wjones127 commented on a change in pull request #166:
URL: https://github.com/apache/arrow-cookbook/pull/166#discussion_r829290778
##########
File path: cpp/code/basic_arrow.cc
##########
@@ -63,3 +66,54 @@ arrow::Status ReturnNotOk() {
TEST(BasicArrow, ReturnNotOkNoMacro) { ASSERT_OK(ReturnNotOkMacro()); }
TEST(BasicArrow, ReturnNotOk) { ASSERT_OK(ReturnNotOk()); }
+
+class TypeCountVisitor : public arrow::TypeVisitor {
+ public:
+ uint64_t nested_count;
+ uint64_t non_nested_count;
+
+ template <typename T>
+ arrow::enable_if_not_nested<T, arrow::Status> Visit(const T&) {
Review comment:
I think I must be doing this wrong, since I am getting NotImplemented
for int8:
https://github.com/apache/arrow-cookbook/runs/5588880880?check_suite_focus=true#step:6:94
Any idea on why?
##########
File path: cpp/code/basic_arrow.cc
##########
@@ -63,3 +66,54 @@ arrow::Status ReturnNotOk() {
TEST(BasicArrow, ReturnNotOkNoMacro) { ASSERT_OK(ReturnNotOkMacro()); }
TEST(BasicArrow, ReturnNotOk) { ASSERT_OK(ReturnNotOk()); }
+
+class TypeCountVisitor : public arrow::TypeVisitor {
+ public:
+ uint64_t nested_count;
+ uint64_t non_nested_count;
+
+ template <typename T>
+ arrow::enable_if_not_nested<T, arrow::Status> Visit(const T&) {
Review comment:
Is it using the default implementation from the parent class instead?
##########
File path: cpp/source/basic.rst
##########
@@ -48,3 +48,29 @@ boilerplate for you. It will run the contained expression
and check the resulti
.. recipe:: ../code/basic_arrow.cc ReturnNotOk
:caption: Using ARROW_RETURN_NOT_OK to check the status
:dedent: 2
+
+
+Using the Visitor Pattern
+=========================
+
+Arrow classes like DataType, Scalar, and Array have specialized subclasses for
+each Arrow type. In order to work with them, use the visitor pattern. These
+types provide an Accept method and have associated Visitor classes.
+
+As an example, below we implement a :cpp:class:`arrow::TypeVisitor` that counts
+the number of primitive and nested types.
+
+To implement a TypeVisitor we have to implement a Visit method for every
possible
+DataType we wish to handle. Fortunately, we can often use templates and type
+traits to make this less verbose.
+
+.. literalinclude:: ../code/basic_arrow.cc
+ :language: cpp
+ :linenos:
+ :start-at: class TypeCountVisitor
+ :end-at: }; // TypeCountVisitor
+ :caption: TypeVisitor that counts nested and non-nested types
Review comment:
Agreed. I'd like to find a better example, but haven't yet thought of
one that wasn't too long. But as a first step I'd like to at least figure out
how to use our API.
##########
File path: cpp/code/basic_arrow.cc
##########
@@ -63,3 +66,54 @@ arrow::Status ReturnNotOk() {
TEST(BasicArrow, ReturnNotOkNoMacro) { ASSERT_OK(ReturnNotOkMacro()); }
TEST(BasicArrow, ReturnNotOk) { ASSERT_OK(ReturnNotOk()); }
+
+class TypeCountVisitor : public arrow::TypeVisitor {
+ public:
+ uint64_t nested_count;
+ uint64_t non_nested_count;
+
+ template <typename T>
+ arrow::enable_if_not_nested<T, arrow::Status> Visit(const T&) {
Review comment:
Ah so if I understand right, if you subclass `TypeVisitor` you can't use
templates to implement the overriding methods.
##########
File path: cpp/source/basic.rst
##########
@@ -48,3 +48,29 @@ boilerplate for you. It will run the contained expression
and check the resulti
.. recipe:: ../code/basic_arrow.cc ReturnNotOk
:caption: Using ARROW_RETURN_NOT_OK to check the status
:dedent: 2
+
+
+Using the Visitor Pattern
+=========================
+
+Arrow classes like DataType, Scalar, and Array have specialized subclasses for
+each Arrow type. In order to work with them, use the visitor pattern. These
+types provide an Accept method and have associated Visitor classes.
+
+As an example, below we implement a :cpp:class:`arrow::TypeVisitor` that counts
+the number of primitive and nested types.
+
+To implement a TypeVisitor we have to implement a Visit method for every
possible
+DataType we wish to handle. Fortunately, we can often use templates and type
+traits to make this less verbose.
+
+.. literalinclude:: ../code/basic_arrow.cc
+ :language: cpp
+ :linenos:
+ :start-at: class TypeCountVisitor
+ :end-at: }; // TypeCountVisitor
+ :caption: TypeVisitor that counts nested and non-nested types
Review comment:
IMO one of the most common uses of the visitors would probably be to
implement a conversion to third-party types. So something like converting a
`std::vector<arrow::Scalar>` to `rapidjson::Document`. But those things aren't
something that could be copied and pasted as-is, since the conversion logic
obviously varies a lot. So maybe this is better for the user guide after-all?
(I partly wanted to do it here since we have a nice build and test setup.)
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