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===================================
Working with the C++ Implementation
===================================

This section of the cookbook goes over basic concepts
that will be needed regardless of how you intend to use
the Arrow C++ implementation.

.. contents::

Working with Status and Result
==============================

C++ libraries often have to choose between throwing exceptions and
returning error codes.  Arrow chooses to return Status and Result
objects as a middle ground.  This makes it clear when a function
can fail and is easier to use than integer arrow codes.

It is important to always check the value of a returned Status object to
ensure that the operation succeeded.  However, this can quickly become
tedious:

.. recipe:: ../code/basic_arrow.cc ReturnNotOkNoMacro
  :caption: Checking the status of every function manually
  :dedent: 2

The macro :c:macro:`ARROW_RETURN_NOT_OK` will take care of some of this
boilerplate for you.  It will run the contained expression and check the resulting
``Status`` or ``Result`` object.  If it failed then it will return the failure.

.. recipe:: ../code/basic_arrow.cc ReturnNotOk
  :caption: Using ARROW_RETURN_NOT_OK to check the status
  :dedent: 2


Using the Visitor Pattern
=========================

Arrow classes :cpp:class:`arrow::DataType`, :cpp:class:`arrow::Scalar`, and
:cpp:class:`arrow::Array` have specialized subclasses for each Arrow type. In 
order to specialize logic for each subclass, you can use the visitor pattern. 
Arrow provides inline template functions that allow you to call visitors 
efficiently:

 * :cpp:func:`arrow::VisitTypeInline`
 * :cpp:func:`arrow::VisitScalarInline`
 * :cpp:func:`arrow::VisitArrayInline`

Generate Random Data
--------------------

See example at :ref:`Generate Random Data Example`.


Generalize Computations Across Arrow Types
------------------------------------------

Array visitors can be useful when writing functions that can handle multiple
array types. However, implementing a visitor for each type individually can be
excessively verbose. Fortunately, Arrow provides type traits that allow you to
write templated functions to handle subsets of types. The example below
demonstrates a table sum function that can handle any integer or floating point 
array with only a single visitor implementation by leveraging
:cpp:type:`arrow::enable_if_number`.

.. literalinclude:: ../code/basic_arrow.cc
   :language: cpp
   :linenos:
   :start-at: class TableSummation
   :end-at: };  // TableSummation
   :caption: Using visitor pattern that can compute sum of table with any numeric type
  

.. recipe:: ../code/basic_arrow.cc VisitorSummationExample
   :dedent: 2