File Input and Output using XML and YAML files {#tutorial_file_input_output_with_xml_yml}
==============================================

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Goal
----

You'll find answers for the following questions:

-   How to print and read text entries to a file and OpenCV using YAML or XML files?
-   How to do the same for OpenCV data structures?
-   How to do this for your data structures?
-   Usage of OpenCV data structures such as @ref cv::FileStorage , @ref cv::FileNode or @ref
    cv::FileNodeIterator .

Source code
-----------

You can [download this from here
](https://github.com/opencv/opencv/tree/3.4/samples/cpp/tutorial_code/core/file_input_output/file_input_output.cpp) or find it in the
`samples/cpp/tutorial_code/core/file_input_output/file_input_output.cpp` of the OpenCV source code
library.

Here's a sample code of how to achieve all the stuff enumerated at the goal list.

@include cpp/tutorial_code/core/file_input_output/file_input_output.cpp

Explanation
-----------

Here we talk only about XML and YAML file inputs. Your output (and its respective input) file may
have only one of these extensions and the structure coming from this. They are two kinds of data
structures you may serialize: *mappings* (like the STL map) and *element sequence* (like the STL
vector). The difference between these is that in a map every element has a unique name through what
you may access it. For sequences you need to go through them to query a specific item.

-#  **XML/YAML File Open and Close.** Before you write any content to such file you need to open it
    and at the end to close it. The XML/YAML data structure in OpenCV is @ref cv::FileStorage . To
    specify that this structure to which file binds on your hard drive you can use either its
    constructor or the *open()* function of this:
    @code{.cpp}
    string filename = "I.xml";
    FileStorage fs(filename, FileStorage::WRITE);
    //...
    fs.open(filename, FileStorage::READ);
    @endcode
    Either one of this you use the second argument is a constant specifying the type of operations
    you'll be able to on them: WRITE, READ or APPEND. The extension specified in the file name also
    determinates the output format that will be used. The output may be even compressed if you
    specify an extension such as *.xml.gz*.

    The file automatically closes when the @ref cv::FileStorage objects is destroyed. However, you
    may explicitly call for this by using the *release* function:
    @code{.cpp}
    fs.release();                                       // explicit close
    @endcode
-#  **Input and Output of text and numbers.** The data structure uses the same \<\< output operator
    that the STL library. For outputting any type of data structure we need first to specify its
    name. We do this by just simply printing out the name of this. For basic types you may follow
    this with the print of the value :
    @code{.cpp}
    fs << "iterationNr" << 100;
    @endcode
    Reading in is a simple addressing (via the [] operator) and casting operation or a read via
    the \>\> operator :
    @code{.cpp}
    int itNr;
    fs["iterationNr"] >> itNr;
    itNr = (int) fs["iterationNr"];
    @endcode
-#  **Input/Output of OpenCV Data structures.** Well these behave exactly just as the basic C++
    types:
    @code{.cpp}
    Mat R = Mat_<uchar >::eye  (3, 3),
        T = Mat_<double>::zeros(3, 1);

    fs << "R" << R;                                      // Write cv::Mat
    fs << "T" << T;

    fs["R"] >> R;                                      // Read cv::Mat
    fs["T"] >> T;
    @endcode
-#  **Input/Output of vectors (arrays) and associative maps.** As I mentioned beforehand, we can
    output maps and sequences (array, vector) too. Again we first print the name of the variable and
    then we have to specify if our output is either a sequence or map.

    For sequence before the first element print the "[" character and after the last one the "]"
    character:
    @code{.cpp}
    fs << "strings" << "[";                              // text - string sequence
    fs << "image1.jpg" << "Awesomeness" << "baboon.jpg";
    fs << "]";                                           // close sequence
    @endcode
    For maps the drill is the same however now we use the "{" and "}" delimiter characters:
    @code{.cpp}
    fs << "Mapping";                              // text - mapping
    fs << "{" << "One" << 1;
    fs <<        "Two" << 2 << "}";
    @endcode
    To read from these we use the @ref cv::FileNode and the @ref cv::FileNodeIterator data
    structures. The [] operator of the @ref cv::FileStorage class returns a @ref cv::FileNode data
    type. If the node is sequential we can use the @ref cv::FileNodeIterator to iterate through the
    items:
    @code{.cpp}
    FileNode n = fs["strings"];                         // Read string sequence - Get node
    if (n.type() != FileNode::SEQ)
    {
        cerr << "strings is not a sequence! FAIL" << endl;
        return 1;
    }

    FileNodeIterator it = n.begin(), it_end = n.end(); // Go through the node
    for (; it != it_end; ++it)
        cout << (string)*it << endl;
    @endcode
    For maps you can use the [] operator again to access the given item (or the \>\> operator too):
    @code{.cpp}
    n = fs["Mapping"];                                // Read mappings from a sequence
    cout << "Two  " << (int)(n["Two"]) << "; ";
    cout << "One  " << (int)(n["One"]) << endl << endl;
    @endcode
-#  **Read and write your own data structures.** Suppose you have a data structure such as:
    @code{.cpp}
    class MyData
    {
    public:
          MyData() : A(0), X(0), id() {}
    public:   // Data Members
       int A;
       double X;
       string id;
    };
    @endcode
    It's possible to serialize this through the OpenCV I/O XML/YAML interface (just as in case of
    the OpenCV data structures) by adding a read and a write function inside and outside of your
    class. For the inside part:
    @code{.cpp}
    void write(FileStorage& fs) const                        //Write serialization for this class
    {
      fs << "{" << "A" << A << "X" << X << "id" << id << "}";
    }

    void read(const FileNode& node)                          //Read serialization for this class
    {
      A = (int)node["A"];
      X = (double)node["X"];
      id = (string)node["id"];
    }
    @endcode
    Then you need to add the following functions definitions outside the class:
    @code{.cpp}
    void write(FileStorage& fs, const std::string&, const MyData& x)
    {
    x.write(fs);
    }

    void read(const FileNode& node, MyData& x, const MyData& default_value = MyData())
    {
    if(node.empty())
        x = default_value;
    else
        x.read(node);
    }
    @endcode
    Here you can observe that in the read section we defined what happens if the user tries to read
    a non-existing node. In this case we just return the default initialization value, however a
    more verbose solution would be to return for instance a minus one value for an object ID.

    Once you added these four functions use the \>\> operator for write and the \<\< operator for
    read:
    @code{.cpp}
    MyData m(1);
    fs << "MyData" << m;                                // your own data structures
    fs["MyData"] >> m;                                 // Read your own structure_
    @endcode
    Or to try out reading a non-existing read:
    @code{.cpp}
    fs["NonExisting"] >> m;   // Do not add a fs << "NonExisting" << m command for this to work
    cout << endl << "NonExisting = " << endl << m << endl;
    @endcode

Result
------

Well mostly we just print out the defined numbers. On the screen of your console you could see:
@code{.bash}
Write Done.

Reading:
100image1.jpg
Awesomeness
baboon.jpg
Two  2; One  1


R = [1, 0, 0;
  0, 1, 0;
  0, 0, 1]
T = [0; 0; 0]

MyData =
{ id = mydata1234, X = 3.14159, A = 97}

Attempt to read NonExisting (should initialize the data structure with its default).
NonExisting =
{ id = , X = 0, A = 0}

Tip: Open up output.xml with a text editor to see the serialized data.
@endcode
Nevertheless, it's much more interesting what you may see in the output xml file:
@code{.xml}
<?xml version="1.0"?>
<opencv_storage>
<iterationNr>100</iterationNr>
<strings>
  image1.jpg Awesomeness baboon.jpg</strings>
<Mapping>
  <One>1</One>
  <Two>2</Two></Mapping>
<R type_id="opencv-matrix">
  <rows>3</rows>
  <cols>3</cols>
  <dt>u</dt>
  <data>
    1 0 0 0 1 0 0 0 1</data></R>
<T type_id="opencv-matrix">
  <rows>3</rows>
  <cols>1</cols>
  <dt>d</dt>
  <data>
    0. 0. 0.</data></T>
<MyData>
  <A>97</A>
  <X>3.1415926535897931e+000</X>
  <id>mydata1234</id></MyData>
</opencv_storage>
@endcode
Or the YAML file:
@code{.yaml}
%YAML:1.0
iterationNr: 100
strings:
   - "image1.jpg"
   - Awesomeness
   - "baboon.jpg"
Mapping:
   One: 1
   Two: 2
R: !!opencv-matrix
   rows: 3
   cols: 3
   dt: u
   data: [ 1, 0, 0, 0, 1, 0, 0, 0, 1 ]
T: !!opencv-matrix
   rows: 3
   cols: 1
   dt: d
   data: [ 0., 0., 0. ]
MyData:
   A: 97
   X: 3.1415926535897931e+000
   id: mydata1234
@endcode
You may observe a runtime instance of this on the [YouTube
here](https://www.youtube.com/watch?v=A4yqVnByMMM) .

@youtube{A4yqVnByMMM}
