don't use std::function in flatbuffers::Parser (#4995)

std::function makes code harder to debug because it requires stepping
through a separate destructor and call operator.  It's use unnecessary
in the Parser since the functions taking functors are private and are
only used within idl_parser.cpp.  Therefore the definitions can stay in
idl_parser.cpp as well.  Only care must be taken that the definitions
appear before use but that's already true and all compilers will
complain equally if it get's violated.  This change might also improve
performance since it might allow inlining where it wasn't possible
before but I haven't measured that.

include/flatbuffers/idl.h

@@ -682,35 +682,15 @@ class Parser : public ParserState {
   FLATBUFFERS_CHECKED_ERROR ParseAnyValue(Value &val, FieldDef *field,
                                           size_t parent_fieldn,
                                           const StructDef *parent_struct_def);
-  // clang-format off
-  #if defined(FLATBUFFERS_CPP98_STL)
-    typedef CheckedError (*ParseTableDelimitersBody)(
-        const std::string &name, size_t &fieldn, const StructDef *struct_def,
-        void *state);
-  #else
-    typedef std::function<CheckedError(const std::string&, size_t&,
-                                       const StructDef*, void*)>
-            ParseTableDelimitersBody;
-  #endif  // defined(FLATBUFFERS_CPP98_STL)
-  // clang-format on
+  template<typename F>
   FLATBUFFERS_CHECKED_ERROR ParseTableDelimiters(size_t &fieldn,
                                                  const StructDef *struct_def,
-                                                 ParseTableDelimitersBody body,
-                                                 void *state);
+                                                 F body);
   FLATBUFFERS_CHECKED_ERROR ParseTable(const StructDef &struct_def,
                                        std::string *value, uoffset_t *ovalue);
   void SerializeStruct(const StructDef &struct_def, const Value &val);
-  // clang-format off
-  #if defined(FLATBUFFERS_CPP98_STL)
-    typedef CheckedError (*ParseVectorDelimitersBody)(size_t &count,
-                                                      void *state);
-  #else
-    typedef std::function<CheckedError(size_t&, void*)>
-            ParseVectorDelimitersBody;
-  #endif  // defined(FLATBUFFERS_CPP98_STL)
-  // clang-format on
-  FLATBUFFERS_CHECKED_ERROR ParseVectorDelimiters(
-      size_t &count, ParseVectorDelimitersBody body, void *state);
+  template<typename F>
+  FLATBUFFERS_CHECKED_ERROR ParseVectorDelimiters(size_t &count, F body);
   FLATBUFFERS_CHECKED_ERROR ParseVector(const Type &type, uoffset_t *ovalue);
   FLATBUFFERS_CHECKED_ERROR ParseNestedFlatbuffer(Value &val, FieldDef *field,
                                                   size_t fieldn,
@@ -761,14 +741,7 @@ class Parser : public ParserState {
   Namespace *UniqueNamespace(Namespace *ns);
 
   FLATBUFFERS_CHECKED_ERROR RecurseError();
-  template<typename F> CheckedError Recurse(F f) {
-    if (recurse_protection_counter >= (FLATBUFFERS_MAX_PARSING_DEPTH))
-      return RecurseError();
-    recurse_protection_counter++;
-    auto ce = f();
-    recurse_protection_counter--;
-    return ce;
-  }
+  template<typename F> CheckedError Recurse(F f);
 
  public:
   SymbolTable<Type> types_;

src/idl_parser.cpp

@@ -119,6 +119,15 @@ CheckedError Parser::RecurseError() {
                NumToString(FLATBUFFERS_MAX_PARSING_DEPTH) + " reached");
 }
 
+template<typename F> CheckedError Parser::Recurse(F f) {
+  if (recurse_protection_counter >= (FLATBUFFERS_MAX_PARSING_DEPTH))
+    return RecurseError();
+  recurse_protection_counter++;
+  auto ce = f();
+  recurse_protection_counter--;
+  return ce;
+}
+
 CheckedError Parser::InvalidNumber(const char *number, const std::string &msg) {
   return Error("invalid number: \"" + std::string(number) + "\"" + msg);
 }
@@ -927,10 +936,10 @@ void Parser::SerializeStruct(const StructDef &struct_def, const Value &val) {
   builder_.AddStructOffset(val.offset, builder_.GetSize());
 }
 
+template <typename F>
 CheckedError Parser::ParseTableDelimiters(size_t &fieldn,
                                           const StructDef *struct_def,
-                                          ParseTableDelimitersBody body,
-                                          void *state) {
+                                          F body) {
   // We allow tables both as JSON object{ .. } with field names
   // or vector[..] with all fields in order
   char terminator = '}';
@@ -958,7 +967,7 @@ CheckedError Parser::ParseTableDelimiters(size_t &fieldn,
       }
       if (!opts.protobuf_ascii_alike || !(Is('{') || Is('['))) EXPECT(':');
     }
-    ECHECK(body(name, fieldn, struct_def, state));
+    ECHECK(body(name, fieldn, struct_def));
     if (Is(terminator)) break;
     ECHECK(ParseComma());
   }
@@ -974,66 +983,60 @@ CheckedError Parser::ParseTable(const StructDef &struct_def, std::string *value,
   size_t fieldn_outer = 0;
   auto err = ParseTableDelimiters(
       fieldn_outer, &struct_def,
-      [](const std::string &name, size_t &fieldn,
-         const StructDef *struct_def_inner, void *state) -> CheckedError {
-        auto *parser = static_cast<Parser *>(state);
+      [&](const std::string &name, size_t &fieldn,
+          const StructDef *struct_def_inner) -> CheckedError {
         if (name == "$schema") {
-          ECHECK(parser->Expect(kTokenStringConstant));
+          ECHECK(Expect(kTokenStringConstant));
           return NoError();
         }
         auto field = struct_def_inner->fields.Lookup(name);
         if (!field) {
-          if (!parser->opts.skip_unexpected_fields_in_json) {
-            return parser->Error("unknown field: " + name);
+          if (!opts.skip_unexpected_fields_in_json) {
+            return Error("unknown field: " + name);
           } else {
-            ECHECK(parser->SkipAnyJsonValue());
+            ECHECK(SkipAnyJsonValue());
           }
         } else {
-          if (parser->IsIdent("null") &&
-              !IsScalar(field->value.type.base_type)) {
-            ECHECK(parser->Next());  // Ignore this field.
+          if (IsIdent("null") && !IsScalar(field->value.type.base_type)) {
+            ECHECK(Next());  // Ignore this field.
           } else {
             Value val = field->value;
             if (field->flexbuffer) {
               flexbuffers::Builder builder(1024,
                                            flexbuffers::BUILDER_FLAG_SHARE_ALL);
-              ECHECK(parser->ParseFlexBufferValue(&builder));
+              ECHECK(ParseFlexBufferValue(&builder));
               builder.Finish();
               // Force alignment for nested flexbuffer
-              parser->builder_.ForceVectorAlignment(builder.GetSize(), sizeof(uint8_t),
-                                                    sizeof(largest_scalar_t));
-              auto off = parser->builder_.CreateVector(builder.GetBuffer());
+              builder_.ForceVectorAlignment(builder.GetSize(), sizeof(uint8_t),
+                                            sizeof(largest_scalar_t));
+              auto off = builder_.CreateVector(builder.GetBuffer());
               val.constant = NumToString(off.o);
             } else if (field->nested_flatbuffer) {
-              ECHECK(parser->ParseNestedFlatbuffer(val, field, fieldn,
-                                                   struct_def_inner));
+              ECHECK(
+                  ParseNestedFlatbuffer(val, field, fieldn, struct_def_inner));
             } else {
-              ECHECK(parser->Recurse([&]() {
-                  return parser->ParseAnyValue(val, field, fieldn,
-                                               struct_def_inner);
+              ECHECK(Recurse([&]() {
+                return ParseAnyValue(val, field, fieldn, struct_def_inner);
               }));
             }
             // Hardcoded insertion-sort with error-check.
             // If fields are specified in order, then this loop exits
             // immediately.
-            auto elem = parser->field_stack_.rbegin();
-            for (; elem != parser->field_stack_.rbegin() + fieldn; ++elem) {
+            auto elem = field_stack_.rbegin();
+            for (; elem != field_stack_.rbegin() + fieldn; ++elem) {
               auto existing_field = elem->second;
               if (existing_field == field)
-                return parser->Error("field set more than once: " +
-                                     field->name);
+                return Error("field set more than once: " + field->name);
               if (existing_field->value.offset < field->value.offset) break;
             }
             // Note: elem points to before the insertion point, thus .base()
             // points to the correct spot.
-            parser->field_stack_.insert(elem.base(),
-                                        std::make_pair(val, field));
+            field_stack_.insert(elem.base(), std::make_pair(val, field));
             fieldn++;
           }
         }
         return NoError();
-      },
-      this);
+      });
   ECHECK(err);
 
   // Check if all required fields are parsed.
@@ -1130,13 +1133,12 @@ CheckedError Parser::ParseTable(const StructDef &struct_def, std::string *value,
   return NoError();
 }
 
-CheckedError Parser::ParseVectorDelimiters(size_t &count,
-                                           ParseVectorDelimitersBody body,
-                                           void *state) {
+template <typename F>
+CheckedError Parser::ParseVectorDelimiters(size_t &count, F body) {
   EXPECT('[');
   for (;;) {
     if ((!opts.strict_json || !count) && Is(']')) break;
-    ECHECK(body(count, state));
+    ECHECK(body(count));
     count++;
     if (Is(']')) break;
     ECHECK(ParseComma());
@@ -1147,22 +1149,13 @@ CheckedError Parser::ParseVectorDelimiters(size_t &count,
 
 CheckedError Parser::ParseVector(const Type &type, uoffset_t *ovalue) {
   size_t count = 0;
-  std::pair<Parser *, const Type &> parser_and_type_state(this, type);
-  auto err = ParseVectorDelimiters(
-      count,
-      [](size_t &, void *state) -> CheckedError {
-        auto *parser_and_type =
-            static_cast<std::pair<Parser *, const Type &> *>(state);
-        auto *parser = parser_and_type->first;
-        Value val;
-        val.type = parser_and_type->second;
-        ECHECK(parser->Recurse([&]() {
-                 return parser->ParseAnyValue(val, nullptr, 0, nullptr);
-        }));
-        parser->field_stack_.push_back(std::make_pair(val, nullptr));
-        return NoError();
-      },
-      &parser_and_type_state);
+  auto err = ParseVectorDelimiters(count, [&](size_t &) -> CheckedError {
+    Value val;
+    val.type = type;
+    ECHECK(Recurse([&]() { return ParseAnyValue(val, nullptr, 0, nullptr); }));
+    field_stack_.push_back(std::make_pair(val, nullptr));
+    return NoError();
+  });
   ECHECK(err);
 
   builder_.StartVector(count * InlineSize(type) / InlineAlignment(type),
@@ -2220,28 +2213,18 @@ CheckedError Parser::SkipAnyJsonValue() {
       size_t fieldn_outer = 0;
       return ParseTableDelimiters(
           fieldn_outer, nullptr,
-          [](const std::string &, size_t &fieldn, const StructDef *,
-             void *state) -> CheckedError {
-            auto *parser = static_cast<Parser *>(state);
-            ECHECK(parser->Recurse([&]() {
-              return parser->SkipAnyJsonValue();
-            }));
+          [&](const std::string &, size_t &fieldn,
+              const StructDef *) -> CheckedError {
+            ECHECK(Recurse([&]() { return SkipAnyJsonValue(); }));
             fieldn++;
             return NoError();
-          },
-          this);
+          });
     }
     case '[': {
       size_t count = 0;
-      return ParseVectorDelimiters(
-          count,
-          [](size_t &, void *state) -> CheckedError {
-            auto *parser = static_cast<Parser *>(state);
-            return parser->Recurse([&]() {
-              return parser->SkipAnyJsonValue();
-            });
-          },
-          this);
+      return ParseVectorDelimiters(count, [&](size_t &) -> CheckedError {
+        return Recurse([&]() { return SkipAnyJsonValue(); });
+      });
     }
     case kTokenStringConstant:
     case kTokenIntegerConstant:
@@ -2258,25 +2241,17 @@ CheckedError Parser::SkipAnyJsonValue() {
 CheckedError Parser::ParseFlexBufferValue(flexbuffers::Builder *builder) {
   switch (token_) {
     case '{': {
-      std::pair<Parser *, flexbuffers::Builder *> parser_and_builder_state(
-          this, builder);
       auto start = builder->StartMap();
       size_t fieldn_outer = 0;
-      auto err = ParseTableDelimiters(
-          fieldn_outer, nullptr,
-          [](const std::string &name, size_t &fieldn, const StructDef *,
-             void *state) -> CheckedError {
-            auto *parser_and_builder =
-                static_cast<std::pair<Parser *, flexbuffers::Builder *> *>(
-                    state);
-            auto *parser = parser_and_builder->first;
-            auto *current_builder = parser_and_builder->second;
-            current_builder->Key(name);
-            ECHECK(parser->ParseFlexBufferValue(current_builder));
-            fieldn++;
-            return NoError();
-          },
-          &parser_and_builder_state);
+      auto err =
+          ParseTableDelimiters(fieldn_outer, nullptr,
+                               [&](const std::string &name, size_t &fieldn,
+                                   const StructDef *) -> CheckedError {
+                                 builder->Key(name);
+                                 ECHECK(ParseFlexBufferValue(builder));
+                                 fieldn++;
+                                 return NoError();
+                               });
       ECHECK(err);
       builder->EndMap(start);
       break;
@@ -2284,18 +2259,9 @@ CheckedError Parser::ParseFlexBufferValue(flexbuffers::Builder *builder) {
     case '[': {
       auto start = builder->StartVector();
       size_t count = 0;
-      std::pair<Parser *, flexbuffers::Builder *> parser_and_builder_state(
-          this, builder);
-      ECHECK(ParseVectorDelimiters(
-          count,
-          [](size_t &, void *state) -> CheckedError {
-            auto *parser_and_builder =
-                static_cast<std::pair<Parser *, flexbuffers::Builder *> *>(
-                    state);
-            return parser_and_builder->first->ParseFlexBufferValue(
-                parser_and_builder->second);
-          },
-          &parser_and_builder_state));
+      ECHECK(ParseVectorDelimiters(count, [&](size_t &) -> CheckedError {
+        return ParseFlexBufferValue(builder);
+      }));
       builder->EndVector(start, false, false);
       break;
     }