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Difference between revisions of "cpp/container/map"

From cppreference.com
< cpp‎ | container
(when the example made sense, it showed four three basic operations: create, insert, iterate (was missing erase but ok). Now it's 90% history lesson/beginner tutorial)
m (→‎Example: +clear, +empty; fmt.)
Line 115: Line 115:
 
#include <string>
 
#include <string>
 
#include <string_view>
 
#include <string_view>
−
+
 
 
void print_map(std::string_view comment, const std::map<std::string, int>& m)
 
void print_map(std::string_view comment, const std::map<std::string, int>& m)
 
{
 
{
−
     std::cout << comment << "\n";
+
     std::cout << comment ;
 
     // iterate using C++17 facilities
 
     // iterate using C++17 facilities
 
     for (const auto& [key, value] : m) {
 
     for (const auto& [key, value] : m) {
−
         std::cout << key << " = " << value << "; ";
+
         std::cout << '[' << key << "] = " << value << "; ";
 
     }
 
     }
−
    std::cout << "\n";
 
 
// C++11 alternative:
 
// C++11 alternative:
 
//  for (const auto& n : m) {
 
//  for (const auto& n : m) {
Line 132: Line 131:
 
//      std::cout << it->first << " = " << it->second << "; ";
 
//      std::cout << it->first << " = " << it->second << "; ";
 
//  }
 
//  }
 +
    std::cout << "\n\n";
 
}
 
}
−
+
 
 
int main()
 
int main()
 
{
 
{
 
     // Create a map of three (strings, int) pairs
 
     // Create a map of three (strings, int) pairs
 
     std::map<std::string, int> m { {"CPU", 10}, {"GPU", 15}, {"RAM", 20}, };
 
     std::map<std::string, int> m { {"CPU", 10}, {"GPU", 15}, {"RAM", 20}, };
−
+
 
 
     print_map("Initial map: ", m);
 
     print_map("Initial map: ", m);
−
    std::cout << "\n";
+
 
−
+
 
     m["CPU"] = 25;  // update an existing value
 
     m["CPU"] = 25;  // update an existing value
 
     m["SSD"] = 30;  // insert a new value
 
     m["SSD"] = 30;  // insert a new value
 
     print_map("Updated map: ", m);
 
     print_map("Updated map: ", m);
−
    std::cout << "\n";
+
 
−
+
 
     // using operator[] with non-existent key always performs an insert
 
     // using operator[] with non-existent key always performs an insert
−
     std::cout << "m[new_key] = " << m["new_key"] << "\n";
+
     std::cout << "m[UPS] = " << m["UPS"] << '\n';
 
     print_map("Updated map: ", m);
 
     print_map("Updated map: ", m);
 +
 +
    m.clear();
 +
    std::cout << std::boolalpha << "Map is empty: " << m.empty() << '\n';
 
}
 
}
−
| output=
+
| output=
−
Initial map:  
+
Initial map: [CPU] = 10; [GPU] = 15; [RAM] = 20;
−
CPU = 10; GPU = 15; RAM = 20;  
+
 
 +
Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30;  
  
−
Updated map:  
+
m[UPS] = 0
−
CPU = 25; GPU = 15; RAM = 20; SSD = 30;  
+
Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30; [UPS] = 0;  
  
−
m[new_key] = 0
+
Map is empty: true
−
Updated map:  
+
−
CPU = 25; GPU = 15; RAM = 20; SSD = 30; new_key = 0;
+
 
}}
 
}}
  

Revision as of 14:33, 26 January 2022

 
C++
 
Containers library
 
std::map
Member functions
Non-member functions
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides (C++17)
 
Defined in header <map>
template<

    class Key,
    class T,
    class Compare = std::less<Key>,
    class Allocator = std::allocator<std::pair<const Key, T> >

> class map;
 (1)
namespace pmr {

    template <class Key, class T, class Compare = std::less<Key>>
    using map = std::map<Key, T, Compare,
                         std::pmr::polymorphic_allocator<std::pair<const Key,T>>>

}
 (2) (since C++17)

std::map is a sorted associative container that contains key-value pairs with unique keys. Keys are sorted by using the comparison function Compare. Search, removal, and insertion operations have logarithmic complexity. Maps are usually implemented as red-black trees.

Everywhere the standard library uses the Compare requirements, uniqueness is determined by using the equivalence relation. In imprecise terms, two objects a and b are considered equivalent (not unique) if neither compares less than the other: !comp(a, b) && !comp(b, a).

std::map meets the requirements of Container, AllocatorAwareContainer, AssociativeContainer and ReversibleContainer.

Contents

Member types

Member type Definition
key_type Key[edit]
mapped_type T[edit]
value_type std::pair<const Key, T>[edit]
size_type Unsigned integer type (usually std::size_t)[edit]
difference_type Signed integer type (usually std::ptrdiff_t)[edit]
key_compare Compare[edit]
allocator_type Allocator[edit]
reference value_type&[edit]
const_reference const value_type&[edit]
pointer

Allocator::pointer

(until C++11)

std::allocator_traits<Allocator>::pointer

(since C++11)
[edit]
const_pointer

Allocator::const_pointer

(until C++11)

std::allocator_traits<Allocator>::const_pointer

(since C++11)
[edit]
iterator LegacyBidirectionalIterator to value_type[edit]
const_iterator LegacyBidirectionalIterator to const value_type[edit]
reverse_iterator std::reverse_iterator<iterator>[edit]
const_reverse_iterator std::reverse_iterator<const_iterator>[edit]
node_type (since C++17) a specialization of node handle representing a container node[edit]
insert_return_type (since C++17) type describing the result of inserting a node_type, a specialization of

template<class Iter, class NodeType>
struct /*unspecified*/
{
    Iter     position;
    bool     inserted;
    NodeType node;
};
instantiated with template arguments iterator and node_type.[edit]

Member classes

 compares objects of type value_type
(class) [edit]

Member functions

 constructs the map
(public member function) [edit]
destructs the map
(public member function) [edit]
assigns values to the container
(public member function) [edit]
returns the associated allocator
(public member function) [edit]
Element access
access specified element with bounds checking
(public member function) [edit]
access or insert specified element
(public member function) [edit]
Iterators
(C++11)
 returns an iterator to the beginning
(public member function) [edit]
(C++11)
 returns an iterator to the end
(public member function) [edit]
(C++11)
 returns a reverse iterator to the beginning
(public member function) [edit]
(C++11)
 returns a reverse iterator to the end
(public member function) [edit]
Capacity
checks whether the container is empty
(public member function) [edit]
returns the number of elements
(public member function) [edit]
returns the maximum possible number of elements
(public member function) [edit]
Modifiers
clears the contents
(public member function) [edit]
inserts elements or nodes(since C++17)
(public member function) [edit]
(C++17)
 inserts an element or assigns to the current element if the key already exists
(public member function) [edit]
(C++11)
 constructs element in-place
(public member function) [edit]
(C++11)
 constructs elements in-place using a hint
(public member function) [edit]
(C++17)
 inserts in-place if the key does not exist, does nothing if the key exists
(public member function) [edit]
erases elements
(public member function) [edit]
swaps the contents
(public member function) [edit]
(C++17)
 extracts nodes from the container
(public member function) [edit]
(C++17)
 splices nodes from another container
(public member function) [edit]
Lookup
returns the number of elements matching specific key
(public member function) [edit]
finds element with specific key
(public member function) [edit]
(C++20)
 checks if the container contains element with specific key
(public member function) [edit]
returns range of elements matching a specific key
(public member function) [edit]
returns an iterator to the first element not less than the given key
(public member function) [edit]
returns an iterator to the first element greater than the given key
(public member function) [edit]
Observers
returns the function that compares keys
(public member function) [edit]
returns the function that compares keys in objects of type value_type
(public member function) [edit]

Non-member functions

(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
 lexicographically compares the values of two maps
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]
(C++20)
 erases all elements satisfying specific criteria
(function template) [edit]

Deduction guides (since C++17)

Example

Run this code
#include <iostream>
#include <map>
#include <string>
#include <string_view>
 
void print_map(std::string_view comment, const std::map<std::string, int>& m)
{
    std::cout << comment ;
    // iterate using C++17 facilities
    for (const auto& [key, value] : m) {
        std::cout << '[' << key << "] = " << value << "; ";
    }
// C++11 alternative:
//  for (const auto& n : m) {
//      std::cout << n.first << " = " << n.second << "; ";
//  }
// C++98 alternative
//  for (std::map<std::string, int>::const_iterator it = m.begin(); it != m.end(); it++) {
//      std::cout << it->first << " = " << it->second << "; ";
//  }
    std::cout << "\n\n";
}
 
int main()
{
    // Create a map of three (strings, int) pairs
    std::map<std::string, int> m { {"CPU", 10}, {"GPU", 15}, {"RAM", 20}, };
 
    print_map("Initial map: ", m);
 
    m["CPU"] = 25;  // update an existing value
    m["SSD"] = 30;  // insert a new value
    print_map("Updated map: ", m);
 
    // using operator[] with non-existent key always performs an insert
    std::cout << "m[UPS] = " << m["UPS"] << '\n';
    print_map("Updated map: ", m);
 
    m.clear();
    std::cout << std::boolalpha << "Map is empty: " << m.empty() << '\n';
}

Output: 

Initial map: [CPU] = 10; [GPU] = 15; [RAM] = 20; 
 
Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30; 
 
m[UPS] = 0
Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30; [UPS] = 0; 
 
Map is empty: true

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 464 C++98 accessing a const map by key was inconvenient at function provided

See also

(C++11)
 collection of key-value pairs, hashed by keys, keys are unique
(class template) [edit]
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