What advantages does it have compared to others? Or should we use something like Ideone while a contest is going on? Which one is best among these?
I won’t recommend online editors. (Due to recent case where many contestants’ code got public due to website issues…
About offline editors, its all about which one you are comfortable with. 
Almost all of them have very similar features, so the thing that makes the difference is you and your coding skills, not editors. (As i believe).
I prefer atom or sublime but it depends on you.
It’s always better to code offline as there’s no chance of codes ever getting leaked. And if you absolutely have to, better use Codechef IDE.
As stated before, all the rest have similar features and you need to try them all out to find your perfect match. Each of them have their own learning curves.
I normally prefer something lightweight, like a text editor with built-in build and run option. Sublime Text 3 suits the best for me. It’s fast and highly customizable and has appealing looks and design. You don’t need to learn any commands to start typing there.
But if you want super speed go for something like vim/emacs. They do have a higher learning curve but it’s worth it.
However, I would like to point out that for longer or more complex codes debugging is difficult with text editors. You may wanna switch to a dedicated IDE with debuggers for that.
Lastly, it all depends on how comfortable you are with your system, and how much effort you want to give in adapting yourself with other editors.
I think sublime text or atom is far better than notepad++. And regarding vim the problem is there is no auto completion and it will take some time to get comfortable with vim.
For C++, CLion by JetBrains is quite good. Give it a try!
Link: https://www.jetbrains.com/clion/
You can get it for free by signing up using your college mail Id from here, https://www.jetbrains.com/student/
Hey… Do you know how to set some code that we want as our default code in CLion. I mean if I create a new source file or project, my file comes empty. Is there a way to set some piece of code as default??
No, my friend, I don’t know. I once tried using Live Templates, but failed. 
Currently, I keep open one tab which consists of the template code and then copy it in each file.
You can learn about live template here, https://www.jetbrains.com/help/clion/creating-and-editing-live-templates.html .
I use Sublime Text. I split the window into four parts - Code, Input, Output and Compile info.
Screenshot
U can use sublime text or atom both are good offline editors, u can also try codechef ide where u can submit soln ,view question ,code on same page;
I too use DevC++. If you are coding in Low Level language like C/C++ then go for DevC++. Else if you are coding in high level language like Python then i suggest to go far “Sublime Text”. Its ahead of DevC++ for Python. Mainly because of folder organisation and because of Auto-Suggestion for keywords. It is faster and less error prone that way.
As far as Notepad++ again same utility as DevC++ but i would go with DevC++ given the choice. And Atom/Emacs/Vim/Codeblocks i have not tried them. So cant comment on it.
Hope it helps ! And yes i wouldn’t recommend online editors.