Add flatc '--cpp_std' switch (#5656)

* Add flatc '--cpp_std' switch and sandbox for C++17 code generator

- Added 'flac --cpp_std legacy' for compatibility with old compilers (VS2010);
- Added experimental switch 'flac --cpp_std c++17' for future development;
- Added C++17 sandbox test_cpp17.cpp;
- C++ code generator generates enums with explicit underlying type to avoid problems with the forward and backward schema compatibility;
- Adjusted CMakeLists.txt, CI and generate code scripts to support of introduced '--cpp_std';

* Fix --cpp_std values: c++0x, c++11, c++17

* Add 'cpp::CppStandard' enum

* Add testing engine into test_cpp17

* Rebase to upstream/master

* Set default '--cpp-std C++0x'

* Fix code generation (--cpp_std C++11) in CMakeLists.txt

- Fix dependency declaration of grpctest target

* Revert --cpp-std for the tests from explicit C++11 to flatc default value (C++0x)

include/flatbuffers/flatbuffers.h

@@ -43,6 +43,20 @@ template<> inline bool IsTheSameAs<double>(double e, double def) {
 }
 #endif
 
+// Check 'v' is out of closed range [low; high].
+// Workaround for GCC warning [-Werror=type-limits]:
+// comparison is always true due to limited range of data type.
+template<typename T>
+inline bool IsOutRange(const T &v, const T &low, const T &high) {
+  return (v < low) || (high < v);
+}
+
+// Check 'v' is in closed range [low; high].
+template<typename T>
+inline bool IsInRange(const T &v, const T &low, const T &high) {
+  return !IsOutRange(v, low, high);
+}
+
 // 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 {
@@ -351,6 +365,7 @@ template<typename T> class Vector {
   // This class is a pointer. Copying will therefore create an invalid object.
   // Private and unimplemented copy constructor.
   Vector(const Vector &);
+  Vector& operator=(const Vector&);
 
   template<typename K> static int KeyCompare(const void *ap, const void *bp) {
     const K *key = reinterpret_cast<const K *>(ap);
@@ -381,6 +396,7 @@ class VectorOfAny {
 
  private:
   VectorOfAny(const VectorOfAny &);
+  VectorOfAny &operator=(const VectorOfAny &);
 };
 
 #ifndef FLATBUFFERS_CPP98_STL
@@ -2377,6 +2393,12 @@ class Struct FLATBUFFERS_FINAL_CLASS {
   uint8_t *GetAddressOf(uoffset_t o) { return &data_[o]; }
 
  private:
+  // private constructor & copy constructor: you obtain instances of this
+  // class by pointing to existing data only
+  Struct();
+  Struct(const Struct &);
+  Struct &operator=(const Struct &);
+
   uint8_t data_[1];
 };
 
@@ -2489,6 +2511,7 @@ class Table {
   // class by pointing to existing data only
   Table();
   Table(const Table &other);
+  Table &operator=(const Table &);
 
   uint8_t data_[1];
 };

include/flatbuffers/flatc.h

@@ -14,6 +14,9 @@
  * limitations under the License.
  */
 
+#ifndef FLATBUFFERS_FLATC_H_
+#define FLATBUFFERS_FLATC_H_
+
 #include <functional>
 #include <limits>
 #include <string>
@@ -22,11 +25,11 @@
 #include "flatbuffers/idl.h"
 #include "flatbuffers/util.h"
 
-#ifndef FLATC_H_
-#  define FLATC_H_
-
 namespace flatbuffers {
 
+extern void LogCompilerWarn(const std::string &warn);
+extern void LogCompilerError(const std::string &err);
+
 class FlatCompiler {
  public:
   // Output generator for the various programming languages and formats we
@@ -94,4 +97,4 @@ class FlatCompiler {
 
 }  // namespace flatbuffers
 
-#endif  // FLATC_H_
+#endif  // FLATBUFFERS_FLATC_H_

include/flatbuffers/idl.h

@@ -556,6 +556,7 @@ struct IDLOptions {
   bool force_defaults;
   bool java_primitive_has_method;
   std::vector<std::string> cpp_includes;
+  std::string cpp_std;
 
   // Possible options for the more general generator below.
   enum Language {

include/flatbuffers/reflection_generated.h

@@ -97,7 +97,7 @@ inline const char * const *EnumNamesBaseType() {
 }
 
 inline const char *EnumNameBaseType(BaseType e) {
-  if (e < None || e > Array) return "";
+  if (flatbuffers::IsOutRange(e, None, Array)) return "";
   const size_t index = static_cast<size_t>(e);
   return EnumNamesBaseType()[index];
 }