Write the content of an object into a file in C++

Question

I'm using OpenCV, and I am computing the histogram of some images, which is contained inside a class named CvHist. I have a CvHist object, but I want to store it to use it later on in another file. How can I do that?

Thank you

Answer 1

Actually, in OpenCV there is a specific way to do that. You can write an object in an XML file as follow:

 CvFileStorage* storage = cvOpenFileStorage("globalHistogram.xml", 0, CV_STORAGE_WRITE);
 cvWrite(storage, "histogram", global_histogram);

and read is as such:

  CvHistogram* global_histogram;
  CvFileStorage* storage = cvOpenFileStorage("globalHistogram.xml", 0, CV_STORAGE_READ);
  global_histogram = (CvHistogram *)cvReadByName(storage, 0, "histogram" ,0);

Answer 2

The Boost Serialization library is pretty nice. It may do what you want. http://www.boost.org/doc/libs/1_49_0/libs/serialization/doc/index.html

Answer 3

1) Decide on a file format, plan it at the byte level (If an existing format is suitable, prefer it).

2) Write out the data in the file format you decided on.

Answer 4

You can add a method to the class called Serialise , something along the lines of the following:

CvHist::Serialise( std::string fName, bool read )
{
    if ( read )
    {
        std::ifstream fStream( fName );
        // Read in values from file, eg:
        fStream >> this->param1;
        fStream >> this->param2;
        // ...etc
    }
    else
    {
        std::ofstream fStream( fName, ios::trunc ); // (ios::trunc clears file)
        // Read out values into file, eg:
        fStream << this->param1;
        fStream << this->param2;
        // ...etc
    }
}

Note, the order is important - the order that you read the various parameters from the file must match the order you write parameters to the file. Also remember to #include <fstream>

Now, to create a CvHist object populated with data from a file data.txt you can simply write this:

CvHist object;
object.Serialise( "data.txt", true );

If you've populated an object and want to store it in a file, this time, say, bob.dat , write this:

// (object has been populated with data previously)
object.Serialise( "bob.dat", false );