Add template version for As method. (#4443)

* Add template version for As method.

* Fix compilation error on Linux.

* Fix linux issue (2).

* Linux fix (3)

* Add few tests.

include/flatbuffers/flexbuffers.h

@@ -579,6 +579,8 @@ class Reference {
     }
   }
 
+  template<typename T> T As();
+
   // Experimental: Mutation functions.
   // These allow scalars in an already created buffer to be updated in-place.
   // Since by default scalars are stored in the smallest possible space,
@@ -688,6 +690,31 @@ class Reference {
   Type type_;
 };
 
+// Template specialization for As().
+template<> inline bool Reference::As<bool>() { return AsBool(); }
+
+template<> inline int8_t Reference::As<int8_t>() { return AsInt8(); }
+template<> inline int16_t Reference::As<int16_t>() { return AsInt16(); }
+template<> inline int32_t Reference::As<int32_t>() { return AsInt32(); }
+template<> inline int64_t Reference::As<int64_t>() { return AsInt64(); }
+
+template<> inline uint8_t Reference::As<uint8_t>() { return AsUInt8(); }
+template<> inline uint16_t Reference::As<uint16_t>() { return AsUInt16(); }
+template<> inline uint32_t Reference::As<uint32_t>() { return AsUInt32(); }
+template<> inline uint64_t Reference::As<uint64_t>() { return AsUInt64(); }
+
+template<> inline double Reference::As<double>() { return AsDouble(); }
+template<> inline float Reference::As<float>() { return AsFloat(); }
+
+template<> inline String Reference::As<String>() { return AsString(); }
+template<> inline std::string Reference::As<std::string>() { return AsString().str(); }
+
+template<> inline Blob Reference::As<Blob>() { return AsBlob(); }
+template<> inline Vector Reference::As<Vector>() { return AsVector(); }
+template<> inline TypedVector Reference::As<TypedVector>() { return AsTypedVector(); }
+template<> inline FixedTypedVector Reference::As<FixedTypedVector>() { return AsFixedTypedVector(); }
+template<> inline Map Reference::As<Map>() { return AsMap(); }
+
 inline uint8_t PackedType(BitWidth bit_width, Type type) {
   return static_cast<uint8_t>(bit_width | (type << 2));
 }

tests/test.cpp

@@ -1687,6 +1687,13 @@ void FlexBuffersTest() {
   TEST_EQ(vec[2].AsString().IsTheEmptyString(), true);  // Wrong Type.
   TEST_EQ_STR(vec[2].AsString().c_str(), "");  // This still works though.
   TEST_EQ_STR(vec[2].ToString().c_str(), "4.0");  // Or have it converted.
+
+  // Few tests for templated version of As.
+  TEST_EQ(vec[0].As<int64_t>(), -100);
+  TEST_EQ_STR(vec[1].As<std::string>().c_str(), "Fred");
+  TEST_EQ(vec[1].As<int64_t>(), 0);  // Number parsing failed.
+  TEST_EQ(vec[2].As<double>(), 4.0);
+
   // Test that the blob can be accessed.
   TEST_EQ(vec[3].IsBlob(), true);
   auto blob = vec[3].AsBlob();