File Input and Output using XML and YAML files

#include <opencv2/core/core.hpp>
#include <iostream>
#include <string>

using namespace cv;
using namespace std;

static void help(char** av)
{
	cout << endl
		<< av[0] << " shows the usage of the OpenCV serialization functionality." << endl
		<< "usage: " << endl
		<< av[0] << " outputfile.yml.gz" << endl
		<< "The output file may be either XML (xml) or YAML (yml/yaml). You can even compress it by "
		<< "specifying this in its extension like xml.gz yaml.gz etc... " << endl
		<< "With FileStorage you can serialize objects in OpenCV by using the << and >> operators" << endl
		<< "For example: - create a class and have it serialized" << endl
		<< "             - use it to read and write matrices." << endl;
	system("pause");
}

class MyData
{
public:
	MyData() : A(0), X(0), id()
	{}
	explicit MyData(int) : A(97), X(CV_PI), id("mydata1234") // explicit to avoid implicit conversion
	{}
	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"];
	}
public:   // Data Members
	int A;
	double X;
	string id;
};

//These write and read functions must be defined for the serialization in FileStorage to work
static void write(FileStorage& fs, const std::string&, const MyData& x)
{
	x.write(fs);
}
static void read(const FileNode& node, MyData& x, const MyData& default_value = MyData()) {
	if (node.empty())
		x = default_value;
	else
		x.read(node);
}

// This function will print our custom class to the console
static ostream& operator<<(ostream& out, const MyData& m)
{
	out << "{ id = " << m.id << ", ";
	out << "X = " << m.X << ", ";
	out << "A = " << m.A << "}";
	return out;
}

int main(int ac, char** av)
{


	//if (ac != 2)
	//{
	//	help(av);
	//	return 1;
	//}

	//string filename = av[1];
	string filename = "YML.yml";
	{ //write
		Mat R = Mat_<uchar>::eye(3, 3),
			T = Mat_<double>::zeros(3, 1);
		MyData m(1);

		FileStorage fs(filename, FileStorage::WRITE);

		fs << "iterationNr" << 100;
		fs << "strings" << "[";                              // text - string sequence
		fs << "image1.jpg" << "Awesomeness" << "baboon.jpg";
		fs << "]";                                           // close sequence

		fs << "Mapping";                              // text - mapping
		fs << "{" << "One" << 1;
		fs << "Two" << 2 << "}";

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

		fs << "MyData" << m;                                // your own data structures

		fs.release();                                       // explicit close
		cout << "Write Done." << endl;
	}

	{//read
		cout << endl << "Reading: " << endl;
		FileStorage fs;
		fs.open(filename, FileStorage::READ);

		int itNr;
		//fs["iterationNr"] >> itNr;
		itNr = (int)fs["iterationNr"];
		cout << itNr;
		if (!fs.isOpened())
		{
			cerr << "Failed to open " << filename << endl;
			help(av);
			return 1;
		}

		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;


		n = fs["Mapping"];                                // Read mappings from a sequence
		cout << "Two  " << (int)(n["Two"]) << "; ";
		cout << "One  " << (int)(n["One"]) << endl << endl;


		MyData m;
		Mat R, T;

		fs["R"] >> R;                                      // Read cv::Mat
		fs["T"] >> T;
		fs["MyData"] >> m;                                 // Read your own structure_

		cout << endl
			<< "R = " << R << endl;
		cout << "T = " << T << endl << endl;
		cout << "MyData = " << endl << m << endl << endl;

		//Show default behavior for non existing nodes
		cout << "Attempt to read NonExisting (should initialize the data structure with its default).";
		fs["NonExisting"] >> m;
		cout << endl << "NonExisting = " << endl << m << endl;
	}

	cout << endl
		<< "Tip: Open up " << filename << " with a text editor to see the serialized data." << endl;

	return 0;
}

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 FileStorage, FileNode or FileNodeIterator.

Source code
You can download this from here or find it in the samples/cpp/tutorial_code/core/file_input_output/file_input_outpu
of the OpenCV source code library.

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.

1. 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 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:
string filename = "I.xml";
FileStorage fs(filename, FileStorage::WRITE);
//...
fs.open(filename, FileStorage::READ);
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 FileStorage objects is destroyed. However, you may explicitly call for
this by using the release function:

fs.release(); // explicit close

2. 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 :
fs << "iterationNr" << 100;
Reading in is a simple addressing (via the [] operator) and casting operation or a read via the >> operator :
int itNr;
fs["iterationNr"] >> itNr;

itNr = (int) fs["iterationNr"];

3. Input/Output of OpenCV Data structures. Well these behave exactly just as the basic C++ types:
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;

4. 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:
fs << "strings" << "["; // text - string sequence
fs << "image1.jpg" << "Awesomeness" << "baboon.jpg";
fs << "]"; // close sequence
For maps the drill is the same however now we use the “{” and “}” delimiter characters:
fs << "Mapping"; // text - mapping
fs << "{" << "One" << 1;
fs << "Two" << 2 << "}";
To read from these we use the FileNode and the FileNodeIterator data structures. The [] operator of the FileStorage class returns a FileNode data type. If the node is sequential we can use the FileNodeIterator to iterate through
the items:
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;
For maps you can use the [] operator again to acces the given item (or the >> operator too)
n = fs["Mapping"]; // Read mappings from a sequence
cout << "Two " << (int)(n["Two"]) << "; ";

cout << "One " << (int)(n["One"]) << endl << endl;

5. Read and write your own data structures. Suppose you have a data structure such as:
class MyData
{
public:
MyData() : A(0), X(0), id() {}
public: // Data Members
int A;
double X;
string id;
};
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:

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"];
}

Then you need to add the following functions definitions outside the class:
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);
}
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:
MyData m(1);
fs << "MyData" << m; // your own data structures
fs["MyData"] >> m; // Read your own structure_
Or to try out reading a non-existing read:
fs["NonExisting"] >> m; // Do not add a fs << "NonExisting" << m command for this to work
cout << endl << "NonExisting = " << endl << m << endl;

Result