Screenplay: std::map Initialization And Inserting

Definining The Type

• Inline as you need it

  code/map-instantiate-explicit.cpp

  #include <map>
  #include <string>
  #include <iostream>
  
  int main()
  {
      std::map<int, std::string> my_map;         // <--- much typing (if done repeatedly)
      std::cout << my_map.size() << std::endl;
      return 0;
  }
  

• Better, mostly, using using

  code/map-instantiate-using.cpp

  #include <map>
  #include <string>
  #include <iostream>
  
  using MyMap = std::map<int, std::string>;
  
  int main()
  {
      MyMap my_map;         // <--- more readable, at least
      std::cout << my_map.size() << std::endl;
      return 0;
  }
  

• Ah, member types

  code/map-instantiate-member-types.cpp

  #include <map>
  #include <string>
  #include <iostream>
  
  using MyMap = std::map<int, std::string>;
  
  int main()
  {
      MyMap my_map;
  
      MyMap::key_type k{666};                                  // <--- same as "int k{666};"
      MyMap::mapped_type v{"sechshundersechsundsechzig"};      // <--- same as "std::string v{...};"
      MyMap::value_type kv{666, "sechshundersechsundsechzig"}; // <--- same as "std::pair<...>{666, ...};"
  
      // avoid "unused" warnings
      (void)k;
      (void)v;
      (void)kv;
  
      return 0;
  }
  

Initialization

• Pre C++11, there is only default ctor

• Elements are added at runtime, using push_back()

  code/map-initialize-pre11.cpp

  #include <map>
  #include <string>
  #include <iostream>
  
  using MyMap = std::map<int, std::string>;    // <--- pre C++11 has no "using"
  
  int main()
  {
      MyMap my_map;                            // <--- pre C++11, there is only default initialization
      my_map[2] = "zwei";
      my_map[51] = "einundfuenfzig";
      my_map[54] = "vierundfuenfzig";
      my_map[346] = "dreihundertsechsundvierzig";
      my_map[1001] = "tausendeins";
      my_map[1002] = "tausendzwei";
  
      std::cout << my_map.size() << std::endl;
  
      return 0;
  }
  

• C++11: Brace Initialization (Uniform Initialization)

• ⟶ const non-default-constructed object

  code/map-initialize-11.cpp

  #include <map>
  #include <string>
  #include <iostream>
  
  using MyMap = std::map<int, std::string>;
  
  int main()
  {
      const MyMap my_map = {                    // <--- this is *initialization*, hence can be const
          { 2, "zwei" },                        // <--- value_type: std::pair<int, std::string>
          { 51, "einundfuenfzig" },             // <--- value_type: std::pair<int, std::string>
          { 54, "vierundfuenfzig" },            // <--- value_type: std::pair<int, std::string>
          { 346, "dreihundertsechsundvierzig" },// <--- value_type: std::pair<int, std::string>
          { 1001, "tausendeins" },              // <--- value_type: std::pair<int, std::string>
          { 1002, "tausendzwei" },              // <--- value_type: std::pair<int, std::string>
      };
  
      std::cout << my_map.size() << std::endl;
  
      return 0;
  }