How to use range-based for loop with std map

Question

The common example for C  11 range-based for   loops is always something simple like this   std  vector lt int gt  numbers     1  2  3  4  5  6  7    for   auto xyz   numbers          std  cout  lt  lt  xyz  lt  lt  std  endl      In which case xyz is an int   But  what happens when we have something like a map   What is the type of the variable in this example   std  map lt  foo  bar  gt  testing          blah         for   auto abc   testing         std  cout  lt  lt  abc  lt  lt  std  endl               should this give a foo  a bar      std  cout  lt  lt  abc- gt first  lt  lt  std  endl        or is abc an iterator      When the container being traversed is something simple  it looks like range-based for   loops will give us each item  not an iterator   Which is nice   if it was iterator  first thing we d always have to do is to dereference it anyway   But I m confused as to what to expect when it comes to things like maps and multimaps    I m still on g   4 4  while range-based loops are in g   4 6   so I haven t had the chance to try it yet

User · Answer

From this paper  http   www open-std org jtc1 sc22 wg21 docs papers 2006 n2049 pdf  for  type-speci er-seq simple-declarator   expression   statement  is syntactically equivalent to        typedef decltype expression  C      auto amp  amp  rng expression       for  auto begin std  For lt C gt   begin rng    end std  For lt C gt   end rng    begin    end     begin            type-speci er-seq simple-declarator  begin           statement           So you can clearly see that what is abc in your case will be std  pair lt key type  value type  gt    So for printing you can do access each element by abc first and abc second

User · Answer

If copy assignment operator of foo and bar is cheap  eg  int  char  pointer etc   you can do the following   foo f  bar b  BOOST FOREACH boost  tie f b  testing      cout  lt  lt   Foo is    lt  lt  f  lt  lt    Bar is    lt  lt  b

User · Answer

In C  17 this is called structured bindings  which allows for the following   std  map lt  foo  bar  gt  testing          blah         for   const auto amp    k  v     testing       std  cout  lt  lt  k  lt  lt       lt  lt  v  lt  lt    n

User · Answer

Each element of the container is a map lt K  V gt   value type  which is a typedef for std  pair lt const K  V gt    Consequently  in C  17 or higher  you can write  for  auto amp   key  value   myMap        std  cout  lt  lt  key  lt  lt    has value    lt  lt  value  lt  lt  std  endl      or as  for  const auto amp   key  value   myMap        std  cout  lt  lt  key  lt  lt    has value    lt  lt  value  lt  lt  std  endl      if you don t plan on modifying the values   In C  11 and C  14  you can use enhanced for loops to extract out each pair on its own  then manually extract the keys and values   for  const auto amp  kv   myMap        std  cout  lt  lt  kv first  lt  lt    has value    lt  lt  kv second  lt  lt  std  endl      You could also consider marking the kv variable const if you want a read-only view of the values

User · Answer

If you only want to see the keys values from your map and like using boost  you can use the boost adaptors with the range based loops   for  const auto amp  value   myMap   boost  adaptors  map values        std  cout  lt  lt  value  lt  lt  std  endl      there is an equivalent boost  adaptors  key values  http   www boost org doc libs 1 51 0 libs range doc html range reference adaptors reference map values html

[c++] How to use range-based for() loop with std::map?

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