Add support of Optional<T> scalars to C++ code generator (#6092)

include/flatbuffers/base.h

@@ -47,22 +47,17 @@
 #include <memory>
 
 #if defined(__unix__) && !defined(FLATBUFFERS_LOCALE_INDEPENDENT)
-#include <unistd.h>
-#endif 
+  #include <unistd.h>
+#endif
 
 #ifdef _STLPORT_VERSION
   #define FLATBUFFERS_CPP98_STL
 #endif
-#ifndef FLATBUFFERS_CPP98_STL
-  #include <functional>
-#endif
 
 #ifdef __ANDROID__
   #include <android/api-level.h>
 #endif
 
-#include "flatbuffers/stl_emulation.h"
-
 #if defined(__ICCARM__)
 #include <intrinsics.h>
 #endif
@@ -162,10 +157,12 @@ namespace flatbuffers {
     defined(__clang__)
   #define FLATBUFFERS_FINAL_CLASS final
   #define FLATBUFFERS_OVERRIDE override
+  #define FLATBUFFERS_EXPLICIT_CPP11 explicit
   #define FLATBUFFERS_VTABLE_UNDERLYING_TYPE : flatbuffers::voffset_t
 #else
   #define FLATBUFFERS_FINAL_CLASS
   #define FLATBUFFERS_OVERRIDE
+  #define FLATBUFFERS_EXPLICIT_CPP11
   #define FLATBUFFERS_VTABLE_UNDERLYING_TYPE
 #endif
 
@@ -173,10 +170,14 @@ namespace flatbuffers {
     (!defined(__GNUC__) || (__GNUC__ * 100 + __GNUC_MINOR__ >= 406)) || \
     (defined(__cpp_constexpr) && __cpp_constexpr >= 200704)
   #define FLATBUFFERS_CONSTEXPR constexpr
+  #define FLATBUFFERS_CONSTEXPR_CPP11 constexpr
+  #define FLATBUFFERS_CONSTEXPR_DEFINED
 #else
   #define FLATBUFFERS_CONSTEXPR const
+  #define FLATBUFFERS_CONSTEXPR_CPP11
 #endif
 
+// This macro is never used in code!
 #if (defined(__cplusplus) && __cplusplus >= 201402L) || \
     (defined(__cpp_constexpr) && __cpp_constexpr >= 201304)
   #define FLATBUFFERS_CONSTEXPR_CPP14 FLATBUFFERS_CONSTEXPR
@@ -202,6 +203,16 @@ namespace flatbuffers {
   #define FLATBUFFERS_DELETE_FUNC(func) private: func;
 #endif
 
+// Check if we can use template aliases
+// Not possible if Microsoft Compiler before 2012
+// Possible is the language feature __cpp_alias_templates is defined well
+// Or possible if the C++ std is C+11 or newer
+#if (defined(_MSC_VER) && _MSC_VER > 1700 /* MSVC2012 */) \
+    || (defined(__cpp_alias_templates) && __cpp_alias_templates >= 200704) \
+    || (defined(__cplusplus) && __cplusplus >= 201103L)
+  #define FLATBUFFERS_TEMPLATES_ALIASES
+#endif
+
 #ifndef FLATBUFFERS_HAS_STRING_VIEW
   // Only provide flatbuffers::string_view if __has_include can be used
   // to detect a header that provides an implementation

include/flatbuffers/flatbuffers.h

@@ -18,6 +18,11 @@
 #define FLATBUFFERS_H_
 
 #include "flatbuffers/base.h"
+#include "flatbuffers/stl_emulation.h"
+
+#ifndef FLATBUFFERS_CPP98_STL
+  #include <functional>
+#endif
 
 #if defined(FLATBUFFERS_NAN_DEFAULTS)
 #  include <cmath>
@@ -1303,6 +1308,11 @@ class FlatBufferBuilder {
     TrackField(field, off);
   }
 
+  template<typename T> void AddElement(voffset_t field, T e) {
+    auto off = PushElement(e);
+    TrackField(field, off);
+  }
+
   template<typename T> void AddOffset(voffset_t field, Offset<T> off) {
     if (off.IsNull()) return;  // Don't store.
     AddElement(field, ReferTo(off.o), static_cast<uoffset_t>(0));
@@ -2460,12 +2470,26 @@ class Table {
     return field_offset ? reinterpret_cast<P>(p) : nullptr;
   }
 
+  template<typename Raw, typename Face>
+  flatbuffers::Optional<Face> GetOptional(voffset_t field) const {
+    auto field_offset = GetOptionalFieldOffset(field);
+    auto p = data_ + field_offset;
+    return field_offset ? Optional<Face>(static_cast<Face>(ReadScalar<Raw>(p)))
+                        : Optional<Face>();
+  }
+
   template<typename T> bool SetField(voffset_t field, T val, T def) {
     auto field_offset = GetOptionalFieldOffset(field);
     if (!field_offset) return IsTheSameAs(val, def);
     WriteScalar(data_ + field_offset, val);
     return true;
   }
+  template<typename T> bool SetField(voffset_t field, T val) {
+    auto field_offset = GetOptionalFieldOffset(field);
+    if (!field_offset) return false;
+    WriteScalar(data_ + field_offset, val);
+    return true;
+  }
 
   bool SetPointer(voffset_t field, const uint8_t *val) {
     auto field_offset = GetOptionalFieldOffset(field);
@@ -2533,6 +2557,17 @@ class Table {
   uint8_t data_[1];
 };
 
+// This specialization allows avoiding warnings like:
+// MSVC C4800: type: forcing value to bool 'true' or 'false'.
+template<>
+inline flatbuffers::Optional<bool> Table::GetOptional<uint8_t, bool>(
+    voffset_t field) const {
+  auto field_offset = GetOptionalFieldOffset(field);
+  auto p = data_ + field_offset;
+  return field_offset ? Optional<bool>(ReadScalar<uint8_t>(p) != 0)
+                      : Optional<bool>();
+}
+
 template<typename T>
 void FlatBufferBuilder::Required(Offset<T> table, voffset_t field) {
   auto table_ptr = reinterpret_cast<const Table *>(buf_.data_at(table.o));

include/flatbuffers/idl.h

@@ -847,6 +847,11 @@ class Parser : public ParserState {
 
   FLATBUFFERS_CHECKED_ERROR Error(const std::string &msg);
 
+  // @brief Verify that any of 'opts.lang_to_generate' supports Optional scalars
+  // in a schema.
+  // @param opts Options used to parce a schema and generate code.
+  static bool SupportsOptionalScalars(const flatbuffers::IDLOptions &opts);
+
  private:
   void Message(const std::string &msg);
   void Warning(const std::string &msg);

include/flatbuffers/stl_emulation.h

@@ -18,6 +18,7 @@
 #define FLATBUFFERS_STL_EMULATION_H_
 
 // clang-format off
+#include "flatbuffers/base.h"
 
 #include <string>
 #include <type_traits>
@@ -25,6 +26,17 @@
 #include <memory>
 #include <limits>
 
+// Detect C++17 compatible compiler.
+// __cplusplus >= 201703L - a compiler has support of 'static inline' variables.
+#if defined(FLATBUFFERS_USE_STD_OPTIONAL) \
+    || (defined(__cplusplus) && __cplusplus >= 201703L) \
+    || (defined(_MSVC_LANG) &&  (_MSVC_LANG >= 201703L))
+  #include <optional>
+  #ifndef FLATBUFFERS_USE_STD_OPTIONAL
+    #define FLATBUFFERS_USE_STD_OPTIONAL
+  #endif
+#endif
+
 #if defined(_STLPORT_VERSION) && !defined(FLATBUFFERS_CPP98_STL)
   #define FLATBUFFERS_CPP98_STL
 #endif  // defined(_STLPORT_VERSION) && !defined(FLATBUFFERS_CPP98_STL)
@@ -33,16 +45,6 @@
   #include <cctype>
 #endif  // defined(FLATBUFFERS_CPP98_STL)
 
-// Check if we can use template aliases
-// Not possible if Microsoft Compiler before 2012
-// Possible is the language feature __cpp_alias_templates is defined well
-// Or possible if the C++ std is C+11 or newer
-#if (defined(_MSC_VER) && _MSC_VER > 1700 /* MSVC2012 */) \
-    || (defined(__cpp_alias_templates) && __cpp_alias_templates >= 200704) \
-    || (defined(__cplusplus) && __cplusplus >= 201103L)
-  #define FLATBUFFERS_TEMPLATES_ALIASES
-#endif
-
 // This header provides backwards compatibility for C++98 STLs like stlport.
 namespace flatbuffers {
 
@@ -302,6 +304,146 @@ inline void vector_emplace_back(std::vector<T> *vector, V &&data) {
 
 #endif  // !FLATBUFFERS_CPP98_STL
 
+#ifdef FLATBUFFERS_USE_STD_OPTIONAL
+template<class T>
+using Optional = std::optional<T>;
+using nullopt_t = std::nullopt_t;
+inline constexpr nullopt_t nullopt = std::nullopt;
+
+#else
+// Limited implementation of Optional<T> type for a scalar T.
+// This implementation limited by trivial types compatible with
+// std::is_arithmetic<T> or std::is_enum<T> type traits.
+
+// A tag to indicate an empty flatbuffers::optional<T>.
+struct nullopt_t {
+  explicit FLATBUFFERS_CONSTEXPR_CPP11 nullopt_t(int) {}
+};
+
+#if defined(FLATBUFFERS_CONSTEXPR_DEFINED)
+  namespace internal {
+    template <class> struct nullopt_holder {
+      static constexpr nullopt_t instance_ = nullopt_t(0);
+    };
+    template<class Dummy>
+    constexpr nullopt_t nullopt_holder<Dummy>::instance_;
+  }
+  static constexpr const nullopt_t &nullopt = internal::nullopt_holder<void>::instance_;
+
+#else
+  namespace internal {
+    template <class> struct nullopt_holder {
+      static const nullopt_t instance_;
+    };
+    template<class Dummy>
+    const nullopt_t nullopt_holder<Dummy>::instance_  = nullopt_t(0);
+  }
+  static const nullopt_t &nullopt = internal::nullopt_holder<void>::instance_;
+
+#endif
+
+template<class T>
+class Optional FLATBUFFERS_FINAL_CLASS {
+  // Non-scalar 'T' would extremely complicated Optional<T>.
+  // Use is_scalar<T> checking because flatbuffers flatbuffers::is_arithmetic<T>
+  // isn't implemented.
+  static_assert(flatbuffers::is_scalar<T>::value, "unexpected type T");
+
+ public:
+  ~Optional() {}
+
+  FLATBUFFERS_CONSTEXPR_CPP11 Optional() FLATBUFFERS_NOEXCEPT
+    : value_(), has_value_(false) {}
+
+  FLATBUFFERS_CONSTEXPR_CPP11 Optional(nullopt_t) FLATBUFFERS_NOEXCEPT
+    : value_(), has_value_(false) {}
+
+  FLATBUFFERS_CONSTEXPR_CPP11 Optional(T val) FLATBUFFERS_NOEXCEPT
+    : value_(val), has_value_(true) {}
+
+  FLATBUFFERS_CONSTEXPR_CPP11 Optional(const Optional &other) FLATBUFFERS_NOEXCEPT
+    : value_(other.value_), has_value_(other.has_value_) {}
+
+  FLATBUFFERS_CONSTEXPR_CPP14 Optional &operator=(const Optional &other) FLATBUFFERS_NOEXCEPT {
+    value_ = other.value_;
+    has_value_ = other.has_value_;
+    return *this;
+  }
+
+  FLATBUFFERS_CONSTEXPR_CPP14 Optional &operator=(nullopt_t) FLATBUFFERS_NOEXCEPT {
+    value_ = T();
+    has_value_ = false;
+    return *this;
+  }
+
+  FLATBUFFERS_CONSTEXPR_CPP14 Optional &operator=(T val) FLATBUFFERS_NOEXCEPT {
+    value_ = val;
+    has_value_ = true;
+    return *this;
+  }
+
+  void reset() FLATBUFFERS_NOEXCEPT {
+    *this = nullopt;
+  }
+
+  void swap(Optional &other) FLATBUFFERS_NOEXCEPT {
+    std::swap(value_, other.value_);
+    std::swap(has_value_, other.has_value_);
+  }
+
+  FLATBUFFERS_CONSTEXPR_CPP11 FLATBUFFERS_EXPLICIT_CPP11 operator bool() const FLATBUFFERS_NOEXCEPT {
+    return has_value_;
+  }
+
+  FLATBUFFERS_CONSTEXPR_CPP11 bool has_value() const FLATBUFFERS_NOEXCEPT {
+    return has_value_;
+  }
+
+  FLATBUFFERS_CONSTEXPR_CPP11 const T& operator*() const FLATBUFFERS_NOEXCEPT {
+    return value_;
+  }
+
+  const T& value() const {
+    FLATBUFFERS_ASSERT(has_value());
+    return value_;
+  }
+
+  T value_or(T default_value) const FLATBUFFERS_NOEXCEPT {
+    return has_value() ? value_ : default_value;
+  }
+
+ private:
+  T value_;
+  bool has_value_;
+};
+
+template<class T>
+FLATBUFFERS_CONSTEXPR_CPP11 bool operator==(const Optional<T>& opt, nullopt_t) FLATBUFFERS_NOEXCEPT {
+  return !opt;
+}
+template<class T>
+FLATBUFFERS_CONSTEXPR_CPP11 bool operator==(nullopt_t, const Optional<T>& opt) FLATBUFFERS_NOEXCEPT {
+  return !opt;
+}
+
+template<class T, class U>
+FLATBUFFERS_CONSTEXPR_CPP11 bool operator==(const Optional<T>& lhs, const U& rhs) FLATBUFFERS_NOEXCEPT {
+  return static_cast<bool>(lhs) && (*lhs == rhs);
+}
+
+template<class T, class U>
+FLATBUFFERS_CONSTEXPR_CPP11 bool operator==(const T& lhs, const Optional<U>& rhs) FLATBUFFERS_NOEXCEPT {
+  return static_cast<bool>(rhs) && (lhs == *rhs);
+}
+
+template<class T, class U>
+FLATBUFFERS_CONSTEXPR_CPP11 bool operator==(const Optional<T>& lhs, const Optional<U>& rhs) FLATBUFFERS_NOEXCEPT {
+  return static_cast<bool>(lhs) != static_cast<bool>(rhs)
+              ? false
+              : !static_cast<bool>(lhs) ? false : (*lhs == *rhs);
+}
+#endif // FLATBUFFERS_USE_STD_OPTIONAL
+
 }  // namespace flatbuffers
 
 #endif  // FLATBUFFERS_STL_EMULATION_H_

include/flatbuffers/util.h

@@ -20,6 +20,7 @@
 #include <errno.h>
 
 #include "flatbuffers/base.h"
+#include "flatbuffers/stl_emulation.h"
 
 #ifndef FLATBUFFERS_PREFER_PRINTF
 #  include <sstream>