std::map::map

	(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 InputIterator > map( InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& alloc = Allocator() );
template< class InputIterator > map( InputIterator first, InputIterator 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). 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
- `InputIterator` must meet the requirements of `InputIterator`.
- `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().

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

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

array (C++11)
vector
deque
forward_list (C++11)
list
set
multiset
map
multimap
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
stack
queue
priority_queue

Member functions
map::map
map::~map
map::operator=
map::get_allocator
Element access
map::at
map::operator[]
Iterators
map::beginmap::cbegin (C++11)
map::endmap::cend (C++11)
map::rbeginmap::crbegin (C++11)
map::rendmap::crend (C++11)
Capacity
map::empty
map::size
map::max_size
Modifiers
map::clear
map::insert
map::insert_or_assign (C++17)
map::emplace (C++11)
map::emplace_hint (C++11)
map::try_emplace (C++17)
map::erase
map::swap
map::extract (C++17)
map::merge (C++17)
Lookup
map::count
map::find
map::equal_range
map::lower_bound
map::upper_bound
Observers
map::key_comp
map::value_comp

std::map::map

Parameters

Complexity

Notes

Example

See also