C++ Initializing a std::map or std::multimap


Example

std::map and std::multimap both can be initialized by providing key-value pairs separated by comma. Key-value pairs could be provided by either {key, value} or can be explicitly created by std::make_pair(key, value). As std::map does not allow duplicate keys and comma operator performs right to left, the pair on right would be overwritten with the pair with same key on the left.

std::multimap < int, std::string > mmp { std::make_pair(2, "stackoverflow"),
                                     std::make_pair(1, "docs-beta"),
                                     std::make_pair(2, "stackexchange")  };
// 1 docs-beta
// 2 stackoverflow
// 2 stackexchange

std::map < int, std::string > mp {  std::make_pair(2, "stackoverflow"),
                                std::make_pair(1, "docs-beta"),
                                std::make_pair(2, "stackexchange")  }; 
// 1 docs-beta
// 2 stackoverflow

Both could be initialized with iterator.

// From std::map or std::multimap iterator
std::multimap< int , int > mmp{ {1, 2}, {3, 4}, {6, 5}, {8, 9}, {6, 8}, {3, 4}, 
                               {6, 7} };
                       // {1, 2}, {3, 4}, {3, 4}, {6, 5}, {6, 8}, {6, 7}, {8, 9}
auto it = mmp.begin();
std::advance(it,3); //moved cursor on first {6, 5}
std::map< int, int > mp(it, mmp.end()); // {6, 5}, {8, 9}

//From std::pair array
std::pair< int, int > arr[10];
arr[0] = {1, 3};
arr[1] = {1, 5};
arr[2] = {2, 5};
arr[3] = {0, 1};
std::map< int, int > mp(arr,arr+4); //{0 , 1}, {1, 3}, {2, 5}

//From std::vector of std::pair
std::vector< std::pair<int, int> > v{ {1, 5}, {5, 1}, {3, 6}, {3, 2} };
std::multimap< int, int > mp(v.begin(), v.end()); 
                        // {1, 5}, {3, 6}, {3, 2}, {5, 1}