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std::map::map(3) C++ Standard Libary 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 option- ally 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_con- struction( C++11) other.get_allocator()). The template parameter Allocator is only deduced from the first ar- gument (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, lin- ear 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 vari- ables, 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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct be- havior LWG 2193 C++11 the default constructor is explicit made non-ex- plicit See also operator= assigns values to the container (public member function) http://cppreference.com 2022.07.31 std::map::map(3)
NAME | Synopsis | Parameters | Type requirements | Complexity | Exceptions | Notes | Example | Output: | See also
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