Add NaN and Inf defaults to the C++ generated code. (#5102)

* Add `NaN` and `Inf` defaults to the C++ generated code.

* Refactoring: add FloatConstantGenerator

* Refactoring-2:

- remove isnan checking for all float/double values
- add most probable implementation of virtual methods of FloatConstantGenerator

* Add conditional (FLATBUFFERS_NAN_DEFAULTS) isnan checking

include/flatbuffers/code_generators.h

@@ -130,6 +130,33 @@ extern void GenComment(const std::vector<std::string> &dc,
                        std::string *code_ptr, const CommentConfig *config,
                        const char *prefix = "");
 
+class FloatConstantGenerator {
+ public:
+  virtual ~FloatConstantGenerator(){};
+  std::string GenFloatConstant(const FieldDef &field) const;
+
+ private:
+  virtual std::string Inf(double v) const = 0;
+  virtual std::string NaN(double v) const = 0;
+  virtual std::string Value(double v, const std::string &src) const {
+    (void)v;
+    return src;
+  }
+
+  virtual std::string Inf(float v) const {
+    return this->Inf(static_cast<double>(v));
+  }
+  virtual std::string NaN(float v) const {
+    return this->NaN(static_cast<double>(v));
+  }
+  virtual std::string Value(float v, const std::string &src) const {
+    return this->Value(static_cast<double>(v), src);
+  }
+
+  template<typename T>
+  std::string GenFloatConstantImpl(const FieldDef &field) const;
+};
+
 }  // namespace flatbuffers
 
 #endif  // FLATBUFFERS_CODE_GENERATORS_H_

include/flatbuffers/flatbuffers.h

@@ -19,7 +19,25 @@
 
 #include "flatbuffers/base.h"
 
+#if defined(FLATBUFFERS_NAN_DEFAULTS)
+#include <cmath>
+#endif
+
 namespace flatbuffers {
+// Generic 'operator==' with conditional specialisations.
+template<typename T> inline bool IsTheSameAs(T e, T def) { return e == def; }
+
+#if defined(FLATBUFFERS_NAN_DEFAULTS) && \
+    (!defined(_MSC_VER) || _MSC_VER >= 1800)
+// Like `operator==(e, def)` with weak NaN if T=(float|double).
+template<> inline bool IsTheSameAs<float>(float e, float def) {
+  return (e == def) || (std::isnan(def) && std::isnan(e));
+}
+template<> inline bool IsTheSameAs<double>(double e, double def) {
+  return (e == def) || (std::isnan(def) && std::isnan(e));
+}
+#endif
+
 // Wrapper for uoffset_t to allow safe template specialization.
 // Value is allowed to be 0 to indicate a null object (see e.g. AddOffset).
 template<typename T> struct Offset {
@@ -1087,7 +1105,7 @@ class FlatBufferBuilder {
   // Like PushElement, but additionally tracks the field this represents.
   template<typename T> void AddElement(voffset_t field, T e, T def) {
     // We don't serialize values equal to the default.
-    if (e == def && !force_defaults_) return;
+    if (IsTheSameAs(e, def) && !force_defaults_) return;
     auto off = PushElement(e);
     TrackField(field, off);
   }
@@ -2241,7 +2259,7 @@ class Table {
 
   template<typename T> bool SetField(voffset_t field, T val, T def) {
     auto field_offset = GetOptionalFieldOffset(field);
-    if (!field_offset) return val == def;
+    if (!field_offset) return IsTheSameAs(val, def);
     WriteScalar(data_ + field_offset, val);
     return true;
   }