YouCompleteMe is a fast, as-you-type, fuzzy-search code completion engine for Vim. It has two completion engines: an identifier-based engine that works with every programming language and a semantic, Clang-based engine that provides semantic code completion for C/C++/Objective-C/Objective-C++ (from now on referred to as “the C-family languages”).

Here’s an explanation of what happens in the short GIF demo above.

First, realize that no keyboard shortcuts had to be pressed to get the list of completion candidates at any point in the demo. The user just types and the suggestions pop up by themselves. If the user doesn’t find the completion suggestions relevant and/or just wants to type, he can do so; the completion engine will not interfere.

When the user sees a useful completion string being offered, he presses the TAB key to accept it. This inserts the completion string. Repeated presses of the TAB key cycle through the offered completions.

If the offered completions are not relevant enough, the user can continue typing to further filter out unwanted completions.

A critical thing to notice is that the completion filtering is NOT based on the input being a string prefix of the completion (but that works too). The input needs to be a subsequence match of a completion. This is a fancy way of saying that any input characters need to be present in a completion string in the order in which they appear in the input. So abc is a subsequence of xaybgc , but not of xbyxaxxc . After the filter, a complicated sorting system ranks the completion strings so that the most relevant ones rise to the top of the menu (so you usually need to press TAB just once).

All of the above works with any programming language because of the identifier-based completion engine. It collects all of the identifiers in the current file and other files you visit and searches them when you type (identifiers are put into per-filetype groups).

The demo also shows the semantic engine in use. The current semantic engine supports only C-family languages. When the user presses . , -> or :: while typing in insert mode, the semantic engine is triggered (it can also be triggered with a keyboard shortcut; see the docs).

The last thing that you can see in the demo is YCM’s integration with Syntastic (the little red X that shows up in the left gutter) if you are editing a file with semantic engine support. As Clang compiles your file and detects warnings or errors, they will be piped to Syntastic for display. You don’t need to save your file or press any keyboard shortcut to trigger this, it “just happens” in the background.

In essence, YCM obsoletes the following Vim plugins because it has all of their features plus extra:

clang_complete

AutoComplPop

Supertab

neocomplcache

Why Did You Build This?

Several reasons, but mostly because a long time ago, in a galaxy far, far away I used to write C# code in Visual Studio. Its IntelliSense engine is a wonder to behold, and I missed it in Vim. Don’t get me wrong, Vim is a much better editor than VS (that’s why I switched to it), but Vim’s various built-in code completion systems are an absolute joke. As in, not-even-funny bad. I’m not going to press some complicated keyboard shortcut every single time I’m typing an identifier; no, screw that, the system should trigger itself and (unobtrusively) offer me completions that I can then either use or ignore.

The awesome Vim community valiantly tries to make the best it can out of the built-in “omni-completion”, but these efforts boil down to putting lipstick on a pig.

You could say that AutoComplPop (ACP) already offered us this type of completion auto-triggering as the user types. Fair point, but there are several issues with that plugin. First, it’s dead. Last update was in 2009. Even worse, it’s slow. Last, there’s no fuzzy search. I really, really like fuzzy search. Oh, and there’s no semantic completion with ACP.

The plugin that really made me see the viability of the subsequence match approach was the truly magnificent Command-T plugin. It does subsequence-based matching on file paths for easy file opening. This is probably my favorite Vim plugin, right up there with EasyMotion. I couldn’t (and wouldn’t want to) imagine writing code without them.

We also want our completion system to be smart about ranking whatever completion candidates survive the filtering step. In a perfect world, the candidate you want is exactly at the top of the menu so you need to press TAB only once. The whole point of a code completion system is to save you keystrokes and if you have to cycle through the menu to get to your candidate the point is lost.

Usually, you know exactly what string you want to type, but its long and you don’t want to type 10 characters if 3 plus a TAB will do the same job.

Well, that’s one use case for a code completion system. There’s another. That’s if you don’t actually know what string you want to use. This is the “API exploration” use case. You might have a vague idea of a function you want to call, maybe you even called it a few times before, but you can’t remember the full name.

This is where semantic completion comes into play. It allows you to explore what methods are available on the object you’re handling, what functions are accessible from the scope you’re in etc. A really good semantic completion engine will even help you with the function’s parameters.

Oh and we also want our completion system to be lightning-fast. If we’re waiting for it to return a response… well it’s hardly doing its job of speeding us up then, is it?

So, let’s see where we are now with requirements for a completion system:

Auto-triggers as you type. Uses subsequence-based completion filtering. Uses smart heuristics to intelligently rank whatever candidates survive the filtering step. Offers semantic completions. Fast, fast, fast!

clang_complete fails everything except #4 (for C-family languages), which it excels at (it uses libclang, much like YCM). IMO it also fails #5 pretty badly because it’s both slow (lots of Python logic, whereas YCM only uses Python as glue code) and it blocks the GUI thread (something YCM does its best to avoid).

ACP as mentioned fails everything except #1. Neocomplcache can be configured to provide #1 and it kinda sorta sometimes does a half-decent job at #4. I’ve used neocomplcache for over eight months and my frustrations with it directly led to the decision to build YCM.

SuperTab I have limited experience with, but from what I understand, it does a half-decent job at #4, much like neocomplcache.

YCM fulfills all of these requirements. #4 is currently only fulfilled for C-family languages, but YCM has an internal Completer API that is designed so that completers for other filetypes can easily be implemented and hooked-up to the rest of the system. A Python semantic completer will probably be built some time in the future, and others can help out with completers for other languages. In the mean time, there will probably be a “generic” filetype completer that queries Vim’s omni-completion system for completions if a specific completer has not yet been written. YCM would provide the other components on top of the results coming from omni-complete. The “generic” completer hasn’t been built yet but it’s on the roadmap.

Things We Can’t Build in Vim yet but Would Like to

I can’t show a box with help text related to the function right next to the completion menu in Vim. There’s just no API for that. libclang can actually extract doxygen information from function comment blocks, but there’s no way to spawn such a “sub-window” arbitrarily on the screen in Vim.

The best Vim offers is the “preview” window that can show up at the top of the file. YCM supports this, but it’s just not the same. Your eyes have to leave the small area of the screen where you’re currently typing and on a 30 inch monitor this is annoying.

So no, we can’t build the full VS experience. I’ll take what I’ve built so far though.

Closing Words

I’ve been building YCM on-and-off for a year now in my spare time; I’d guess about 4 months of work went into it. It’s not a terribly complicated system to build, the main causes of slowdowns were bugs in Vim I encountered while building this. Those were incredibly annoying and put me off the whole thing for months.

But it’s reached a very releasable state. I’ve been using the identifier completer for roughly eight months now and the semantic completer for about two months (with C++ code). In my experience, they’re both rock-solid. I’ve had co-workers bang on YCM and lots of bugs were fixed as a result; hopefully it will be pretty stable for a wider audience as well.

Enjoy the plugin and feel free to report any issues you encounter!