std::map<Key,T,Compare,Allocator>::map - cppreference.com (2024)

#include <iomanip>#include <iostream>#include <map>#include <string>template<typename Key, typename Value, typename Cmp>std::ostream& operator<<(std::ostream& os, std::map<Key, Value, Cmp> const& map){ os << "{ "; for (auto comma{map.size()}; auto const& 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'; std::cout << "Custom Key class option 2:\n"; // Use a comparison lambda // This lambda sorts points according to their magnitudes, where // 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}); std::cout << "magy = " << magy << '\n'; std::cout << "Construction from a range:\n"; using PS = std::pair<const std::string, int>; const auto rg = {PS{"one", 1}, {"one", 101}, {"two", 2}, {"three", 3}};#if __cpp_lib_containers_ranges std::map<std::string, int> nums(std::from_range, rg); // overload (12)#else std::map<std::string, int> nums(rg.begin(), rg.end()); // fallback to (4)#endif std::cout << "nums = " << nums << '\n';}

map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }iter = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }copied = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }moved = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }map1 = { }init = { 'be' is 100, 'can' is 100, 'const' is 100, 'this' is 100 }Custom Key class option 1:mag = { '(-8, -15)' is 17, '(3, 4)' is 5, '(5, -12)' is 13 }Custom Key class option 2:magy = { '(3, 4)' is 5, '(5, -12)' is 13, '(-8, -15)' is 17 }Construction from a range:nums = { 'one' is 1, 'three' is 3, 'two' is 2 }