Make associative container out-of-range exception provide missing key

[folly.git] / folly / Format.h

diff --git a/folly/Format.h b/folly/Format.h
index 3837d1079e9636b31d529a32d39f990e0805057d..b8df86fbfa1e8d02c54251011ea3b570df62e892 100644 (file)
--- a/folly/Format.h
+++ b/folly/Format.h
@@ -18,6 +18,7 @@
 #define FOLLY_FORMAT_H_
 
 #include <cstdio>
 #define FOLLY_FORMAT_H_
 
 #include <cstdio>

+#include <stdexcept>

 #include <tuple>
 #include <type_traits>
 
 #include <tuple>
 #include <type_traits>
 


@@ -306,6 +307,33 @@ inline std::string svformat(StringPiece fmt, Container&& container) {
   return vformat(fmt, std::forward<Container>(container)).str();
 }
 
   return vformat(fmt, std::forward<Container>(container)).str();
 }
 

+/**
+ * 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
+

 /**
  * Wrap a sequence or associative container so that out-of-range lookups
  * return a default value rather than throwing an exception.
 /**
  * Wrap a sequence or associative container so that out-of-range lookups
  * return a default value rather than throwing an exception.