make Resize and Init virtual on builder

cpp/src/arrow/builder.h

@@ -69,14 +69,19 @@ class ArrayBuilder {
   // Set the next length bits to not null (i.e. valid).
   Status SetNotNull(int32_t length);
 
-  // Allocates requires memory at this level, but children need to be
-  // initialized independently
-  Status Init(int32_t capacity);
-
-  // Resizes the null_bitmap array
-  Status Resize(int32_t new_bits);
-
-  Status Reserve(int32_t extra_bits);
+  // Allocates initial capacity requirements for the builder.  In most
+  // cases subclasses should override and call there parent classes
+  // method as well.
+  virtual Status Init(int32_t capacity);
+
+  // Resizes the null_bitmap array.  In most
+  // cases subclasses should override and call there parent classes
+  // method as well.
+  virtual Status Resize(int32_t new_bits);
+
+  // Ensures there is enough space for adding the number of elements by checking
+  // capacity and calling Resize if necessary.
+  Status Reserve(int32_t elements);
 
   // For cases where raw data was memcpy'd into the internal buffers, allows us
   // to advance the length of the builder. It is your responsibility to use

cpp/src/arrow/types/list.cc

@@ -50,7 +50,7 @@ Status ListArray::Validate() const {
   }
   if (offset_buf_->size() / sizeof(int32_t) < length_) {
     std::stringstream ss;
-    ss << "offset buffer size: " << offset_buf_->size()
+    ss << "offset buffer size (bytes): " << offset_buf_->size()
        << " isn't large enough for length: " << length_;
     return Status::Invalid(ss.str());
   }

cpp/src/arrow/types/list.h

@@ -112,26 +112,18 @@ class ListBuilder : public ArrayBuilder {
         offset_builder_(pool),
         values_(values) {}
 
-  Status Init(int32_t elements) {
+  Status Init(int32_t elements) override {
     RETURN_NOT_OK(ArrayBuilder::Init(elements));
     // one more then requested for offsets
-    return offset_builder_.Resize(elements + 1);
+    return offset_builder_.Resize((elements + 1) * sizeof(int32_t));
   }
 
-  Status Resize(int32_t capacity) {
-    // +1 because we Need space for the end offset
+  Status Resize(int32_t capacity) override {
+    // one more then requested for offsets
     RETURN_NOT_OK(offset_builder_.Resize((capacity + 1) * sizeof(int32_t)));
     return ArrayBuilder::Resize(capacity);
   }
 
-  Status Reserve(int32_t elements) {
-    if (length_ + elements > capacity_) {
-      int32_t new_capacity = util::next_power2(length_ + elements);
-      return Resize(new_capacity);
-    }
-    return Status::OK();
-  }
-
   // Vector append
   //
   // If passed, valid_bytes is of equal length to values, and any zero byte

cpp/src/arrow/types/primitive.cc

@@ -102,19 +102,10 @@ Status PrimitiveBuilder<T>::Resize(int32_t capacity) {
   return Status::OK();
 }
 
-template <typename T>
-Status PrimitiveBuilder<T>::Reserve(int32_t elements) {
-  if (length_ + elements > capacity_) {
-    int32_t new_capacity = util::next_power2(length_ + elements);
-    return Resize(new_capacity);
-  }
-  return Status::OK();
-}
-
 template <typename T>
 Status PrimitiveBuilder<T>::Append(
     const value_type* values, int32_t length, const uint8_t* valid_bytes) {
-  RETURN_NOT_OK(PrimitiveBuilder<T>::Reserve(length));
+  RETURN_NOT_OK(Reserve(length));
 
   if (length > 0) {
     memcpy(raw_data_ + length_, values, type_traits<T>::bytes_required(length));

cpp/src/arrow/types/primitive.h

@@ -114,17 +114,13 @@ class PrimitiveBuilder : public ArrayBuilder {
   Status Append(
       const value_type* values, int32_t length, const uint8_t* valid_bytes = nullptr);
 
-  // Ensure that builder can accommodate an additional number of
-  // elements. Resizes if the current capacity is not sufficient
-  Status Reserve(int32_t elements);
-
   std::shared_ptr<Array> Finish() override;
 
-  Status Init(int32_t capacity);
+  Status Init(int32_t capacity) override;
 
   // Increase the capacity of the builder to accommodate at least the indicated
   // number of elements
-  Status Resize(int32_t capacity);
+  Status Resize(int32_t capacity) override;
 
  protected:
   std::shared_ptr<PoolBuffer> data_;
@@ -143,7 +139,7 @@ class NumericBuilder : public PrimitiveBuilder<T> {
 
   // Scalar append.
   void Append(value_type val) {
-    PrimitiveBuilder<T>::Reserve(1);
+    ArrayBuilder::Reserve(1);
     UnsafeAppend(val);
   }