How to initialize a private static const map in C

Question

I need just dictionary or associative array string    int   There is type map C   for this case   But I need only one map forall instances -  static  and this map can t be changed -  const    I have found this way with boost library   std  map lt int  char gt  example          boost  assign  map list of 1   a    2   b    3   c      Is there other solution without this lib  I have tried something like this  but there are always some issues with map initialization   class myClass  private      static map lt int int gt  create map                       map lt int int gt  m            m 1    2            m 3    4            m 5    6            return m                static map lt int int gt  myMap    create map

User · Answer

I did it      Works fine without C  11  class MyClass       typedef std  map lt std  string  int gt  MyMap       struct T           const char  Name          int Num           operator MyMap  value type   const               return std  pair lt std  string  int gt  Name  Num                         static const T MapPairs        static const MyMap TheMap      const MyClass  T MyClass  MapPairs              Jan   1       Feb   2       Mar   3       const MyClass  MyMap MyClass  TheMap MapPairs  MapPairs   3

User · Answer

A different approach to the problem   struct A       static const map lt int  string gt    singleton map             static map lt int  string gt   m   NULL          if   m                m   new map lt int  string gt               m 42     42                     other initializations                   return m                rest of the class     This is more efficient  as there is no one-type copy from stack to heap  including constructor  destructors on all elements   Whether this matters or not depends on your use case  Does not matter with strings   but you may or may not find this version  cleaner

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include  lt map gt  using namespace std   struct A      static map lt int int gt  create map                       map lt int int gt  m            m 1    2            m 3    4            m 5    6            return m                static const map lt int int gt  myMap       const map lt int int gt  A   myMap    A  create map     int main

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If the map is to contain only entries that are known at compile time and the keys to the map are integers  then you do not need to use a map at all   char get value int key        switch  key                case 1              return  a           case 2              return  b           case 3              return  c           default                 Do whatever is appropriate when the key is not valid

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A function call cannot appear in a constant expression   try this   just an example    include  lt map gt   include  lt iostream gt   using std  map  using std  cout   class myClass   public   static map lt int int gt  create map               map lt int int gt  m        m 1    2        m 3    4        m 5    6        return m         const static map lt int int gt  myMap      const map lt int int gt myClass  myMap    create map     int main        map lt int int gt  t myClass  create map       std  cout lt  lt t 1     prints 2

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If you are using a compiler which still doesn t support universal initialization or you have reservation in using Boost  another possible alternative would be as follows  std  map lt int  int gt  m               std  pair lt int int gt   m               std  make pair 1   sizeof 2            std  make pair 3   sizeof 4            std  make pair 5   sizeof 6         std  map lt int  int gt  m      for  auto data   m                m data first    data second            return m

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I often use this pattern and recommend you to use it as well   class MyMap   public std  map lt int  int gt    public      MyMap                   either         insert make pair 1  2            insert make pair 3  4            insert make pair 5  6              or           this  1    2            this  3    4            this  5    6          const static my map    Sure it is not very readable  but without other libs it is best we can do  Also there won t be any redundant operations like copying from one map to another like in your attempt   This is even more useful inside of functions  Instead of   void foo        static bool initComplete   false     static Map map     if   initComplete             initComplete   true        map                Use the following   void bar         struct MyMap   Map             MyMap                                      static mymap      Not only you don t need here to deal with boolean variable anymore  you won t have hidden global variable that is checked if initializer of static variable inside function was already called

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You could try this   MyClass h  class MyClass   private      static const std  map lt key  value gt  m myMap       static const std  map lt key  value gt  createMyStaticConstantMap    public      static std  map lt key  value gt  getMyConstantStaticMap  return m myMap         MyClass   MyClass cpp   include  MyClass h   const std  map lt key  value gt  MyClass  m myMap   MyClass  createMyStaticConstantMap     const std  map lt key  value gt  MyClass  createMyStaticConstantMap         std  map lt key  value gt  mMap      mMap insert  std  make pair  key1  value1          mMap insert  std  make pair  key2  value2                      mMap insert  std  make pair  lastKey  lastValue           return mMap       createMyStaticConstantMap   With this implementation your classes constant static map is a private member and can be accessible to other classes using a public get method  Otherwise since it is constant and can not change  you can remove the public get method and move the map variable into the classes public section  I would however leave the createMap method private or protected if inheritance and or polymorphism is required  Here are some samples of use    std  map lt key value gt  m1   MyClass  getMyMap        then do work on m1 or  unsigned index   some predetermined value  MyClass  getMyMap   at  index       As long as index is valid this will      retun map second or map- gt second value so if in this case key is an     unsigned and value is a std  string then you could do  std  cout  lt  lt  std  string  MyClass  getMyMap   at  some index that exists in map          and it will print out to the console the string locted in the map at this index     You can do this before any class object is instantiated or declared       If you are using a pointer to your class such as   std  shared ptr lt MyClass gt     std  unique ptr lt MyClass gt      Then it would look like this   pMyClass- gt getMyMap   at  index       And Will do the same as above     Even if you have not yet called the std pointer s reset method on     this class object        This will only work on static methods only  and all data in static methods must be available first    I had edited my original post  there was nothing wrong with the original code in which I posted for it compiled  built and ran correctly  it was just that my first version I presented as an answer the map was declared as public and the map was const but wasn t static

User · Answer

If you find boost  assign  map list of useful  but can t use it for some reason  you could write your own   template lt class K  class V gt  struct map list of type     typedef std  map lt K  V gt  Map    Map data    map list of type K k  V v    data k    v      map list of type amp  operator   K k  V v    data k    v  return  this      operator Map const amp    const   return data       template lt class K  class V gt  map list of type lt K  V gt  my map list of K k  V v      return map list of type lt K  V gt  k  v      int main       std  map lt int  char gt  example        my map list of 1   a    2   b    3   c      cout  lt  lt  example  lt  lt    n       It s useful to know how such things work  especially when they re so short  but in this case I d use a function   a hpp  struct A     static map lt int  int gt  const m       a cpp  namespace   map lt int int gt  create map       map lt int  int gt  m    m 1    2     etc    return m       map lt int  int gt  const A  m   create map

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The C  11 standard introduced uniform initialization which makes this much simpler if your compiler supports it     myClass hpp class myClass     private      static map lt int int gt  myMap           myClass cpp map lt int int gt  myClass  myMap         1  2       3  4       5  6       See also this section from Professional C    on unordered maps

[c++] How to initialize a private static const map in C++?

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