podio/utilities/StringKeyMap.h

0001 #ifndef PODIO_UTILITIES_STRINGKEYMAP_H
0002 #define PODIO_UTILITIES_STRINGKEYMAP_H
0003
0004 #include <functional>
0005 #include <string>
0006 #include <string_view>
0007 #include <unordered_map>
0008
0009 namespace podio {
0010
0011 namespace detail {
0012
0013   /// Transparent hash for @c std::string keyed @c std::unordered_map that enables
0014   /// heterogeneous lookup with @c std::string_view (and @c const @c char*) without
0015   /// constructing a temporary @c std::string for each lookup. Must be used together
0016   /// with @c std::equal_to<> as the key equality comparator so that the same
0017   /// key-comparison transparency applies.
0018   struct TransparentStringHash {
0019     using is_transparent = void;
0020     // We only need the string_View overload because a string converts to that
0021     std::size_t operator()(std::string_view sv) const noexcept {
0022       return std::hash<std::string_view>{}(sv);
0023     }
0024   };
0025
0026 } // namespace detail
0027
0028 /// A convenience alias for @c std::unordered_map<std::string, Value> that
0029 /// supports heterogeneous lookup.
0030 ///
0031 /// Keys can be looked up (via @c find, @c count, @c contains, ...) using any
0032 /// type that is implicitly convertible to @c std::string_view — most notably
0033 /// @c std::string_view and plain string literals — without constructing a
0034 /// temporary @c std::string. This is achieved by pairing @c
0035 /// podio::detail::TransparentStringHash with @c std::equal_to<> (the
0036 /// "transparent" equality comparator introduced in C++14).
0037 ///
0038 /// @note Insertion operations (@c try_emplace, @c operator[], …) still require
0039 /// a @c std::string key since that is the map's @c key_type. Pass
0040 /// @c std::string(sv) explicitly when inserting from a @c std::string_view.
0041 template <typename Value>
0042 using StringKeyMap = std::unordered_map<std::string, Value, detail::TransparentStringHash, std::equal_to<>>;
0043 } // namespace podio
0044
0045 #endif