SDF readFile not parsing world file [closed]

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I am using ROS Indigo with Ubuntu 14.04.

I need to parse the elements of a simple_environment.world file using sdf::readFile. The simple_environment.world file is as follows:

<?xml version="1.0" ?>
<sdf version="1.4">
  <world name="default">
    <include>
      <uri>model://ground_plane</uri>
    </include>
    <include>
      <uri>model://sun</uri>
    </include>

    <model name="box">
    <pose>1.4 0.8 0.14 0 0 0</pose>
    <static>true</static>
    <link name='link_box'>
        <pose>0 0 0 0 0 0</pose>
        <collision name="collision_box">
            <pose>0 0 0 0 0 0</pose>
            <geometry>
                  <box>
                    <size>1 2 1.3</size>
                  </box>
                </geometry>
            </collision>

            <visual name="visual_box">
            <geometry>
                  <box>
                    <size>1 2 1.3</size>
                  </box>
                </geometry>
            </visual>

        <self_collide>0</self_collide>      
        <gravity>1</gravity>
    </link>
    </model>

  </world>
</sdf>

In a portion of my C++ code I try to parse the content of the world file as follows:

// Read the sdf
sdf::SDFPtr sdfParsed(new sdf::SDF());
sdf::init(sdfParsed);

const std::string sdfStringPath(argv[1]);

assert(sdf::readFile(sdfStringPath, sdfParsed));

But this does not seem to function as expected because if I try to print sdfParsed with

cout << "THE PARSED SDF IS: " << sdfParsed->ToString() << endl;

the output is the following:

THE PARSED SDF IS: <?xml version='1.0'?> <sdf version='1.5'/>

The argv[1] is clearly the path to the above mentioned .world file. What's wrong with my code?

Thanks in advance.