* You have to wait for compilation of the whole library at least once every time you do a build of your program - certainly every time you do a clean build, and potentially even incremental builds if it's header only.
* If the library is header only (many of the linked libraries are) then you you potentially have to pay that compilation cost more than once per compilation of your program - once per every one of your source files that include it.
* Again this is specific to header-only libraries, but to avoid code bloat you'll need to turn on link-time optimisation which is far slower than just allowing the linker to do its job by only compiling definitions into a single object file. (Admittedly LTO is a good idea anyway, but adding a bunch of duplicated symbols is avoidable extra work for it.)
* Some useful libraries are realistically just too big for their authors to write the whole thing in one file (e.g. protobuf, opencv, ... in fact most libraries I use on a regular basis seem to fall into that). They could "release" the library in single-file format, similar to SQLite's amalgam, but then if there are any problems (either a bug in their code or something in your code that makes you want to look at their code) you're now not looking at the original source but some mangled version of it.
* If the library is so large that its interface needs to be split over multiple headers (think Boost or OpenCV) then you're now bang out of luck. Hopefully the library has cleanly-enough separated modules you could potentially release these separately (e.g. OpenCV core, imgproc, imgcodecs, highgui, ...) but then you're essentially back to multi-file libraries.
* Adding a library with a lot of its own transitive depedencies takes proportionally the amount of effort as the number of those dependencies, rather than being handled automatically.
One interesting thing about all of these problems is that they get worse and worse as you need more libraries in your program, or need a larger library for your program. In contrast, using a package manager (I'm thinking particularly vcpkg here) tends to add a one-time cost at the start but allows you to scale your dependency list almost for free.
If you're writing your own library, rather than a application, then there are even more problems with this approach, but I won't quite open that can of worms.
In addition, although the name of the repo is "single_file_libs", it links to many double-file libraries consisting of a pair of .c and .h files. These libraries won't have the issues you are talking about. Developers are well aware of the potential linking problems.
That said, you are right that single/double-file libraries tend to be small. It is hard to work with a file with >10,000 LOCs anyway.
I never claimed to be writing only about his libraries, but instead about single-file libraries in general. Sorry if I wasn't clear about that.
> For his (and many others') libraries, you "instantiate" the implementation in a .c file and declare functions in other source files. This way you only compile the library implementation once.
I tried to make clear which of the bullet points only applied to header-only libraries (again, apologies if it wasn't clear). For example, my objection to compiling the same library multiple times specifically applied only header only libraries. As you say, with two-file (or single .c file, if you like) libraries you only have to compile the library once per build of your program - but as I made clear in a separate point, that is still potentially a pain if you do a clean build (but it could be mitigated by having a separate CMake target, or whatever, that contains all your library files).
> In addition, although the name of the repo is "single_file_libs", it links to many double-file libraries consisting of a pair of .c and .h files. These libraries won't have the issues you are talking about. Developers are well aware of the potential linking problems.
As I just said, I had tried to distinguish those cases. They're less problematic, but still somewhat problematic.
> That said, you are right that single/double-file libraries tend to be small. It is hard to work with a file with >10,000 LOCs anyway.
Yes, and if you only need a small number of small libraries then all my objections are a lot less severe. To spin the conclusion on its head - you can avoid going down the package manager route if you avoid having any large dependencies. For some projects that's acceptable. Personally, I find the pain of working without basic largish libraries like protobuf to be enormous compared to the relatively simple process of using a package manager.
The owner of this repo is a well known figure on game development. His libraries are somewhat widely used and have probably inspired quite a few single-header libraries which adopt similar strategies. My comments are not specific to his libraries, either.
I know. In my original comment I tried to make it clear that some (but not all) of my points applied to these libraries that have some separate compilation. In my second comment I apologised for not making that clear and tried to stress it even harder. I'm not sure what else I can do to make it clearer.
> The owner of this repo ... His libraries
If you just use this guy's libraries, and that works out for you, then all power to you.
But I'm worried about C++ beginners stumbling across this page on Hacker News and thinking, hmm, using C++ libraries is so hard that I need to restrict myself to libraries like these. That is simply a myth - using vcpkg is really not hard at all, and well worth it for all the other libraries it gives you easy access to. Again, if that doesn't matter to you then that's fine, but I want readers to know that the option is available.
By the way, that the vast majority of libraries linked to from this article are not by the author and do not have the separate compilation model you talked about - they're simply header only with inline functions.
Even visual studio's compiler compiles sqlite's 6MB in under a second.
> Again this is specific to header-only libraries, but to avoid code bloat you'll need to turn on link-time optimisation
This is nonsense. This list seems like you are trying to invent problems that aren't there. The vast majority of the time you decide what compilation unit to put the definitions into and that's it.
SQLite is notably pure C, which compiles orders of magnitude faster than C++, or at least certain types of C++.
I've worked on a project - not particularly large - where using precompiled headers reduced compilation time from something like an hour and a half to more like 15 minutes. Admittedly that's a very old crusty laptop, but that's without building the libraries, which are compiled separately! Even on much faster modern machines, building the dependencies is at least an hour, maybe more.
I wonder if our opinions are so strongly at odds because we're simply working in totally different situations to start with. As I hope I've illustrated, compilation time definitely IS an issue for us, but obviously it's not for the projects you're working on - which must surely be C-based or at least C-like C++ code. I would wager more people are in my situation, but to some extent that's irrelevant. The advice to other devs has to be: if you're happy to restrict yourself to small C or C-like libraries then single file libraries are fine, but if you want to take advantage of the full C++ ecosystem be aware that there package managers (again, I'm mainly thinking vcpkg) you can use with only a little initial effort. I think it's dangerous for beginners to see articles like the one linked to here in case it makes them think that using C++ has to be like that.
> > Again this is specific to header-only libraries, but to avoid code bloat you'll need to turn on link-time optimisation
> This is nonsense. ... The vast majority of the time you decide what compilation unit to put the definitions into and that's it.
I said that this paricular objection obly applies to header-only libraries, and that qualification is even in the bit you quoted. I never claimed that there are no single-file libraries that allow separate compilation.
But I do dispute that the "vast majority" allow or require separate compilation (either as a separately supplied .c/.cpp file, or by #defining something before one of the uses of the header). That is the exact opposite of my experience. As an experiment I looked at all the libraries listed under "argv" in the linked article, and 9 of them were header only with no option for separate compilation (Argh!, Clara, CLI11, cmdline, flags, kgflags, linkom, optionparser, ProgramOptions.hxx) while only 1 (parg) allowed separate compilation.
But is it due to 'single file libraries'? What I've seen is that having fewer, fatter compilation units speeds up compilation the same way 'unity builds' do. Instead of having a single monolithic compilation unit, a balance can be used to iterate faster and use multiple cores. Compilation units can be groups of things that are changed more or less frequently.
> But I do dispute that the "vast majority" allow or require separate compilation
It isn't about the library always being made for separate compilation. The reality is that unless you are using something fairly fundamental it will probably only need to be in one compilation unit in the first place.
What does need to be shared are data structures and those ideally have as little logic and dependencies as possible.
> But is it due to 'single file libraries'?
That's a fair point, they're generally not. I just meant that if those libraries were single file, but otherwise unchanged, then the problem would only be worse. I see what you're saying about unity builds, but if you build your dependencies totally separately from the rest of your program then it's not so important exactly how fast they build. I'm imagining a situation where you leave vcpkg to build all your libraries overnight, then you spend the next few weeks iteratively working on your application code (which is how it usually works for us).
> The reality is that unless you are using something fairly fundamental it will probably only need to be in one compilation unit in the first place.
In the projects I work on, which are really quite varied, many (but not all) libraries are used throughout the codebase as vocabulary types. Again, I think this just comes down to very different codebases and uses of libraries for us.
I don't know what 'vocabulary types' are, but I think this is an oversimplification. My guess is that most big programs end up like yours but my point is that it doesn't have to be this way. Single file libraries are not the problem and can actually help, although the real issue is deeper.
I think the problem is actually dependencies and many times single file libraries go to great lengths to not depend on anything else. To keep it as simple as possible, I think classes / data structures get over used and instead of just making an interface to some data, people stuff everything they are doing into some class they have.
Transformations from one type to another put into data structures means that the class now has dependencies on those other types. If those types have transformations then they have dependencies and so on. The extrapolation is that everything depends on everything else and even compilation units that should be tiny end up pulling in large parts of the entire program as source code. Lots of compilation units can then mean compiling huge chunks of the source hundreds of times.
This approach lacks any features that reasonable dependency manager provides, such as providing updates compliant with semver.
Can trivially support multiple incompatible versions of the library, nothing needs to be fetched or resolved (once you've got the single file of course), can send and distribute it easily over any channel you want (web, email, free file host, google drive, your own website, etc).