Problem with libraries

[eluser]Unknown[/eluser]
Welcome and sorry for my poor english.
I have big problem with libraries.
I writed my own library:

Code:

class Log_in
{

    function Log_in()
    {
        echo $this->input->post('username');

    }

}

This library is loaded by autoload.php file.
When I run this class in controller, I receive this error:

Fatal error: Call to a member function post() on a non-object in...

Can you help me?