Use a function rather than a macro

Author: pitrou

cpp/src/arrow/ipc/metadata_internal.h

@@ -96,8 +96,10 @@ struct FileBlock {
                            " in flatbuffer-encoded metadata"); \
   }
 
-#define FLATBUFFERS_VECTOR_SIZE(fb_vector) \
-  ((fb_vector == NULLPTR) ? 0 : fb_vector->size())
+template <typename T>
+inline uint32_t FlatBuffersVectorSize(const flatbuffers::Vector<T>* vec) {
+  return (vec == NULLPTR) ? 0 : vec->size();
+}
 
 inline std::string StringFromFlatbuffers(const flatbuffers::String* s) {
   return (s == NULLPTR) ? "" : s->str();

cpp/src/arrow/ipc/reader.cc

@@ -657,11 +657,11 @@ class RecordBatchFileReader::RecordBatchFileReaderImpl {
   }
 
   int num_dictionaries() const {
-    return FLATBUFFERS_VECTOR_SIZE(footer_->dictionaries());
+    return static_cast<int>(internal::FlatBuffersVectorSize(footer_->dictionaries()));
   }
 
   int num_record_batches() const {
-    return FLATBUFFERS_VECTOR_SIZE(footer_->recordBatches());
+    return static_cast<int>(internal::FlatBuffersVectorSize(footer_->recordBatches()));
   }
 
   MetadataVersion version() const {