cpp/container/map

{"CPU", 10}, {"GPU", 15}, {"RAM", 20}};

   print_map("1) Initial map: ", m);

   m["CPU"] = 25; // update an existing value
   m["SSD"] = 30; // insert a new value
   print_map("2) Updated map: ", m);

   // using operator[] with non-existent key always performs an insert
   std::cout << "3) m[UPS] = " << m["UPS"] << '\n';
   print_map("4) Updated map: ", m);

   m.erase("GPU");
   print_map("5) After erase: ", m);

   std::erase_if(m, [](const auto& pair){ return pair.second > 25; });
   print_map("6) After erase: ", m);
   std::cout << "7) m.size() = " << m.size() << '\n';

   m.clear();
   std::cout << std::boolalpha << "8) Map is empty: " << m.empty() << '\n';

} |output= 1) Initial map: [CPU] = 10; [GPU] = 15; [RAM] = 20; 2) Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30; 3) m[UPS] = 0 4) Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30; [UPS] = 0; 5) After erase: [CPU] = 25; [RAM] = 20; [SSD] = 30; [UPS] = 0; 6) After erase: [CPU] = 25; [RAM] = 20; [UPS] = 0; 7) m.size() = 3 8) 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 230	C++98	Key was not required to be CopyConstructible (a key of type Key might not be able to be constructed)	Key is also required to be CopyConstructible
LWG 464	C++98	accessing a const map by key was inconvenient	at function provided

See also

unordered_map (C++11)

collection of key-value pairs, hashed by keys, keys are unique (class template)

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