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# std::map::map

From cppreference.com
< cpp‎ | container‎ | map

[edit template]

| (1) |
explicit map( const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); |  | (until C++14)
map() : map( Compare() ) {} explicit map( const Compare& comp, const Allocator& alloc = Allocator() ); |  | (since C++14)
explicit map( const Allocator& alloc ); | (1) | (since C++11)
| (2) |
template< class InputIt > map( InputIt first, InputIt last, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); |  |
template< class InputIt > map( InputIt first, InputIt last, const Allocator& alloc ); |  | (since C++14)
map( const map& other ); | (3) |
map( const map& other, const Allocator& alloc ); | (3) | (since C++11)
map( map&& other ); | (4) | (since C++11)
map( map&& other, const Allocator& alloc ); | (4) | (since C++11)
| (5) |
map( std::initializer_list<value_type> init, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); |  | (since C++11)
map( std::initializer_list<value_type> init, const Allocator& ); |  | (since C++14)

Constructs new container from a variety of data sources and optionally using user supplied allocator `alloc` or comparison function object `comp`.

1) Default constructor. Constructs empty container.
2) Constructs the container with the contents of the range `[first, last)`.
3) Copy constructor. Constructs the container with the copy of the contents of `other`. If `alloc` is not provided, allocator is obtained by calling std::allocator_traits<allocator_type>::select_on_copy_construction(other).
4) Move constructor. Constructs the container with the contents of `other` using move semantics. If `alloc` is not provided, allocator is obtained by move-construction from the allocator belonging to `other`.
5) Constructs the container with the contents of the initializer list `init`.

### [edit] Parameters

alloc | - | allocator to use for all memory allocations of this container
comp | - | comparison function object to use for all comparisons of keys
first, last | - | the range to copy the elements from
other | - | another container to be used as source to initialize the elements of the container with
init | - | initializer list to initialize the elements of the container with
Type requirements
- `InputIt` must meet the requirements of `InputIterator`.
- `Compare` must meet the requirements of `Compare`.
- `Allocator` must meet the requirements of `Allocator`.

### [edit] Complexity

1) Constant
2) N log(N) where N = std::distance(first, last) in general, linear in `N` if the range is already sorted by `value_comp()`.
3) Linear in size of `other`
4) Constant. If `alloc` is given and alloc != other.get_allocator(), then linear.
5) N log(N) where N = init.size()) in general, linear in `N` if `init` is already sorted by `value_comp()`.

### [edit] Example

Run this code

    #include <iostream>
    #include <string>
    #include <iomanip>
    #include <map>

    // helper function templates for printing each element
    template<typename CharT, typename Traits, typename T>
    void print_item(std::basic_ostream<CharT, Traits>& stream, const T& item)
    {
      stream << item;
    }

    template<typename CharT, typename Traits, typename Alloc>
    void print_item(std::basic_ostream<CharT, Traits>& stream,
        const std::basic_string<CharT, Traits, Alloc>& item)
    {
      stream << std::quoted(item);
    }

    // A printer for unordered maps
    template<typename Key, typename T, typename Compare, typename Allocator>
    std::ostream& operator<<(std::ostream& stream,
        const std::map<Key, T, Compare, Allocator>& map)
    {
      stream << '{';
      char comma[3] = {'\0', ' ', '\0'};
      for (const auto& pair : map) {
        stream << comma;
        print_item(stream, pair.first);
        stream << ':';
        print_item(stream, pair.second);
        comma[0] = ',';
      }
      stream << '}';
      return stream;
    }

    int main()
    {
      // (1) Default constructor
      std::map<std::string, int> map1;
      map1["something"] = 69;
      map1["anything"] = 199;
      map1["that thing"] = 50;
      std::cout << std::string(80, '-') << '\n';
      std::cout << "map1 = " << map1 << '\n';

      // (2) Iterator constructor
      std::map<std::string, int> iter(map1.find("anything"), map1.end());
      std::cout << std::string(80, '-') << '\n';
      std::cout << "iter = " << iter << '\n';
      std::cout << "map1 = " << map1 << '\n';

      // (3) Copy constructor
      std::map<std::string, int> copied(map1);
      std::cout << std::string(80, '-') << '\n';
      std::cout << "copied = " << copied << '\n';
      std::cout << "map1 = " << map1 << '\n';

      // (4) Move constructor
      std::map<std::string, int> moved(std::move(map1));
      std::cout << std::string(80, '-') << '\n';
      std::cout << "moved = " << moved << '\n';
      std::cout << "map1 = " << map1 << '\n';

      // (5) Initializer list constructor
      const std::map<std::string, int> init {
        {"this", 100},
        {"can", 100},
        {"be", 100},
        {"const", 100},
      };
      std::cout << std::string(80, '-') << '\n';
      std::cout << "init = " << init << '\n';
    }

Output:

    --------------------------------------------------------------------------------
    map1 = {"anything":199, "something":69, "that thing":50}
    --------------------------------------------------------------------------------
    iter = {"anything":199, "something":69, "that thing":50}
    map1 = {"anything":199, "something":69, "that thing":50}
    --------------------------------------------------------------------------------
    copied = {"anything":199, "something":69, "that thing":50}
    map1 = {"anything":199, "something":69, "that thing":50}
    --------------------------------------------------------------------------------
    moved = {"anything":199, "something":69, "that thing":50}
    map1 = {}
    --------------------------------------------------------------------------------
    init = {"be":100, "can":100, "const":100, "this":100}

### [edit] See also

operator= | assigns values to the container (public member function) [edit]

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