Is there any way to have a simple edit box in plain X11 C or C++ code?
By "plain X11 C or C++ code" I mean creating the control on a XCreateSimpleWindow created window as the CreateWindow("edit"..) equivalent in Win32 API.
If not, what are my options to have a simple edit box with the minimum amount of dependencies (i.e. no Qt please) for a simple edit box input dialog?
On
X11 is a very low level protocol. It provides the ability to create individual windows, draw on them, and receive individual input events, and nothing more. That's it.
The job of providing high level user interface, like edit boxes, is for higher level toolkits, like Qt or Gtk. Those are the most common toolkits. There are a few less-popular ones too.
But the bottom line is that there are no "edit controls" of any kind, that you get with Xlib, the library you're using to talk X11 protocol. If the only dependency you want is Xlib, then it's up to you to actually implement the edit control, using your own code. If you don't want to use any common font handling libraries, like fontconfig and freetype, you will have to write the code to enumerate the X server's built-in fonts, handle keyboard input, and draw your edit control. Just the job of translating the somewhat obscure keyboard encodings used in X11 into something usable, like UTF-8 or unicode, will take a few hundreds of lines of code.
So the answer here is: use Qt or Gtk.
Plain X11 doesn't have such functionality - It is a low-level windowing toolkit (opening and maintaining windows, receiving input events, drawing to windows).
In case you are prepared to write such an edit window from the available primitives, you are fine. If not, you need to use some toolkit that does it for you. The simplest and most lightweight one providing such functions would probably be Athena widgets (in case you are not particularily choosy about look and feel).