std::unordered_map vs std::map - When to Use Each

The choice between std::unordered_map and std::map depends on your specific requirements and the characteristics of your data. Here are some guidelines to help you decide when to use each:

Use std::unordered_map when:

1. You need fast average-case lookup, insertion, and deletion operations. std::unordered_map provides average constant-time complexity (O(1)) for these operations.
2. The order of elements is not important. std::unordered_map does not maintain any particular order of elements.
3. You don't need to perform range-based operations or retrieve elements in sorted order.

Use std::map when:

1. You need to maintain elements in sorted order based on their keys. std::map is an ordered associative container that keeps elements sorted by keys.
2. You require logarithmic-time complexity (O(log N)) for lookup, insertion, and deletion operations. std::map provides these guarantees.
3. You need to perform range-based operations, such as finding all elements within a specific range of keys.

Here's an example illustrating a scenario where std::map is preferred:

1#include <iostream>
2#include <map>
3#include <string>
4
5int main() {
6  std::map<std::string, int> ageMap{
7    {"Alice", 25}, {"Bob", 30},
8    {"Charlie", 35}, {"David", 40}};
9
10  auto lower = ageMap.lower_bound("B");  
11  auto upper = ageMap.upper_bound("D");  
12
13  for (auto it = lower; it != upper; ++it) {
14    std::cout << it->first << ": "
15      << it->second << '\n';
16  }
17}

1Bob: 30
2Charlie: 35

In this example, std::map is chosen because we want to retrieve people within a specific range of names (keys). The lower_bound() and upper_bound() functions efficiently find the range of elements based on the sorted order of keys.

On the other hand, if fast lookup is the primary concern and the order of elements is not important, std::unordered_map would be a better choice.

1#include <iostream>
2#include <unordered_map>
3#include <string>
4
5int main() {
6  std::unordered_map<
7    std::string, std::string> phoneBook{
8    {"Alice", "1234567890"},
9    {"Bob", "9876543210"},
10    {"Charlie", "5555555555"}
11  };
12
13  // Lookup phone number by name
14  std::string name = "Bob";
15  auto it = phoneBook.find(name);
16  if (it != phoneBook.end()) {
17    std::cout << "Phone number of " << name
18      << ": " << it->second << '\n';
19  }
20}

1Phone number of Bob: 9876543210

In this case, std::unordered_map provides fast lookup of phone numbers based on names, which is the primary operation required.

Consider the specific requirements of your application, such as the need for ordering, range-based operations, and the expected performance of different operations, when deciding between std::unordered_map and std::map.