Use of for each on map elements

Question

I have a map where I d like to perform a call on every data type object member function  I yet know how to do this on any sequence but  is it possible to do it on an associative container   The closest answer I could find was this  Boost Bind to access std  map elements in std  for each  But I cannot use boost in my project so  is there an STL alternative that I m missing to boost  bind   If not possible  I thought on creating a temporary sequence for pointers to the data objects and then  call for each on it  something like this   class MyClass   public   void Method   const     std  map lt int  MyClass gt  Map         std  vector lt MyClass  gt  Vector  std  transform Map begin    Map end    std  back inserter Vector   std  mem fun ref  amp std  map lt int  MyClass gt   value type  second    std  for each Vector begin    Vector end    std  mem fun  amp MyClass  Method      It looks too obfuscated and I don t really like it  Any suggestions

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I wrote this awhile back to do just what you re looking for   namespace STLHelpers                 iterator helper type for iterating through the  values  of key value collections                                                               template lt typename  traits gt      struct  value iterator               explicit  value iterator typename  traits  iterator type  it                it  it                                value iterator const  value iterator  amp  other                it  other it                               friend bool operator   const  value iterator  amp lhs  const  value iterator  amp rhs                        return lhs it    rhs it                     friend bool operator   const  value iterator  amp lhs  const  value iterator  amp rhs                        return   lhs    rhs                       value iterator  amp operator                             it              return  this                      value iterator operator   int                         value iterator t  this                  this              return t                     typename  traits  value type  amp operator- gt                          return   this                     typename  traits  value type  amp operator                          return it- gt second                     typename  traits  iterator type it              template lt typename  tyMap gt      struct  map iterator traits               typedef typename  tyMap  iterator iterator type          typedef typename  tyMap  mapped type value type              template lt typename  tyMap gt      struct  const map iterator traits               typedef typename  tyMap  const iterator iterator type          typedef const typename  tyMap  mapped type value type

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Here is an example of how you can use for each for a map    std  map lt int  int gt  map   map insert std  pair lt int  int gt  1  2    map insert std  pair lt int  int gt  2  4    map insert std  pair lt int  int gt  3  6     auto f      std  pair lt int int gt  it   std  cout  lt  lt  it first   it second  lt  lt  std  endl     std  for each map begin    map end    f

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C  14 brings generic lambdas  Meaning we can use std  for each very easily   std  map lt int  int gt  myMap  1  2    3  4    5  6    7  8     std  for each myMap begin    myMap end       const auto  amp myMapPair        std  cout  lt  lt   first    lt  lt  myMapPair first  lt  lt    second                  lt  lt  myMapPair second  lt  lt  std  endl        I think std  for each is sometimes better suited than a simple range based for loop  For example when you only want to loop through a subset of a map

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It s unfortunate that you don t have Boost however if your STL implementation has the  extensions then you can compose mem fun ref and select2nd to create a single functor suitable for use with for each   The code would look something like this    include  lt algorithm gt   include  lt map gt   include  lt ext functional gt       GNU-specific extension for functor classes missing from standard STL  using namespace   gnu cxx      for compose1 and select2nd  class MyClass   public      void Method   const      std  map lt int  MyClass gt  Map   int main void        std  for each Map begin    Map end    compose1 std  mem fun ref  amp MyClass  Method   select2nd lt std  map lt int  MyClass gt   value type gt           Note that if you don t have access to compose1  or the unary compose template  and select2nd  they are fairly easy to write

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You can iterate through a std  map object  Each iterator will point to a std  pair lt const T S gt  where T and S are the same types you specified on your map   Here this would be   for  std  map lt int  MyClass gt   iterator it   Map begin    it    Map end      it      it- gt second Method        If you still want to use std  for each  pass a function that takes a std  pair lt const int  MyClass gt  amp  as an argument instead   Example   void CallMyMethod std  pair lt const int  MyClass gt  amp  pair     could be a class static method as well     pair second Method        And pass it to std  for each   std  for each Map begin    Map end    CallMyMethod

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Just an example   template  lt class key  class value gt  class insertIntoVec   public      insertIntoVec std  vector lt value gt  amp  vec in  m vec vec in              void operator     const std  pair lt key  value gt  amp  rhs                     m vec push back rhs second          private      std  vector lt value gt  amp  m vec      int main     std  map lt int  std  string gt  aMap  aMap 1     test1   aMap 2     test2   aMap 3     test3   aMap 4     test4    std  vector lt std  string gt  aVec   aVec reserve aMap size     std  for each aMap begin    aMap end              insertIntoVec lt int  std  string gt  aVec

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Will it work for you     class MyClass  typedef std  pair lt int MyClass gt  MyPair  class MyClass     private    void foo   const    public  static void Method MyPair const amp  p                          p second foo                  std  map lt int  MyClass gt  Map          std  for each Map begin    Map end      amp MyClass  Method

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For fellow programmers who stumble upon this question from google  there is a good way using boost   Explained here   Is it possible to use boost  foreach with std  map   Real example for your convenience       typedef in include  given here for info    typedef std  map lt std  string  std  string gt  Wt  WEnvironment  CookieMap  Wt  WEnvironment  CookieMap cookie map   environment cookies     BOOST FOREACH  const Wt  WEnvironment  CookieMap  value type  amp cookie  cookie map         std  cout  lt  lt   cookie      lt  lt  cookie first  lt  lt         lt  lt  cookie second  lt  lt  endl      enjoy

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How about a plain C     example fixed according to the note by  Noah Roberts   for std  map lt int  MyClass gt   iterator itr   Map begin    itr end   Map end    itr    itr end    itr      itr- gt second Method

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From what I remembered  C   map can return you an iterator of keys using map begin    you can use that iterator to loop over all the keys until it reach map end    and get the corresponding value  C   map

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C  11 allows you to do   for  const auto amp  kv   myMap        std  cout  lt  lt  kv first  lt  lt    has value    lt  lt  kv second  lt  lt  std  endl      C  17 allows you to do   for  const auto amp   key  value    myMap        std  cout  lt  lt  key  lt  lt    has value    lt  lt  value  lt  lt  std  endl      using structured binding   UPDATE   const auto is safer if you don t want to modify the map

[c++] Use of for_each on map elements

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