No, that title isn’t a mistake. You read it right.
Note: If you don’t use CMake, you may not be satisfied with the CMake-centric tool pertinent to this post.
If you want to skip a lot of pontificating and bemoaning, jump to the Get to the Point section below.
C++ developers are stubborn. Me included. For people that write in what may arguably be the most complex programming language in history, they also seem to want their tools to “just work” without having to sink a lot of effort.
You know what I’m talking about.
C++ developers want a package and dependency management tool that will work with their existing codebase without them having to make any changes to their project and build system. Well I have some bad news: They don’t, they won’t, and (in my opinion) they shouldn’t. But… that’s a subject for another post.
The Basic Hurdles
Adopting a package manager for a project presents a few challenges. I’ll talk about Conan and vcpkg for now (and you’ll see why in a moment).
- How do I install it?
- For Conan, this requires that you have a Python installation, and that you use Pip. C++ developers, in their stubbornness, don’t want to be required to learn another tool.
- For vcpkg, you need to obtain a particular revision of their Git repository, run the bootstrap script, then optionally deploy integrations for it for your shell.
- How do I use it?
- For Conan, you write a
conanfile.pyand place it in your project, then run
conan installon that directory, then include
CMakeLists.txt, then call
conan_basic_setup(TARGETS), then link against the Conan-generated targets
- For vcpkg, you
vcpkg install [pkg-name [...]]to install some packages, then pass
-DCMAKE_TOOLCHAIN_FILE=<path-to-vcpkg.cmake>when you run your CMake configure, then
find_package()the requirements you want to import. The way you link to those packages depends on the individual package
- For Conan, you write a
- How do I upgrade the Package Manager?
- For Conan, you use
pip install conan -U, or
pip install conan==<version>for a particular version. You need to run this periodically, as Conan won’t remind you itself when updates are available.
- For vcpkg, you need to manually re-download and re-bootstrap the repository.
- For Conan, you use
- How do I package my software?
- This is out of scope for this post, but I’ll come back on a future date.
This is actually quite a lot of extra stuff that one needs to do to use these
tools. As such, both Conan and vcpkg support being used as an “optional” tool
for a project. In particular, this is why vcpkg uses
do its bidding: It’s an easy way to shoe-horn arbitrary code into an existing
CMake project, as the toolchain file will be
include()‘d by CMake when it
does compiler checks. The CMake project need not necessarily be aware that
this extra tool is modifying the build process.
To find the root of the problem, let’s follow the 5 Whys. Ask “Why?” a few times, and you’ll eventually find a root cause.
Fear of Commitment
But why? Why do C++ developers avoid committing to these tools? Why not just
put your foot down and require these tools? “To work with my project, you
I’ve been watching closely, and I have a hypothesis:
C++ developers don’t want to use to these tools because no one else uses these tools.
Of course it’s an exaggeration to say “no one uses these tools.” There are
plenty of projects in the wild with a
conanfile.py in their repository, but
not nearly enough to counter the network effects. Adopting vcpkg doesn’t
require changes to your project, and many projects will work with it “out of the
box,” but its adoption is still far lower than its potential.
Let’s say I run a moderately successful open source library, and I’m tired of getting bug reports about trouble using the library with dependency versions that I don’t support. I could go the route of simply documenting exactly what versions of dependencies are supported, but documentation is to users as code comments are to compilers. And we know how to fix useless comments: Make them tool-enforced!
So I drop a
conanfile.txt into my project and update the
check that the user has run
conan install, and links to the
CONAN_PKG::SomeLibrary generated targets. By doing this, I am effectively
dropping support for building my library using
apt-get as a dependency
If you’re like many C++ developers, you probably just recoiled in horror at the thought. Let’s explore those feelings, and some common objections:
yumare perfectly fine package managers for open source libraries.”
I could spend a good few posts just talking about everything wrong with this sentiment. It’s just wrong. I won’t even bother explaining it here, but maybe I’ll write another post if I get enough hate-mail.
“My dependencies aren’t available in Conan/vcpkg.”
This is a valid objection, but I can’t help but feel it causes the problem to perpetuate. People don’t use the package manager, and therefore their libraries will not appear in the package repositories, and downstream packages won’t use the package manager either since their dependency isn’t in the repository, and even further downstream packages won’t use the package manager…
The only way to fix this is to start contributing!
“Conan/vcpkg doesn’t work exactly how I want it to.”
Understandable, and I say the same thing: Conan and vcpkg don’t work exactly how I want them to either.
The problem is the same, and the solution is the same: Contribute!
“I don’t want to install another tool when I’m already using CMake!”
This objection simply makes no sense. If you have ten dependencies, you’re already downloading and managing ten external items (plus any transitive dependencies). You could instead hand over those 10+ tasks to a single tool dedicated to the job. It’ll probably do the task better than you anyway.
“I don’t need something new. What I have already works.”
This sounds very familiar…
“Conan/vcpkg doesn’t support my very special and unique codebase.”
I’ve seen some wacky codebases, and I’ve written several on my own. I can almost guarantee: You aren’t that special, and if your code is built in such a bizarre and abnormal way, I am 98% certain it doesn’t need to be.
“I don’t want to force my users to use a single version of
This is a common objection to committing to a dependency manager, and it raises some interesting questions that may warrant its own “5-whys” and another post. I’ll just say here that I feel this fear is unnecessary and detrimental to C++ development. If I get enough hate-mail I’ll write some more on the subject.
“I don’t want to force my users to learn and use
There it is.
This is an objection that you can’t readily refute, so let’s go again: Why? Why
do you not want to force your users to learn and use
The Cost of Commitment
It boils down to empathy. We do not want to force our user to do something difficult: It will hurt our adoption. Now why do we say that adopting these tools is difficult? Well, we can go back to The Basic Hurdles section above and see why.
The root problem is not that we ourselves refuse to jump these hurdles, but that we know that our users will prefer a solution which does not require one. If the onboarding and consumption process of my project is just slightly more difficult than a competing project, users will opt for the easier solution.
To compensate for this additional difficulty of consumption, the product you offer must be sufficiently superior to the alternatives to justify the additional work required to use it. This is why header-only libraries are so popular: Cost of consumption is (seemingly) very low.
Unless we can offer something sufficiently superior to the competition, the cost of commitment [to a packaging tool] is the most important measure of a (non-commercial) product’s success: The size of its userbase.
Get to the Point
I’ve blabbered on for a while now, but this has all been to illustrate the problem that I have set out on solving:
Adopting package and dependency management tools creates too much friction.
I have a few long-term concepts and plans on how that fiction my be reduced, but those plans require cooperation between build systems and package managers that is currently absent. So in the meantime I’ve developed an alternative: an addition to CMake that makes using a package and dependency manager drop-dead simple for both developers and consumers.
Why CMake? It’s the most popular build system for C++ code these days (for better or worse). It is fairly well supported by packaging tools, and it is the build system in which I have the most expertise. Similar tools could be created for other build systems, but for now I’ve implemented one for CMake.
It is called PMM, The Package Manager Manager.
What Does PMM Do?
PMM does what it says on the tin: It manages package managers. Currently it supports Conan and vcpkg, but other packaging systems could also be supported in the future.
In particular, PMM is implemented as pure CMake script code that will automatically download, bootstrap, extract, and control package managers.
For Conan, all you need is a relatively recent Python installation. Any recent Linux distribution will be ready to use Conan in PMM. Windows users will need to have Python 3, if they haven’t already.
For vcpkg, Python obviously is not required. All you need is a relatively recent C++ compiler.
What Does PMM Look Like? How Do I Use It?
To use PMM, you will need to make the following three changes to your project:
pmm.cmakeand place it in your project repository. Do not download this file automatically as a build step. Do not make this file optional for your build. Do not install this file externally. Do not embed this file’s contents within another file.
pmm.cmakefile from your root
- Call the
That’s it! Of course, there will be additional steps depending on which package manager you wish to use.
I Have an Objection to this Tool!
I anticipated a lot of possible objections to PMM, and I’ve written answers to many of them in the project README. Give it a look before sending me hate-mail.
PMM addresses a few huge concerns with using an additional tool:
Keeping your Package Manager Up-To-Date
PMM is able to automatically install and manage the version of your package manager. It will never automatically update anything, but it will notify you when there are updates available.
Ensuring Your Environment
Since PMM automatically obtains and executes the package manager, there is no
need to document or enforce additional setup steps for either your users or
your continuous integration system. Running
cmake to configure your project
will do all that is needed to ensure your dependencies are available to your
No longer do you need to direct your users to download dependencies manually and fret about how they might do that depending on their platform. PMM will ensure the dependency manager will be run and do it for them, and your dependencies are encoded in your repository as code, rather than hoping and praying that your user or CI server have the correct versions of the dependencies.
You already require your user to execute
cmake to build your project. Why not
piggy-back off of it and use it to get your dependencies as well?
Call for Feedback
PMM is still very new, and the posting of this page marks the 1.0.0 release. Please try it out and tell me what you think! Here is the repository where you can download it or send me issues and feedback