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

From cppreference.com
< cpp‎ | container‎ | map

[edit template]
 
C++
 
Containers library
 
std::map
Member functions
map::map
             
Non-member functions
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides (C++17)
 
(1)
map();
 (until C++11)
map() : map(Compare()) {}
 (since C++11)
(constexpr since C++26)
explicit map( const Compare& comp,
              const Allocator& alloc = Allocator() );
 (2) (constexpr since C++26)
explicit map( const Allocator& alloc );
 (3) (since C++11)
(constexpr since C++26)
template< class InputIt >

map( InputIt first, InputIt last,
     const Compare& comp = Compare(),

     const Allocator& alloc = Allocator() );
 (4) (constexpr since C++26)
template< class InputIt >

map( InputIt first, InputIt last,
     const Allocator& alloc )

    : map(first, last, Compare(), alloc) {}
 (5) (since C++14)
(constexpr since C++26)
map( const map& other );
 (6) (constexpr since C++26)
map( const map& other, const Allocator& alloc );
 (7) (since C++11)
(constexpr since C++26)
map( map&& other );
 (8) (since C++11)
(constexpr since C++26)
map( map&& other, const Allocator& alloc );
 (9) (since C++11)
(constexpr since C++26)
map( std::initializer_list<value_type> init,

     const Compare& comp = Compare(),

     const Allocator& alloc = Allocator() );
 (10) (since C++11)
(constexpr since C++26)
map( std::initializer_list<value_type> init,

     const Allocator& alloc )

    : map(init, Compare(), alloc) {}
 (11) (since C++14)
(constexpr since C++26)
template< container-compatible-range<value_type> R >

map( std::from_range_t, R&& rg,
     const Compare& comp = Compare(),

     const Allocator& alloc = Allocator() );
 (12) (since C++23)
(constexpr since C++26)
template< container-compatible-range<value_type> R >

map( std::from_range_t, R&& rg,
     const Allocator& alloc )

    : map(std::from_range, std::forward<R>(rg), Compare(), alloc) {}
 (13) (since C++23)
(constexpr since C++26)

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

1-3) Constructs an empty container.
4,5) Constructs the container with the contents of the range [first, last).
If [first, last) is not a valid range, the behavior is undefined.
6,7) 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()).

(since C++11)

During class template argument deduction, only the first argument contributes to the deduction of the container's Allocator template parameter.

(since C++23)
8,9) 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.

During class template argument deduction, only the first argument contributes to the deduction of the container's Allocator template parameter.

(since C++23)
10,11) Constructs the container with the contents of the initializer list init.
12,13) Constructs the container with the contents of rg.

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 pair of iterators defining the source range of elements to copy
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
rg - a container compatible range, that is, an input_range whose elements are convertible to value_type
Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.
-
Compare must meet the requirements of Compare.
-
Allocator must meet the requirements of Allocator.

[edit] Complexity

1-3) Constant.
4,5) N·log(N) where N is std::distance(first, last) in general, linear in N if [first, last) is already sorted by value_comp().
6,7) Linear in size of other.
8,9) Constant. If alloc is given and alloc != other.get_allocator(), then linear.
10,11) N·log(N) where N is init.size() in general, linear in N if init is already sorted by value_comp().
12,13) N·log(N) where N is ranges::distance(rg) in general, linear in N if rg is already sorted by value_comp().

[edit] Exceptions

Calls to Allocator::allocate may throw.

[edit] Notes

After container move construction (overload (8,9)), 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 [container.reqmts]/67, and a more direct guarantee is under consideration via LWG issue 2321.

If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).

Although not formally required until C++23, some implementations has already put the template parameter Allocator into non-deduced contexts in earlier modes.

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion; overloads (12,13)

[edit] Example

Run this code
#include <iomanip>
#include <iostream>
#include <map>
#include <string>
 
template<typename Key, typename Value, typename Cmp>
std::ostream& operator<<(std::ostream& os, const std::map<Key, Value, Cmp>& map)
{
    os << "{ ";
    for (auto comma{map.size()}; const auto& p : map)
        os << '\'' << p.first << "' is " << p.second << (--comma ? ", " : " ");
    return os << "}\n";
}
 
struct Point
{
    double x, y;
 
    friend std::ostream& operator<<(std::ostream& os, Point pt)
    {
        return os << '(' << pt.x << ", " << pt.y << ')';
    }
};
 
struct PointCmp
{
    bool operator()(const Point& lhs, const Point& rhs) const
    {
        return lhs.x < rhs.x; // NB: y is intentionally ignored
    }
};
 
int main()
{
    // (1) Default constructor
    std::map<std::string, int> map1;
    map1["something"] = 69;
    map1["anything"] = 199;
    map1["that thing"] = 50;
    std::cout << "map1 = " << map1;
 
    // (4) Range constructor
    std::map<std::string, int> iter(map1.find("anything"), map1.end());
    std::cout << "\niter = " << iter;
    std::cout << "map1 = " << map1;
 
    // (6) Copy constructor
    std::map<std::string, int> copied(map1);
    std::cout << "\ncopied = " << copied;
    std::cout << "map1 = " << map1;
 
    // (8) Move constructor
    std::map<std::string, int> moved{std::move(map1)};
    std::cout << "\nmoved = " << moved;
    std::cout << "map1 = " << map1;
 
    // (10) Initializer list constructor
    const std::map<std::string, int> init
    {
        {"this", 100},
        {"can", 100},
        {"be", 100},
        {"const", 100}
    };
    std::cout << "\ninit = " << init;
 
    std::cout << "\nCustom Key class option 1:\n";
    // Use a comparison struct
    std::map<Point, double, PointCmp> mag =
    {
        {{5, -12}, 13},
        {{3, 4}, 5},
        {{-8, -15}, 17}
    };
    std::cout << "mag = " << mag << '\n';