Add json::print_error and use it for serialization errors

Summary: To differentiate serialization and deserialization errors.

Reviewed By: yfeldblum

Differential Revision: D27743086

fbshipit-source-id: 8eb4fe1a1733e3effd0aa0478f59160381738db2

folly/json.cpp

@@ -63,11 +63,11 @@ struct Printer {
       case dynamic::DOUBLE:
         if (!opts_.allow_nan_inf) {
           if (std::isnan(v.asDouble())) {
-            throw json::parse_error(
+            throw json::print_error(
                 "folly::toJson: JSON object value was a NaN");
           }
           if (std::isinf(v.asDouble())) {
-            throw json::parse_error(
+            throw json::print_error(
                 "folly::toJson: JSON object value was an INF");
           }
         }
@@ -107,7 +107,7 @@ struct Printer {
  private:
   void printKV(const std::pair<const dynamic, dynamic>& p) const {
     if (!opts_.allow_non_string_keys && !p.first.isString()) {
-      throw json::parse_error(
+      throw json::print_error(
           "folly::toJson: JSON object key was not a "
           "string");
     }

folly/json.h

@@ -168,11 +168,20 @@ void escapeString(
  */
 std::string stripComments(StringPiece jsonC);
 
+// Parse error can be thrown when deserializing json (ie., converting string
+// into json).
 class FOLLY_EXPORT parse_error : public std::runtime_error {
  public:
   using std::runtime_error::runtime_error;
 };
 
+// Print error can be thrown when serializing json (ie., converting json into
+// string).
+class FOLLY_EXPORT print_error : public std::runtime_error {
+ public:
+  using std::runtime_error::runtime_error;
+};
+
 // may be extened in future to include offset, col, etc.
 struct parse_location {
   uint32_t line{}; // 0-indexed

folly/test/JsonTest.cpp

@@ -26,6 +26,7 @@ using folly::parseJson;
 using folly::parseJsonWithMetadata;
 using folly::toJson;
 using folly::json::parse_error;
+using folly::json::print_error;
 
 TEST(Json, Unicode) {
   auto val = parseJson(u8"\"I \u2665 UTF-8\"");
@@ -389,7 +390,7 @@ TEST(Json, Produce) {
 
   // We're not allowed to have non-string keys in json.
   EXPECT_THROW(
-      toJson(dynamic::object("abc", "xyz")(42.33, "asd")), parse_error);
+      toJson(dynamic::object("abc", "xyz")(42.33, "asd")), print_error);
 
   // Check Infinity/Nan
   folly::json::serialization_opts opts;