std::map::map

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< cpp‎ | container‎ | map

 
C++
 
Containers library
 
std::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 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.

Contents

[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

assigns values to the container
(public member function)
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