std::map::map

 

C++

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)

 

Containers library

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

 

std::map

Member functions

map::map

map::~map

map::operator=

map::get_allocator

Element access

map::at

map::operator[]

Iterators

map::begin map::cbegin (C++11)

map::end map::cend (C++11)

map::rbegin map::crbegin (C++11)

map::rend map::crend (C++11)

Capacity

map::empty

map::size

map::max_size

Modifiers

map::clear

map::insert

map::emplace (C++11)

map::emplace_hint (C++11)

map::erase

map::swap

Lookup

map::count

map::find

map::equal_range

map::lower_bound

map::upper_bound

Observers

map::key_comp

map::value_comp

 

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

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

Parameters

alloc	-	allocator to use for all memory allocations of this container
comp	-	comparison function 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.

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().

Example

run this code

#include <iostream>
#include <functional>
#include <map>
 
template<typename Key, typename T, typename Compare, typename Allocator>
std::ostream& operator<<(std::ostream& stream, const std::map<Key, T, Compare, Allocator>& map);
template<typename T, typename Compare, typename Allocator>
std::ostream& operator<<(std::ostream& stream, const std::map<std::string, T, Compare, Allocator>& map);
template<typename Key, typename Compare, typename Allocator>
std::ostream& operator<<(std::ostream& stream, const std::map<Key, std::string, Compare, Allocator>& map);
template<typename Compare, typename Allocator>
std::ostream& operator<<(std::ostream& stream, const std::map<std::string, std::string, Compare, Allocator>& map);
 
int main()
{
  typedef std::string Key;
  typedef int Mapped;
 
  // (1) Default constructor
  std::map<Key, Mapped> map;
  map["something"] = 69;
  map["anything"] = 199;
  map["that thing"] = 50;
  std::cerr << std::string(80, '-') << std::endl;
  std::cerr << "map = " << map << std::endl;
 
  // (2) Iterator constructor
  std::map<Key, Mapped> iter(map.find("anything"), map.end());
  std::cerr << std::string(80, '-') << std::endl;
  std::cerr << "iter = " << iter << std::endl;
  std::cerr << "map = " << map << std::endl;
 
  // (3) Copy constructor
  std::map<Key, Mapped> copy(map);
  std::cerr << std::string(80, '-') << std::endl;
  std::cerr << "copy = " << copy << std::endl;
  std::cerr << "map = " << map << std::endl;
 
  // (4) Move constructor
  std::map<Key, Mapped> moved(std::move(map));
  std::cerr << std::string(80, '-') << std::endl;
  std::cerr << "moved = " << copy << std::endl;
  std::cerr << "map = " << map << std::endl;
 
  // (5) Initializer list constructor
  const std::map<Key, Mapped> init {
    {"this", 100},
    {"can", 100},
    {"be", 100},
    {"const", 100},
  };
  std::cerr << std::string(80, '-') << std::endl;
  std::cerr << "init = " << init << std::endl;
}
 
// 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 << pair.first << ':' << pair.second;
    comma[0] = ',';
  }
  stream << '}';
  return stream;
}
 
// A specialization for Key strings
template<typename T, typename Compare, typename Allocator>
std::ostream& operator<<(std::ostream& stream,
    const std::map<std::string, T, Compare, Allocator>& map) {
  stream << '{';
  char comma[3] = {'\0', ' ', '\0'};
  for (const auto& pair : map) {
    stream << comma << '"' << pair.first << "\":" << pair.second;
    comma[0] = ',';
  }
  stream << '}';
  return stream;
}
 
// A printer for unordered maps
template<typename Key, typename Compare, typename Allocator>
std::ostream& operator<<(std::ostream& stream,
    const std::map<Key, std::string, Compare, Allocator>& map) {
  stream << '{';
  char comma[3] = {'\0', ' ', '\0'};
  for (const auto& pair : map) {
    stream << comma << pair.first << ":\"" << pair.second << '"';
    comma[0] = ',';
  }
  stream << '}';
  return stream;
}
 
// A specialization for Key and Mapped strings
template<typename Compare, typename Allocator>
std::ostream& operator<<(std::ostream& stream,
    const std::map<std::string, std::string, Compare, Allocator>& map) {
  stream << '{';
  char comma[3] = {'\0', ' ', '\0'};
  for (const auto& pair : map) {
    stream << comma << '"' << pair.first << "\":\"" << pair.second << '"';
    comma[0] = ',';
  }
  stream << '}';
  return stream;
}

Output:

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

See also

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