utility extends far beyond lambdas-as-callables. The entire STL library is built using the Template feature of C++. Templates are far more important than the ability to do "true" object-oriented programming using other features, in my opinion. For a more concrete example that doesn't rely on the "authority" of an externally developed template library, consider that templates are also useful for static/compile-time polymorphism (see: Curiously Recurring Template Pattern).