std::map::map(3)

NAME
std::map::map - std::map::map

Synopsis
   map(); (1)
   explicit map( const Compare& comp, (2)
   const Allocator& alloc = Allocator() );
   explicit map( const Allocator& alloc ); (3) (since C++11)
   template< class InputIt >
   map( InputIt first, InputIt last, (4)
   const Compare& comp = Compare(),
   const Allocator& alloc = Allocator() );
   template< class InputIt >
   map( InputIt first, InputIt last, (5) (since C++14)
   const Allocator& alloc );
   map( const map& other ); (6)
   map( const map& other, const Allocator& alloc ); (7) (since C++11)
   map( map&& other ); (8) (since C++11)
   map( map&& other, const Allocator& alloc ); (9) (since C++11)
   map( std::initializer_list<value_type> init,
   const Compare& comp = Compare(), (10) (since C++11)
   const Allocator& alloc = Allocator() );
   map( std::initializer_list<value_type> init, (11) (since C++14)
   const Allocator& );
   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
   multiple elements in the range have keys that compare equivalent, it is unspecified
   which element is inserted (pending LWG2844).
   6-7) Copy constructor. Constructs the container with the copy of the contents of
   other.
   If alloc is not provided, allocator is obtained by calling (since
   std::allocator_traits<allocator_type>::select_on_container_copy_construction( C++11)
   other.get_allocator()).
   The template parameter Allocator is only deduced from the first argument (since
   while used in class template argument deduction. C++23)
   8-9) 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.
   The template parameter Allocator is only deduced from the first (since C++23)
   argument while used in class template argument deduction.
   10-11) 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
   -
   InputIt must meet the requirements of LegacyInputIterator.
   -
   Compare must meet the requirements of Compare.
   -
   Allocator must meet the requirements of Allocator.

Complexity
   1-3) Constant
   4-5) N log(N) where N = std::distance(first, last) in general, linear in N if the
   range 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 = init.size() in general, linear in N if init is already
   sorted by value_comp().

Exceptions
   Calls to Allocator::allocate may throw.

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.requirements.general]/12, and a more direct guarantee is
   under consideration via LWG 2321.
   Although not formally required until C++23, some implementations has already put the
   template parameter Allocator into non-deduced contexts in earlier modes.

Example
// Run this code
 #include <iomanip>
 #include <iostream>
 #include <string>
 #include <map>
 template<typename Key, typename Value>
 std::ostream& operator<<(std::ostream& os, std::map<Key, Value> const& m)
 {
    os << "{ ";
    for(auto const& p: m)
         os << '(' << p.first << ':' << p.second << ") ";
    return os << "}\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 = " << 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 }
   };
   for(auto p : mag)
       std::cout << "The magnitude of (" << p.first.x
                 << ", " << p.first.y << ") is "
                 << p.second << '\n';
   std::cout << "\nCustom Key class option 2:\n";
   // 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});
   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) }
 Custom Key class option 1:
 The magnitude of (-8, -15) is 17
 The magnitude of (3, 4) is 5
 The magnitude of (5, -12) is 13
 Custom Key class option 2:
 The magnitude of (3, 4) is 5
 The magnitude of (5, -12) is 13
 The magnitude of (-8, -15) is 17
  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 2193 C++11 the default constructor is explicit made non-explicit

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