Callbacks in C++ using template functors (1994)
tutok.sk
tutok.sk
https://www.boost.org/doc/libs/1_68_0/doc/html/signals2.html
It provides a slightly nicer interface in that signals can be defined in a single template argument, vs. splitting args / return type as separate template arguments. For example:
boost::signals2::signal<void (float, float)> sig;
void do_something(float x, float y) { ... }
Qt's approach requires a separate compilation pass woith a Qt-specific tool named "moc" which declares a bunch of info in the signal class that's necessary for the slot implementation.
IMO, the Boost implementation is nicer. It's vanilla C++ (no moc required), is thread safe, and supports some advanced features like "combiners".
In terms of which to use, it depends on your use case.
Lambdas let you easily specify a single callback function, so they're a nice syntactic sugar for passing to STL functions (e.g., for_each). If you're the sender and only have one callback to fire, a lambda is great for this.
If you have to fire n callbacks, you could probably build a signal / slot mechanism with lambdas. But at that point, I'd start to consider using a dedicated library like signals2. It takes care of a bunch of things like object lifetime (e.g., slot deallocation if an object is destructed), efficient management of the list of slots, and thread safety.
it's not comparable, a boost::signal is equivalent to a list of functions which gets called when the signal is triggered, instead of a single function.
If you are willing to accept uglier macros, you can have QObject-compatible signals / slots in pure C++ without any kind of specific tool : https://github.com/woboq/verdigris
Also, Qt's approach is thread-safe (if you call a signal in a thread A on an object attached to a thread B, the slot will be executed in thread B) since it is tied to Qt's event loop.
Back in games we'd usually use some DSL to generate all the boilerplate code for things like this(also including a whole bunch of editor-specific flags for signals/members).
Also, every time I bring in boost headers I watch compile times crater. C++ 11 or 14 is good enough at this point that I don't see a strong reason to use boost. I know of 2-3 projects that migrated from boost -> C++11 for that reason alone.
...or that the code uses templates extensively (it's boost after all) thus everything must be placed in the same compilation unit so that the code can be generated at compile time.
No, you can't. The only case where the declaration of template functions/classes can be separated from definitions is in the rare cases where only a fixed and predetermined set of objects/values are used, thus enabling explicit instantiation.
It is not surprising that boost choose convenience over tradition and fast compile speeds.
on linux / macos it's fine, an apt-something away, but on windows anything else than header-only is a pain if you are providing a C++ library, because you may need :
- static and dynamic builds
- debug and release (changes the std:: ABI by default on windows)
- with static (/MT) and dynamic (/MD) runtimes (for whatever reason you can't link /MT with /MD code)
- x86 / amd64 (and more and more people are using windows on ARM increasing the demand for arm and aarch64).
So you need at least 2 * 2 * 2 * 4 (ie 32) distinct builds of your library if you want to cover all cases, and that's ONLY for vs 2015 and 2017 - older and newer versions break binary compatibility ; likewise for mingw / msys which needs its own set of libraries due to using itanium name mangling.
You should probably be using tools that generate the Visual Studio projects for you. Some of these tools have been around for a long time, many of them are quite mature. You should be using GYP, CMake, Bazel, or something similar for building. These tools all work on Windows in addition to working on macOS and Linux. Hell, if these tools don’t work for you, then writing a Python script which spits out the Visual Studio projects is not out of the question either… I’ve done it, it’s not terrible!
If you are choosing to make a library header-only just because it’s too hard to build otherwise, well, my request is that you spend a day learning a build system which is not Visual Studio, and see if next time you make a library, you don’t restructure the code just to avoid learning how build systems work.
Yes, updating my Bazel build files for third-party dependencies creates extra toil when I upgrade versions. In my experience, this is a small amount of toil compared to the API changes a third-party will introduce (e.g. upgrading libfmt to 5.0 meant touching a hundred files), so I am willing to pay that price to get faster builds.
Or I’d put it this way:
- Most of my time is spent making changes, recompiling, and seeing the results. Small improvements here have a large positive impact on productivity, and header-only libraries make this worse.
- A small amount of my time is spent managing the build system and writing build scripts. Tasks like writing my own Bazel build script for a third-party dependency are not done often, maybe only when porting to a new platform or updating to a new version, so even though I spend extra time here, it is not burdensome.
I don't know why it can't be simpler but apparently it can't.
How many CMake config files export targets for debug and release windows libs for instance ?
A more direct analogue of the type discussed in the article is std::function, which is simply a type that can contain any one callable (function pointer or function object, including lambda functions) or can be empty (like a null function pointer), and can be called in an immediate synchronous way.
Anyway, I find the use of the term "callback" somewhat troubling in the context of OO software. A "callback" is really a procedural concept. In an OO language, it's just a message that you send to one of your arguments, and since message sending is all you can do, the most natural thing in the world.
One example here that's also popular particular in the FRP examples is the button, which needs a "callback". But when you look at a button in Objective-C/Cocoa, it just sends a message to an object. No callbacks in sight, though of course the actual underlying mechanism is very similar.
This turns something like Cocoa into an architectural adapter, adapting the UI into the message-sending architecture of the rest of the program. The user becomes a participant in the program, sending messages to objects in the program by way of the button.
Actually, the fact that C++ has both more and less aesthetically pleasing subsets is a big problem. It means that different people prefer different ways of doing things for purely visual reasons rather than reasons of overall simplicity and maintainability, and that creates needless conflict and churn.
In contrast, Python has very little variation in aesthetic pleasingness. It's all the same Dutch Utilitarian Modernist, perfectly OK if unexciting to look at (@ metaprogramming is an exception.)
And a contrast in the other direction: nobody ever spent a lot of time tweaking their FORTRAN deck for aesthetics.
Just to pick something on my screen at the moment, consider this C++ from mc-stan:
namespace stan {
namespace math {
namespace {
class cos_vari : public op_v_vari {
public:
explicit cos_vari(vari* avi) : op_v_vari(std::cos(avi->val_), avi) {}
void chain() { avi_->adj_ -= adj_ * std::sin(avi_->val_); }
};
} // namespace
But the big problem is that aesthetic choices interact in complicated ways with architectural decisions. For example, using operator << as syntactic sugar over .add() is motivated by aesthetics, but it affects how you do encapsulation.
A good language lets you choose your architecture, and then write an aesthetically pleasing program to match.
On the contrary, C++ treats user-code aesthetics as a first-class design-decision, moreso than many other languages.
In C++ the whole thing is that built-in types aren't special, including ints and operators. This is a deep concept in the design of the language.
The fact that you can express abstract things like matrix multiplication, compile-time unit-conversions, and compile-time range-checking through operator-overloading speaks to how aesthetic the language is.
> It means that different people prefer different ways of doing things for purely visual reasons
I've never seen any sizable project where visual aesthetics of the API aren't relevant. Java's got chained builders and lambdas, Python's got `@property` and various `__foo__` methods, and any sufficiently large Ruby project's syntax is a DSL wrapped in a riddle wrapped in bacon.
User-defined APIs are syntax, so why limit the tokens?
> A good language lets you choose your architecture, and then write an aesthetically pleasing program to match.
C++ does let you do that. You've pointed to an example where there was little or no intentional thought put into the aesthetics of the API. E.g. `avi_` really could own those methods, and the underscores aren't required by the language.
You can argue that C++ gives you too much power or that sometimes it's non-obvoius how to use that power or that you've seen code where the API designers have taken enough rope to hang themselves. But of course it is possible to write "Java" in C++ and avoid any operator-overloading or syntax-sugar if your team decides it wants to do that.
Except it needs to interact with C, which significantly limits the extent of this unification.
SomeType variable;
Libraries like MTL (the matrix template library) are completely architected around providing their nifty syntax. They do insane things like have operators return types representing a symbolic computation, so that template specialization can be used to do a kind of peephole optimization. The end-user experience is aesthetically improved, but do not stare into the abyss of the template definitions.
That is getting better and better with time. C++14 with generic lambda, C++17 with constexpr and C++20 with concepts imrpoved all of that.
It made the writing of template expression switch from "obscure C++ gourou dark magic " to "Kinda readable C++ dude".
Really ? Python would probably be the last language I would quote for "One way to do things" and "little aesthetic pleasingness".
There is around 20 ways to do packaging, unit testing, list iteration, string concatenation, list iterations, network calls, Object Oriented and almost any single action in Python.
The Zen of python (https://www.python.org/dev/peps/pep-0020/) might quote "There should be one-- and preferably only one --obvious way to do it.". That never have been the case in practice.
Rich Hickey did not bring joy and aesthetics to software development, it was already there before he was even born -- https://en.wikipedia.org/wiki/Lisp_(programming_language)
What he did, he made it more popular and more accessible which is at least as important.
template<typename CBT> void addListener(CBT cb) { static_assert(is_invocable_r_v<int, CBT, Button*>, "type mismatch"); }
void addListener(std::function<int(Button*)> cb)std::function can potentially allocate memory and hides stuff behind a vtable, e.g. two indirections. That's not what I would call "frationally less efficient".
But as a fraction of your application's overall runtime, in almost cases it's going to be an absolutely insignificant fraction, like less than a millionth of the runtime. This is certainly true if you're writing a GUI or a network-based server process. For it to be noticeable you'd have to be calling the callback in an extraordinarily tight loop, where the actual function does very little (less than a few pointer indirections!).
Put another way, avoiding std::function is a classic case of premature optimisation.
But are you writing those in C++ ? C++ is used exactly where you have those tight loops. Execution engines, VMs and interpreters... Besides, as soon as you want to have some real-time behaviour you can't have any call to malloc since it may make performance much less deterministic due to locks in malloc implementations. And there is a lot of real-time code being written in C++.