Fix high certainty warnings from PVS-studio (#5115)

* Fix high certainty warnings from PVS-studio

- Introduced FLATBUFFERS_ATTRIBUTE macro to use [[attribute]] if modern C++ compiler used

* Update the note about __cplusplus usage in the MSVC

include/flatbuffers/base.h

@@ -59,6 +59,15 @@
 // Clang 3.4 and later implement all of the ISO C++ 2014 standard.
 // http://clang.llvm.org/cxx_status.html
 
+// Note the MSVC value '__cplusplus' may be incorrect:
+// The '__cplusplus' predefined macro in the MSVC stuck at the value 199711L,
+// indicating (erroneously!) that the compiler conformed to the C++98 Standard.
+// This value should be correct starting from MSVC2017-15.7-Preview-3.
+// The '__cplusplus' will be valid only if MSVC2017-15.7-P3 and the `/Zc:__cplusplus` switch is set.
+// Workaround (for details see MSDN):
+// Use the _MSC_VER and _MSVC_LANG definition instead of the __cplusplus  for compatibility.
+// The _MSVC_LANG macro reports the Standard version regardless of the '/Zc:__cplusplus' switch.
+
 /// @cond FLATBUFFERS_INTERNAL
 #if __cplusplus <= 199711L && \
     (!defined(_MSC_VER) || _MSC_VER < 1600) && \
@@ -224,6 +233,15 @@ template<typename T> FLATBUFFERS_CONSTEXPR inline bool IsConstTrue(T t) {
   return !!t;
 }
 
+// Enable of std:c++17 or higher.
+#if (defined(__cplusplus) &&  (__cplusplus >= 201703L)) || \
+    (defined(_MSVC_LANG) &&  (_MSVC_LANG >= 201703L))
+  // All attributes unknown to an implementation are ignored without causing an error.
+  #define FLATBUFFERS_ATTRIBUTE(attr) // [[attr]] - will be enabled in a future release
+#else
+  #define FLATBUFFERS_ATTRIBUTE(attr)
+#endif
+
 /// @endcond
 
 /// @file

include/flatbuffers/flatbuffers.h

@@ -211,6 +211,7 @@ template<typename T> class Vector {
   uoffset_t size() const { return EndianScalar(length_); }
 
   // Deprecated: use size(). Here for backwards compatibility.
+  FLATBUFFERS_ATTRIBUTE(deprecated("use size() instead"))
   uoffset_t Length() const { return size(); }
 
   typedef typename IndirectHelper<T>::return_type return_type;
@@ -996,8 +997,8 @@ class FlatBufferBuilder {
   /// @warning Do NOT attempt to use this FlatBufferBuilder afterwards!
   /// @return A `FlatBuffer` that owns the buffer and its allocator and
   /// behaves similar to a `unique_ptr` with a deleter.
-  /// Deprecated: use Release() instead
-  DetachedBuffer ReleaseBufferPointer() {
+  FLATBUFFERS_ATTRIBUTE(deprecated("use Release() instead")) DetachedBuffer
+  ReleaseBufferPointer() {
     Finished();
     return buf_.release();
   }
@@ -1205,7 +1206,7 @@ class FlatBufferBuilder {
         auto vt_offset_ptr = reinterpret_cast<uoffset_t *>(it);
         auto vt2 = reinterpret_cast<voffset_t *>(buf_.data_at(*vt_offset_ptr));
         auto vt2_size = *vt2;
-        if (vt1_size != vt2_size || memcmp(vt2, vt1, vt1_size)) continue;
+        if (vt1_size != vt2_size || 0 != memcmp(vt2, vt1, vt1_size)) continue;
         vt_use = *vt_offset_ptr;
         buf_.pop(GetSize() - vtableoffsetloc);
         break;
@@ -1226,7 +1227,7 @@ class FlatBufferBuilder {
     return vtableoffsetloc;
   }
 
-  // DEPRECATED: call the version above instead.
+  FLATBUFFERS_ATTRIBUTE(deprecated("call the version above instead"))
   uoffset_t EndTable(uoffset_t start, voffset_t /*numfields*/) {
     return EndTable(start);
   }
@@ -2114,7 +2115,7 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
     if (!Verify<uoffset_t>(start)) return 0;
     auto o = ReadScalar<uoffset_t>(buf_ + start);
     // May not point to itself.
-    Check(o != 0);
+    if (!Check(o != 0)) return 0;
     // Can't wrap around / buffers are max 2GB.
     if (!Check(static_cast<soffset_t>(o) >= 0)) return 0;
     // Must be inside the buffer to create a pointer from it (pointer outside

include/flatbuffers/idl.h

@@ -326,7 +326,7 @@ inline size_t InlineAlignment(const Type &type) {
 
 struct EnumVal {
   EnumVal(const std::string &_name, int64_t _val) : name(_name), value(_val) {}
-  EnumVal(){};
+  EnumVal() : value(0){};
 
   Offset<reflection::EnumVal> Serialize(FlatBufferBuilder *builder, const Parser &parser) const;
 
@@ -672,7 +672,7 @@ class Parser : public ParserState {
   bool Deserialize(const reflection::Schema* schema);
 
   Type* DeserializeType(const reflection::Type* type);
-  
+
   // Checks that the schema represented by this parser is a safe evolution
   // of the schema provided. Returns non-empty error on any problems.
   std::string ConformTo(const Parser &base);

include/flatbuffers/registry.h

@@ -72,7 +72,7 @@ class Registry {
       return DetachedBuffer();
     }
     // We have a valid FlatBuffer. Detach it from the builder and return.
-    return parser.builder_.ReleaseBufferPointer();
+    return parser.builder_.Release();
   }
 
   // Modify any parsing / output options used by the other functions.

include/flatbuffers/util.h

@@ -66,7 +66,7 @@ inline bool is_digit(char c) { return check_in_range(c, '0', '9'); }
 
 inline bool is_xdigit(char c) {
   // Replace by look-up table.
-  return is_digit(c) | check_in_range(c & 0xDF, 'a' & 0xDF, 'f' & 0xDF);
+  return is_digit(c) || check_in_range(c & 0xDF, 'a' & 0xDF, 'f' & 0xDF);
 }
 
 // Case-insensitive isalnum