C 11 reverse range-based for-loop

Question

Is there a container adapter that would reverse the direction of iterators so I can iterate over a container in reverse with range-based for-loop   With explicit iterators I would convert this   for  auto i   c begin    i    c end      i          into this   for  auto i   c rbegin    i    c rend      i          I want to convert this   for  auto amp  i  c          to this   for  auto amp  i  std  magic reverse adapter c           Is there such a thing or do I have to write it myself

User · Accepted Answer

Actually Boost does have such adaptor  boost  adaptors  reverse     include  lt list gt   include  lt iostream gt   include  lt boost range adaptor reversed hpp gt   int main         std  list lt int gt  x   2  3  5  7  11  13  17  19        for  auto i   boost  adaptors  reverse x           std  cout  lt  lt  i  lt  lt    n       for  auto i   x          std  cout  lt  lt  i  lt  lt    n

User · Answer

This should work in C  11 without boost   namespace std   template lt class T gt  T begin std  pair lt T  T gt  p        return p first    template lt class T gt  T end std  pair lt T  T gt  p        return p second       template lt class Iterator gt  std  reverse iterator lt Iterator gt  make reverse iterator Iterator it        return std  reverse iterator lt Iterator gt  it      template lt class Range gt  std  pair lt std  reverse iterator lt decltype begin std  declval lt Range gt      gt   std  reverse iterator lt decltype begin std  declval lt Range gt      gt  gt  make reverse range Range amp  amp  r        return std  make pair make reverse iterator begin r    make reverse iterator end r        for auto x  make reverse range r

User · Answer

template  lt typename C gt  struct reverse wrapper        C  amp  c       reverse wrapper C  amp  c     c  c          typename C  reverse iterator begin    return c  rbegin         typename C  reverse iterator end    return c  rend          template  lt typename C  size t N gt  struct reverse wrapper lt  C N   gt        C   amp c   N       reverse wrapper  C  amp c  N      c  c          typename std  reverse iterator lt const C   gt  begin     return std  rbegin c          typename std  reverse iterator lt const C   gt  end     return std  rend c           template  lt typename C gt  reverse wrapper lt C gt  r wrap C  amp  c        return reverse wrapper lt C gt  c       eg   int main int argc  const char   argv          std  vector lt int gt  arr 1  2  3  4  5       int arr1      1  2  3  4  5        for  auto i   r wrap arr             printf   d    i             printf   n         for  auto i   r wrap arr1             printf   d    i             printf   n        return 0

User · Answer

Got this example from cppreference  It works with  GCC 10 1  with flag -std c  20  include  lt ranges gt   include  lt iostream gt    int main         static constexpr auto il    3  1  4  1  5  9         std  ranges  reverse view rv  il       for  int i   rv          std  cout  lt  lt  i  lt  lt             std  cout  lt  lt    n         for int i   il   std  views  reverse          std  cout  lt  lt  i  lt  lt

User · Answer

Does this work for you    include  lt iostream gt   include  lt list gt   include  lt boost range begin hpp gt   include  lt boost range end hpp gt   include  lt boost range iterator range hpp gt   int main int argc  char  argv        typedef std  list lt int gt  Nums    typedef Nums  iterator NumIt    typedef boost  range reverse iterator lt Nums gt   type RevNumIt    typedef boost  iterator range lt NumIt gt  irange 1    typedef boost  iterator range lt RevNumIt gt  irange 2     Nums n    1  2  3  4  5  6  7  8     irange 1 r1   boost  make iterator range  boost  begin n   boost  end n       irange 2 r2   boost  make iterator range  boost  end n   boost  begin n            prints  1 2 3 4 5 6 7 8    for auto e   r1      std  cout  lt  lt  e  lt  lt          std  cout  lt  lt  std  endl        prints  8 7 6 5 4 3 2 1   for auto e   r2      std  cout  lt  lt  e  lt  lt          std  cout  lt  lt  std  endl     return 0

User · Answer

If not using C  14  then I find below the simplest solution    define METHOD NAME       auto NAME   VA ARGS   - gt  decltype m T r  NAME    return m T r  NAME    template lt typename T gt  struct Reverse     T amp  m T     METHOD begin       METHOD end       METHOD begin    const     METHOD end    const       undef METHOD  template lt typename T gt  Reverse lt T gt  MakeReverse  T amp  t    return Reverse lt T gt  t       Demo  It doesn t work for the containers data-types  like array   which doesn t have begin rbegin  end rend functions

User · Answer

You could simply use BOOST REVERSE FOREACH which iterates backwards  For example  the code   include  lt iostream gt   include  lt boost foreach hpp gt   int main         int integers       0  1  2  3  4        BOOST REVERSE FOREACH auto i  integers                std  cout  lt  lt  i  lt  lt  std  endl            return 0      generates the following output   4  3  2  1  0

User · Answer

Actually  in C  14 it can be done with a very few lines of code   This is a very similar in idea to  Paul s solution  Due to things missing from C  11  that solution is a bit unnecessarily bloated  plus defining in std smells   Thanks to C  14 we can make it a lot more readable   The key observation is that range-based for-loops work by relying on begin   and end   in order to acquire the range s iterators  Thanks to ADL  one doesn t even need to define their custom begin   and end   in the std   namespace   Here is a very simple-sample solution      -------------------------------------------------------------------    --- Reversed iterable  template  lt typename T gt  struct reversion wrapper   T amp  iterable      template  lt typename T gt  auto begin  reversion wrapper lt T gt  w    return std  rbegin w iterable      template  lt typename T gt  auto end  reversion wrapper lt T gt  w    return std  rend w iterable      template  lt typename T gt  reversion wrapper lt T gt  reverse  T amp  amp  iterable    return   iterable        This works like a charm  for instance   template  lt typename T gt  void print iterable  std  ostream amp  out  const T amp  iterable        for  auto amp  amp  element  iterable          out  lt  lt  element  lt  lt           out  lt  lt    n      int main  int  char          using namespace std          on prvalues     print iterable cout  reverse initializer list lt int gt    1  2  3  4               on const lvalue references     const list lt int gt  ints list   1  2  3  4         for  auto amp  amp  el  reverse ints list           cout  lt  lt  el  lt  lt           cout  lt  lt    n           on mutable lvalue references     vector lt int gt  ints vec   0  0  0  0         size t i   0      for  int amp  el  reverse ints vec           el    i        print iterable cout  ints vec       print iterable cout  reverse ints vec         return 0      prints as expected  4 3 2 1  4 3 2 1  3 2 1 0  0 1 2 3    NOTE std  rbegin    std  rend    and std  make reverse iterator   are not yet implemented in GCC-4 9  I write these examples according to the standard  but they would not compile in stable g    Nevertheless  adding temporary stubs for these three functions is very easy  Here is a sample implementation  definitely not complete but works well enough for most cases      -------------------------------------------------- template  lt typename I gt  reverse iterator lt I gt  make reverse iterator  I i        return std  reverse iterator lt I gt    i          -------------------------------------------------- template  lt typename T gt  auto rbegin  T amp  iterable        return make reverse iterator iterable end        template  lt typename T gt  auto rend  T amp  iterable        return make reverse iterator iterable begin           const container variants  template  lt typename T gt  auto rbegin  const T amp  iterable        return make reverse iterator iterable end        template  lt typename T gt  auto rend  const T amp  iterable        return make reverse iterator iterable begin

User · Answer

If you can use range v3   you can use the reverse range adapter ranges  view  reverse which allows you to view the container in reverse   A minimal working example    include  lt iostream gt   include  lt vector gt   include  lt range v3 view hpp gt   int main         std  vector lt int gt  intVec    1  2  3  4  5  6  7  8  9        for  auto const amp  e   ranges  view  reverse intVec             std  cout  lt  lt  e  lt  lt                    std  cout  lt  lt  std  endl       for  auto const amp  e   intVec            std  cout  lt  lt  e  lt  lt                    std  cout  lt  lt  std  endl      See DEMO 1   Note  As per Eric Niebler  this feature will be available in C  20  This can be used with the  lt experimental ranges range gt  header  Then the for statement will look like this   for  auto const amp  e   view  reverse intVec            std  cout  lt  lt  e  lt  lt              See DEMO 2

[c++] C++11 reverse range-based for-loop

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