how to take screen shot of a output console screen in c++ using graphics.h package

Print screen Alt+PrnScr always works on console applications. You are doing something wrong; or your OS is broken.

Basically, you do it through getimage(int left, int top, int right, int bottom, void *bitmap), please see:
http://www.programmingsimplified.com/c/graphics.h/getimage[^].

Additional problem is how to get the extents of the console window. Here you need to use include <windows.h> as well.

In particular, you can assume that the screenshot code is being executed or you can get screen shot of all windows on the desktop, or use some other selection criteria. You will need some of the following Windows API functions:

http://msdn.microsoft.com/en-us/library/windows/desktop/ms633504%28v=vs.85%29.aspx[^],
http://msdn.microsoft.com/en-us/library/windows/desktop/ms633505%28v=vs.85%29.aspx[^],
http://msdn.microsoft.com/en-us/library/windows/desktop/ms633515%28v=vs.85%29.aspx[^],
http://msdn.microsoft.com/en-us/library/windows/desktop/ms633519%28v=vs.85%29.aspx[^].

The last one GetWindowRect ultimately gives you the size/position parameters you need for a call to getimage.

—SA