# std::map<Key,T,Compare,Allocator>::map

From cppreference.com
< cpp‎ | container‎ | map

map(); explicit map( const Compare& comp, const Allocator& alloc = Allocator() ); | (1) |
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) Constructs an empty container.
2) Constructs the container with the contents of the range `[first, last)`. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).
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_container_copy_construction(other.get_allocator()).
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`. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).

### 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 LegacyInputIterator.
- `Compare` must meet the requirements of Compare.
- `Allocator` must meet the requirements of Allocator.

### 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()`.

### Exceptions

Calls to `Allocator::allocate` may throw.

### Notes

After container move construction (overload (4)), references, pointers, and iterators (other than the end iterator) to `other` remain valid, but refer to elements that are now in *this. The current standard makes this guarantee via the blanket statement in §23.2.1[container.requirements.general]/12, and a more direct guarantee is under consideration via LWG 2321.

### Example

Run this code

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

    template<typename Map>
    void print_map(Map& m)
    {
       std::cout << '{';
       for(auto& p: m)
            std::cout << p.first << ':' << p.second << ' ';
       std::cout << "}\n";
    }

    struct Point { double x, y; };
    struct PointCmp {
        bool operator()(const Point& lhs, const Point& rhs) const {
            return lhs.x < rhs.x; // NB. intentionally ignores y
        }
    };

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

      // (2) Range constructor
      std::map<std::string, int> iter(map1.find("anything"), map1.end());
      std::cout << "\niter = "; print_map(iter);
      std::cout << "map1 = "; print_map(map1);

      // (3) Copy constructor
      std::map<std::string, int> copied(map1);
      std::cout << "\ncopied = "; print_map(copied);
      std::cout << "map1 = "; print_map(map1);

      // (4) Move constructor
      std::map<std::string, int> moved(std::move(map1));
      std::cout << "\nmoved = "; print_map(moved);
      std::cout << "map1 = "; print_map(map1);

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

      // Custom Key class option 1:
      // Use a comparison struct
      std::map<Point, double, PointCmp> mag = {
          { {5, -12}, 13 },
          { {3, 4},   5 },
          { {-8, -15}, 17 }
      };

      for(auto p : mag)
          std::cout << "The magnitude of (" << p.first.x
                    << ", " << p.first.y << ") is "
                    << p.second << '\n';

      // Custom Key class option 2:
      // Use a comparison lambda
      // This lambda sorts points according to their magnitudes, where note that
      //  these magnitudes are taken from the local variable mag
      auto cmpLambda = [&mag](const Point &lhs, const Point &rhs) { return mag[lhs] < mag[rhs]; };
      //You could also use a lambda that is not dependent on local variables, like this:
      //auto cmpLambda = [](const Point &lhs, const Point &rhs) { return lhs.y < rhs.y; };
      std::map<Point, double, decltype(cmpLambda)> magy(cmpLambda);

      //Various ways of inserting elements:
      magy.insert(std::pair<Point, double>({5, -12}, 13));
      magy.insert({ {3, 4}, 5});
      magy.insert({Point{-8.0, -15.0}, 17});

      std::cout << '\n';
      for(auto p : magy)
          std::cout << "The magnitude of (" << p.first.x
                    << ", " << p.first.y << ") is "
                    << p.second << '\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 }
    The magnitude of (-8, -15) is 17
    The magnitude of (3, 4) is 5
    The magnitude of (5, -12) is 13

    The magnitude of (3, 4) is 5
    The magnitude of (5, -12) is 13
    The magnitude of (-8, -15) is 17

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2193 | C++11 | the default constructor is explicit | made non-explicit

### See also

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

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