> The most widespread C++ libraries are ones that are single header files. Because that’s the only thing that’s trivial to implement into an existing project.

I think this has much more to do with templates than anything else. A template can only exist in a header.

Then there is also the performance benefit of inlining. (As well as performance hit of inlining, namely code size.)

There is a separate, much later movement/phenomenon of C (not C++) developed as header-only libraries, which by contrast I don't think there is nearly as good reason for except for that some people are confused at how a linker works. I have seen use of that technique advocated on Hacker News much more frequently than in production.

Header only and single file are slightly different. A headery library may include many many files. A single file or single header may be “header only” or it may have an impl define. People who prefer this distribution mechanism are generally ok with 1 header plus 1 cpp.

These libraries generally have no third party dependencies. Because tracking down and compiling C++ dependencies is a dumpster fire.

Or maybe they don't actually need the dependencies. There is a style of writing code to minimize dependencies and many people value this.

I would take the sqlite amalgamation as an example of good use of "few files, few dependencies". First off, it is above all a solid piece of work. It is distributed with one header file and one .c, even though it is developed as multiple files. Iirc the sqlite website cites a ~10% speed improvement or so due to the optimizer being able to do a better job across the whole library. In my opinion this is a better reason than "tracking dependencies sucks" or "omg too many files".

I'm pretty sure it's the (lack of) package managers.

The presence of a defacto package manager completely changes the library dynamics.

I can't even link and test properly against GMP on Windows :/

Sigh. Keeping sqlite as an example. On most Unix-like OSs (and even windows now) there is a dynamically linked sqlite managed by a package manager.

It also predates the time period when people would whine about a lack of package managers.

As do a bunch of these libraries.