string_view in unordered_map

Yes, std::string_view can be used as a key in std::unordered_map, but it requires a custom hash function and comparison function.

std::string_view does not own the data, so you need to ensure the underlying data remains valid for the map's lifetime.

Custom Hash and Comparison Functions

To use std::string_view as a key, you need to define custom hash and comparison functions:

1#include <iostream>
2#include <string_view>
3#include <unordered_map>
4
5struct StringViewHash {
6  std::size_t operator()(
7    std::string_view str
8  ) const noexcept {
9    return std::hash<std::string_view>{}(str);
10  }
11};
12
13struct StringViewEqual {
14  bool operator()(
15    std::string_view lhs,
16    std::string_view rhs
17  ) const noexcept {
18    return lhs == rhs;
19  }
20};
21
22int main() {
23  std::unordered_map<
24    std::string_view, int,
25    StringViewHash, StringViewEqual
26  > map;
27
28  std::string key{"example"};
29  map[key] = 42;
30
31  std::cout << "Value: " << map["example"];
32}

1Value: 42

Ensuring Validity

The underlying string data must remain valid for the entire lifetime of the std::unordered_map entry. Be cautious with temporary strings:

1#include <iostream>
2#include <string_view>
3#include <unordered_map>
4
5struct StringViewHash {
6  std::size_t operator()(
7    std::string_view str
8  ) const noexcept {
9    return std::hash<std::string_view>{}(str);
10  }
11};
12
13struct StringViewEqual {
14  bool operator()(
15    std::string_view lhs,
16    std::string_view rhs
17  ) const noexcept {
18    return lhs == rhs;
19  }
20};
21
22int main() {
23  std::unordered_map<
24    std::string_view,
25    int, StringViewHash,
26    StringViewEqual
27  > map;
28
29  {
30    std::string key{"example"};
31    map[key] = 42;  // Key is valid here
32  }                 // Key is destroyed here
33
34  // Accessing the map with the destroyed key
35  std::cout << "Value: " << map["example"]; 
36}

1undefined behavior or runtime error

Conclusion

Using std::string_view as a key in std::unordered_map can be efficient, but it requires careful management of the underlying string's lifetime.

Ensure the data outlasts the map entries to avoid dangling references and undefined behavior. Custom hash and comparison functions are necessary to make this work.