Initializing a static std map int int in C

Question

What is the right way of initializing a static map  Do we need a static function that will initialize it

User · Answer

For example   const std  map lt LogLevel  const char  gt  g log levels dsc           LogLevel  Disabled    ---            LogLevel  Info        inf            LogLevel  Warning     wrn            LogLevel  Error       err            LogLevel  Debug       dbg          If map is a data member of a class  you can initialize it directly in header by the following way  since C  17       Example  template lt  gt  class StringConverter lt CacheMode gt  final   public      static auto convert CacheMode mode  - gt  const std  string amp                   validate            return s modes at mode          private      static inline const std  map lt CacheMode  std  string gt  s modes                           CacheMode  All   All                   CacheMode  Selective   Selective                   CacheMode  None   None                   etc

User · Answer

It s not a complicated issue to make something similar to boost  Here s a class with just three functions  including the constructor  to replicate what boost did  almost    template  lt typename T  typename U  class create map   private      std  map lt T  U  m map  public      create map const T  key  const U  val                m map key    val             create map lt T  U   operator   const T  key  const U  val                m map key    val          return  this             operator std  map lt T  U                  return m map             Usage    std  map mymap   create map lt int  int   1 2  3 4  5 6     The above code works best for initialization of global variables or static members of a class which needs to be initialized and you have no idea when it gets used first but you want to assure that the values are available in it   If say  you ve got to insert elements into an existing std  map    here s another class for you   template  lt typename MapType gt  class map add values   private      MapType mMap  public      typedef typename MapType  key type KeyType      typedef typename MapType  mapped type MappedType       map add values const KeyType amp  key  const MappedType amp  val                mMap key    val             map add values amp  operator   const KeyType amp  key  const MappedType amp  val            mMap key    val          return  this             void to  MapType amp  map            map insert mMap begin    mMap end                Usage   typedef std  map lt int  int gt  Int2IntMap  Int2IntMap testMap  map add values lt Int2IntMap gt  1 2  3 4  5 6  to testMap     See it in action with GCC 4 7 2 here  http   ideone com 3uYJiH                  EVERYTHING BELOW THIS IS OBSOLETE                    EDIT  The map add values class below  which was the original solution I had suggested  would fail when it comes to GCC 4 5   Please look at the code above for how to add values to existing map    template lt typename T  typename U  class map add values   private      std  map lt T U   m map  public      map add values std  map lt T  U    map  m map  map        map add values  operator   const T   key  const U   val                m map key    val          return  this            Usage   std  map lt int  int  my map     Later somewhere along the code map add values lt int int  my map  1 2  3 4  5 6    NOTE  Previously I used a operator    for adding the actual values  This is not possible as commented by dalle                         END OF OBSOLETE SECTION

User · Answer

I would wrap the map inside a static object  and put the map initialisation code in the constructor of this object  this way you are sure the map is created before the initialisation code is executed

User · Answer

Best way is to use a function    include  lt map gt   using namespace std   map lt int int gt  create map       map lt int int gt  m    m 1    2    m 3    4    m 5    6    return m     map lt int int gt  m   create map

User · Answer

You have some very good answers here  but I m to me  it looks like a case of  when all you know is a hammer      The simplest answer of to why there is no standard way to initialise a static map  is there is no good reason to ever use a static map      A map is a structure designed for fast lookup  of an unknown set of elements  If you know the elements before hand  simply use a C-array  Enter the values in a sorted manner  or run sort on them  if you can t do this  You can then get log n  performance by using the stl  functions to loop-up entries  lower bound upper bound  When I have tested this previously they normally perform at least 4 times faster than a map   The advantages are many fold    - faster performance   4  I ve measured on many CPU s types  it s always around 4  - simpler debugging  It s just easier to see what s going on with a linear layout  - Trivial implementations of copy operations  should that become necessary  - It allocates no memory at run time  so will never throw an exception  - It s a standard interface  and so is very easy to share across  DLL s  or languages  etc   I could go on  but if you want more  why not look at Stroustrup s many blogs on the subject

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Just wanted to share a pure C   98 work around    include  lt map gt   std  map lt std  string  std  string gt  aka   struct akaInit       akaInit                 aka   George       John           aka   Joe       Al           aka   Phil       Sue           aka   Smitty       Yando           AkaInit

User · Answer

You can try   std  map  lt int  int gt  mymap              std  pair  lt int  int gt   1  1           std  pair  lt int  int gt   2  2           std  pair  lt int  int gt   2  2

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This is similar to PierreBdR  without copying the map    include  lt map gt   using namespace std   bool create map map lt int int gt   amp m      m 1    2    m 3    4    m 5    6    return true     static map lt int int gt  m  static bool  dummy   create map  m

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If you are stuck with C  98 and don t want to use boost  here there is the solution I use when I need to initialize a static map   typedef std  pair lt  int  char  gt  elemPair t  elemPair t elemPairs            elemPair t  1   a         elemPair t  3   b          elemPair t  5   c          elemPair t  7   d        const std  map lt  int  char  gt  myMap   amp elemPairs  0     amp elemPairs  sizeof  elemPairs     sizeof  elemPairs  0

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Using C  11    include  lt map gt  using namespace std   map lt int  char gt  m     1   a     3   b     5   c     7   d       Using Boost Assign    include  lt map gt   include  boost assign hpp  using namespace std  using namespace boost  assign   map lt int  char gt  m   map list of  1   a    3   b    5   c    7   d

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Here is another way that uses the 2-element data constructor  No functions are needed to initialize it  There is no 3rd party code  Boost   no static functions or objects  no tricks  just simple C      include  lt map gt   include  lt string gt   typedef std  map lt std  string  int gt  MyMap   const MyMap  value type rawData          MyMap  value type  hello   42      MyMap  value type  world   88      const int numElems   sizeof rawData   sizeof rawData 0   MyMap myMap rawData  rawData   numElems     Since I wrote this answer C  11 is out  You can now directly initialize STL containers using the new initializer list feature   const MyMap myMap       hello   42     world   88

[c++] Initializing a static std::map<int, int> in C++

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