+/**
+ * Exception class thrown when a format key is not found in the given
+ * associative container keyed by strings. We inherit std::out_of_range for
+ * compatibility with callers that expect exception to be thrown directly
+ * by std::map or std::unordered_map.
+ *
+ * Having the key be at the end of the message string, we can access it by
+ * simply adding its offset to what(). Not storing separate std::string key
+ * makes the exception type small and noexcept-copyable like std::out_of_range,
+ * and therefore able to fit in-situ in exception_wrapper.
+ */
+class FormatKeyNotFoundException : public std::out_of_range {
+ public:
+ explicit FormatKeyNotFoundException(StringPiece key);
+
+ char const* key() const noexcept {
+ return what() + kMessagePrefix.size();
+ }
+
+ private:
+ static constexpr StringPiece const kMessagePrefix = "format key not found: ";
+};
+
+namespace detail {
+[[noreturn]] void throwFormatKeyNotFoundException(StringPiece key);
+} // namespace detail
+