Function capacity

cpp/src/arrow/array/builder_base.h:116–116

  virtual int64_t length() const { return length_; }
  int64_t null_count() const { return null_count_; }
  int64_t capacity() const { return capacity_; }

  /// \brief Ensure that enough memory has been allocated to fit the indicated
  /// number of total elements in the builder, including any that have already