Tag: unordered_map

I often use std::string_view via the sv suffix for string constants in my code. If I need to associate something with those constants at runtime, I put it in an std::unordered_map with the constants as the keys.

Just a few days ago, I was using and std::unordered_map<std::string, ...> and wanted to .find(...) something in it with such a string constant. But that didn’t compile. From long ago, I remember that the type must be identical, and since there is no implicit conversion from std::string_view to std::string, I made that explicit to get it to compile. But wait. Didn’t C++ add support for using a different type than the key_type for the lookup? Indeed it did, in P0919R3 and P1690R1 from last decade. All major compilers seem to support it too. Then why wasn’t this working? It turns out that it’s not enabled by default, you need to explicitly enable it by supplying a special hasher. Here’s how I do it:

struct stringly_hash
{
  using is_transparent = void;
  [[nodiscard]] size_t operator()(char const* rhs) const
  {
    return std::hash<std::string_view>{}(rhs);
  }
  [[nodiscard]] size_t operator()(std::string_view rhs) const
  {
    return std::hash<std::string_view>{}(rhs);
  }
  [[nodiscard]] size_t operator()(std::string const& rhs) const
  {
    return std::hash<std::string>{}(rhs);
  }
};

template <typename ValueType>
using unordered_string_map = std::unordered_map<
  std::string,
  ValueType,
  stringly_hash,
  std::equal_to<>
>;

This is almost the same code as the sample given in the first of the two proposals. The using is_transparent = void; is how the feature is enabled and was changed in the second proposal.