C++ Patterns: The Badge

https://awesomekling.github.io/Serenity-C++-patterns-The-Badge/

by brianush1 • 7 years ago

203 155 7 years ago

awesomekling.github.io

arithma2• 7 years ago

A different pattern I have thought of would be to provide an accessor interface object, and certain functionality is limited to be accessible only through that object. Controlling who has access to that interface can be flexible (possibly returning it along with construction, or passing it in at creation time...) Albeit, this is not without overhead, and for that, Badge is actually better, in terms of performance, if it fits the use case. What happens when you have multiple Device objects though, in this case, they can all access VFS..

C++'s private inheritance can be used here to avoid some of the gymnastics, wherein, that private aspect of the class can be passed out into the appropriate party at creation time or otherwise.

To illustrate: https://gist.github.com/arithma/0d96aec5c07e2d8dc24cfc6cd31f...

Note: I have never used this in production as far as I remember.

mmkos• 7 years ago

Pretty sure you described the attorney-client pattern.

arithma2• 7 years ago

If it is, it's the first time I hear about it. It always gives me a kick when I make up a similar design pattern to some grey beard's invention. Alas, I should read up on those patterns to avoid reinventing stuff and sharing things with people with appropriate names.

nisuni• 7 years ago

How can {} return a Badge object? Some kind of operator overload?

antisemiotic• 7 years ago

Brace initialization, I suppose.

cjhanks2• 7 years ago

It's bracket initialization of a struct, in this case the struct has no variables - the type is implicit from the method declaration.

eMSF• 7 years ago

While it makes no difference in this case, it's worth noting that Badge<T> is not an aggregate type because it has a user-provided constructor. Therefore the object is value initialized, i.e. the braces are equivalent to 'Badge<Device>()'.

edit: what this means is that even if the struct had data members, you couldn't brace initialize it like an aggregate type, and to achieve something syntactically similar, you would need a suitable constructor.

notacoward• 7 years ago

So a badge seems a bit like a capability. Clever; I like it.

monocasa• 7 years ago

Yeah, my thought too. Even the name badge invokes the idea of a capability.

fiddlerwoaroof• 7 years ago

And, I believe you could probably subclass Badge to make capabilities more granular.

zwieback• 7 years ago

I like it but is access restriction really a problem? In my years of C++ programming it always seemed to be a theoretical problem more than something that leads to crashes.

detaro• 7 years ago

I'd guess it's more architectural concern than runtime. Making sure only code that is supposed to uses a semi-private interface so people don't build dependencies that aren't wanted.

bdowling2• 7 years ago

The C++ access restrictions exist solely to teach and enforce proper API usage by other developers.

They're supposed to trigger a reconsideration of the API when a new developer runs into a restriction, but more often in my experience the new developer will just add a new public interface.

kazinator10• 7 years ago

And you can add yourself a public interface even if all you have is a compiled library, and header files.

Flipping a "private:" to "public:" has no effect on the binary compatibility.

jandrewrogers3• 7 years ago

This solves a chronic architectural maintenance problem in complex C++ code bases, which I've also run into countless times.

Any visible interface, public or private (via friend), will eventually be used by other programmers for other than its intended use case simply because it is there and accessible. For the maintainer of an interface with a single intended purpose, what should be a clean, tidy modification within the scope of its intended use ends up breaking all kinds of unrelated code that the maintainer wasn't even aware of that is using the interface in ways the designer didn't anticipate. It is a common form of architectural rot. Reducing the incidence of this in large code bases is usually enforced by soft means, like sternly worded comments, vigorous policing, etc but that only goes so far. Ideally, an access policy would be in the code itself and enforced by the compiler.

The public/private labels don't address this. If you make things public, the world has the ability to do unseemly things with your interface. If you friend a bunch of classes so they can access private methods, those classes have the ability to do unseemly things with your private implementation details. Both cases embrittle the architecture. What you really want is fine-grained ACLs to class members to document both intent and dependencies.

This method of restricting usage of methods to specific classes at compile time is clever and provides good documentation of design intent, I haven't seen that pattern before. I also would guess that the compiler elides the Badge argument since it is never used.

nwallin• 7 years ago

There's a name for this phenomena: Hyrum's Law.

With a sufficient number of users of an API, it does not matter what you promise in the contract: all observable behaviors of your system will be depended on by somebody.

http://www.hyrumslaw.com/

SolarNet• 7 years ago

I also would guess that the compiler elides the Badge argument since it is never used.

It's also zero width, so no temporaries, etc.

kazinator10• 7 years ago

The best guard against unintended uses that hamstring the maintenance of the library is the good old opaque handle:

   typedef struct foo_struct *foo;

   foo foo_make();
   void foo_destroy(foo);
   void foo_frobnicate(foo, int how);

The client executables have no idea how big a foo object is, or what alignment requirements are, let alone what its members might be. The client executables don't control the allocation, construction, clean-up, freeing, or anything else.

The programmers who use this cannot just edit a header file to gain access to something; the don't have a header file which tells them anything. They could reverse engineer the data structure, but to thwart that, you could scramble the order of the structure members with each new release of the component, so their hack wouldn't be backward or forward compatible.

liquidify• 7 years ago

This pattern is defensive by nature so people who use your library in the future will be less prone to making mistakes. Also, it shows intent to protect that asset clearly which generally leads to easier maintainability. If someone sees this guys code, they will definitely ask questions about why he is doing it, and if they are competent, they can draw the same conclusions that his blog leads you to without having to read the blog.

cjhanks2• 7 years ago

I had never seen this before, I cannot think of another way to do this. I suspect it has 1 byte overhead (on the stack) cost, but that's pretty cheap and may be optimized out.

jackewiehose• 7 years ago

I do C++ for almost 20 years but never came across this simple technique before, so I'm quite surprised that I actually like it.

I usually avoid 'friend' in these situations and just write

    //private:

before the method declaration. But it actually happened that co-workers used these private methods because they use code-completion instead of reading the header...

But I might would call the class Friend<T> or something.

derefr• 7 years ago

But it actually happened that co-workers used these private methods because they use code-completion instead of reading the header...

A nice feature I've noticed in Elixir is that giving a function a "@doc false" annotation will actually prevent IDE/REPL autocomplete from completing the function. (It's still there to call if you type it yourself.) Maybe that's something C++ tooling could copy.

jhalstead• 7 years ago

This reminds me of the Passkey idiom: https://arne-mertz.de/2016/10/passkey-idiom/

b0sk• 7 years ago

This is pretty much the PassKey idiom

akling9• 7 years ago

Oh wow, that's so similar! So similar in fact, that I must have seen this blog post at some point and internalized the idea :)

The improvement that Badge<T> makes is the <T> part; you don't need to look up the PassKey declaration to see who can construct PassKeys, it's encoded right there in the type instead.

I do like how PassKey makes the copy ctor private. I think we can improve on that and delete both the copy and move ctors in Badge.

shkurski• 7 years ago

I might be wrong, but doesn't it look like a way to enforce SRP violation? In the provided example we don't only let the device know how to register itself via VFS, but we also make sure there will be no future DeviceManager to do that. Unless we duplicate the entire interface with Badge<DeviceManager> tag. Am I missing any non-obvious benefits of this approach?

dllthomas3• 7 years ago

Up to this point the comments seem to be missing the obligatory "Treasure of the Sierra Madre" reference, so here I am fixing that.

sorenjan• 7 years ago

Could you make the badge the last argument and give it a default value, so you don't need the initialization at the function call? It looks a bit strange and would need an explanation for why the empty initialization list is there IMO.

I'm not sure if it would work, cppreference has this to say about default arguments:

The names used in the default arguments are looked up, checked for accessibility, and bound at the point of declaration, but are executed at the point of the function call

https://en.cppreference.com/w/cpp/language/default_arguments

ryanianian2• 7 years ago

It's an empty struct so the compiler will elide any storage either way, but agreed having it be the first arg is a bit odd. Perhaps it's for regularity in the cases where you want to make variadic functions 'badged' in this way.

dllthomas3• 7 years ago

Empty structs have size 1, per the C++ spec. The compiler probably won't elide the storage unless it can show that no one probably cares.

ncmncm2• 7 years ago

Yes, a default argument would be cleaner.

There are an astonishing number of uses for default arguments that never appear at the call site.

My fav constructs an object that is just storage for a value that needs to live a little bit longer than the call itself.

antoniolin• 7 years ago

Unfortunately, it doesn't work. The reason is that VFS cannot access private constructor of Badge<Device>.

akling9• 7 years ago

Oh man, I got really excited when I saw this suggestion and got out of bed early to try it out.

Unfortunately it seems like default arguments can't use private constructors. Both GCC and Clang immediately fail at the declaration. Not fair, C++!

/home/andreas/src/serenity/Kernel/FileSystem/VirtualFileSystem.h:81:52: error: 'Badge<T>::Badge() [with T = Device]' is private within this context

mwkaufma2• 7 years ago

I like it! The self-documentation of where the call is coming-from is quite nice (though a more descriptive name like CallFrom<T> would make that even clearer, were it the principle intent). I can't help myself, though:

    template<typename T>
    Badge<T> FakeBadge() {
      struct Stub {}
      return reinterpret_cast<Badge<T>>(Stub());
    }

kazinator10• 7 years ago

Doesn't subvert Device badges for me. Complete sample:

  template<typename T>
  class Badge {
      friend T;
      Badge() {}
  };

  template<typename T>
  Badge<T> FakeBadge() {
    struct Stub {};
    return reinterpret_cast<Badge<T>>(Stub());
  };

  class Device {
  public:
    static Badge<Device> getBadge() { return Badge<Device>(); }
  };

  void needDeviceBadge(Badge<Device> badge)
  {
  }

  int main()
  {
     needDeviceBadge(Device::getBadge());   // no error
     needDeviceBadge(FakeBadge<Device>());  // error!
     return 0;
  }

g++ errors:

  badge.cc: In instantiation of ‘Badge<T> FakeBadge() [with T = Device]’:
  badge.cc:25:38:   required from here
  badge.cc:10:10: error: invalid cast from type ‘FakeBadge() [with T = Device]::Stub’ to type ‘Badge<Device>’
     return reinterpret_cast<Badge<T>>(Stub());
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A reinterpret_cast won't turn an instance of the Stub local class into a Badge<Device>.

That doesn't even have to do with friendship/protection. If we move that line into the Device class, the reinterpret_cast still doesn't compile.

a13692099933• 7 years ago

It does, however, work fine if you use a proper "* (Badge<T>* )&stub" cast.

mwkaufma2• 7 years ago

Interesting, I'd always assumed reinterpret_cast<> was YOLO-anything-goes. You learn something every day ;)

microtherion• 7 years ago

That's not a legal C++ cast, but it's conceivable that it could be made to work by casting to a reference type instead.

vnorilo8• 7 years ago

Badge could prevent this by making the copy and move constructors private.

a13692099933• 7 years ago

You actually need:

  template<typename T>
  Badge<T> FakeBadge() {
    static struct Stub {} stub;
    return *(Badge<T>*)&stub;
    };

ryanianian2• 7 years ago

How is this different from the (relatively common afaik) passkey idiom https://arne-mertz.de/2016/10/passkey-idiom/ ?

ncmncm2• 7 years ago

Yes, this has various names. "Ticket" is traditional.

kazinator10• 7 years ago

C++ could solve this without badges, if it didn't have a very tiny, silly misfeature.

The misfeature is this: a class A can only declare a specific member function of class B as a friend, if that B member function is public!

Example:

The idea here is that Device has a static member function called Device::registrationHelper. That specific function (not the entire Device class) is declared a friend to VFS, and so that function can call VFS::registerDevice.

[Edit: this doesn't quite match the Badge solution, though. If this worked, it would have the problem that the registrationHelper has access to all of VFS, which is almost as bad as the entire Device class being a friend of VFS. That is one of the problems which motivate Badge. Nice try, though! The C++ restriction is probably worth fixing anyway. What the C++ friendship mechanism really needs here is to be able to say that a specific member function in B is allowed to access a specific member of A.]

  class Device;

  class VFS;

  class Device {
  public:  // we want this private!
     static void registrationHelper(VFS &v, Device &d);
  private:
     void registerWith(VFS &vfs) { registrationHelper(vfs, *this); }
  };

  class VFS {
  private:
     void registerDevice(const Device &);
     friend void Device::registrationHelper(VFS &, Device &);
  };

  void Device::registrationHelper(VFS &v, Device &d)
  {
     v.registerDevice(d);
  }

But we are forced to make Device::registrationHelper public, which defeats the purpose: anyone can call it and use it is a utility to register devices with VFSs.

If we make Device::registrationHelper private to prevent this, then the "friend void Device::registrationHelper" declaration in VFS fails.

This is an oversight; the privacy of the Device::registrationHelper identifier means that the VFS class is not even allowed to mention its name for the sake of declaring it a friend.

That should be fixed in C++. Declaring a function a friend shouldn't require it to be accessible; we are not lifting its address or calling it; we are just saying that it can call us. Allowing a function to call us is not a form of access to that function, yet is being prevented by access specifiers.

nly3• 7 years ago

Your solution is similar to mine and you might find it helpful:

https://news.ycombinator.com/item?id=20160957

nly3• 7 years ago

Personally I'd move the register_device() and unregister_device() functions outside of the VFS class entirely, perhaps to namespace scope.

Alternatively, there are other ways to leverage friendship and access control in C++. Here's one option:

    template <typename Registry>
    class DeviceManager;

    class Device {
        template<typename Registry> friend class DeviceManager;
        int y;
    };

    class VFS {
        friend struct DeviceManager<VFS>;
        int x;
    };

    template <typename Registry>
    struct DeviceManager {
        static void register_device (Registry& registry, Device& device) {
            registry.x = 1; // accessing privates
            device.y = 2;   // accessing privates
        }
    };

    int main() {
        VFS vfs;
        Device dev;
        DeviceManager<VFS>::register_device (vfs, dev);
    }

Here all DeviceManager<>'s can reach inside Devices, but only the VFS device manager can reach inside VFS.

This is also flexible. Remove 'static' and you get yourself a stateful mediator/manager. Add a variadic template register_devices_s_ member to Devicemanager and you've got yourself a convenience function for registering multiple devices (of heterogeneous types) without bloating the VFS API and, without runtime cost, and without losing type safety (by e.g. casting down to Device& from USBDevice&).

kazinator10• 7 years ago

The thing is, there is no restriction on who calls DeviceManager<VFS>::register_device. main() is able to do it.

This doesn't seem so different from:

   class A { private: friend void foo(A &, B &); int x; };
   class B { private: friend void foo(A &, B &); int y; };

   void foo(A &a, B &b)
   {
      a.x = 1;
      b.y = 2;
   }
   
   int main() {
      A a; B b;
      foo(a, b);
   }

We've just obfuscated it a bit by using a template class with one function in it, and declaring that to be the friend instead of just a non-member function.

nly3• 7 years ago

Yes, your version is what I mean by moving it to namespace scope, and it's probably what I'd do to start off with.

there is no restriction on who calls DeviceManager<VFS>::register_device

Well, you can make it private inside DeviceManager and give the class its own friends :-))

Another benefit of my solution is that adding things to DeviceManager<VFS> doesn't require changing the VFS.h or Device.h headers at all. Anything that links these 2 classes is an entirely separate concern and very much in line with thinking in terms of models or concepts rather than types. What you want at the end of the day is an absolute bare minimum of member functions in any class.

To be honest, whatever the solution, the key is that having the Device class self-register to the VFS in its constructor (as in Andreas's blog post) is, imho, an anti-pattern and is what is really leading to this Badge mess.

lenkite• 7 years ago

Love this. I also felt that having an explicit `register` on a mediator like `DeviceManager` made much more sense for SOC, but this is really elegant. Maybe you should write and publish some C++ Idioms :)

gowld2• 7 years ago

These functions are called by all Device objects when they are constructed and destroyed respectively. They allow the VFS to keep track of the available device objects so they can be opened through files in the /dev directory.
Now, nobody except Device should ever be calling VFS::register_device() or VFS::unregister_device(),

Why?

Either have VFS call the constructor because non-registered devices are banned and RAII is good, or don't create arbitrary restrictions to hamstring how the rest of the system manages Devices.

asveikau• 7 years ago

I kind of agree that it's not clear why a VFS needs to register devices that only register themselves, however, if you suspend disbelief on that one it's a clever trick that is easy to imagine being useful.

Something1234• 7 years ago

Wouldn't -Wextra yell about the Badge being an unused parameter in the register devices? You would have to do some kind of dummy call that the compiler would have to optimize away.

jandrewrogers3• 7 years ago

There are attributes for that, standardized as of C++17 and available in most compilers much earlier. -Wextra won't yell about anything.

akling9• 7 years ago

If an argument is nameless, the compiler won't complain about it being unused.

No warning:

    void VFS::register_device(Badge<Device>, Device& device) { ... }

Warning:

    void VFS::register_device(Badge<Device> badge, Device& device) { ... }

QuadrupleA6• 7 years ago

I used to carefully design my C++ code with public / private / protected / friend (and the pointer-to-impl pattern for "compile time private") but a few years ago I started doing everything all-public ala python (struct instead of class by default) with occasionally an m_ prefix for "intended to be private", and I've never looked back. It's been great, made my programming life much easier.

This badge thing is clever, but seems like yet more accidental/unnecessary complexity on top of the already-probably-unnecessary private/protected/friend - as requirements change you now have this extra layer of access-control cruft to reason about, that you probably wouldn't even miss if it didn't exist.

Not to get too ranty, and no disrespect to Andreas, but the C++ culture seems to thrive in unnecessary / accidental complexity, deep template hacks and the like that add friction to refactoring, obfuscate underlying logic so you miss opportunities to simplify, and turn straightforward programming problems into brain-bending template puzzles.

OOP also; I've seen so many crazy class architectures that could just be a handful of plain-old-functions, complex virutal "type families" that could just be a TypeOfThingThisIs enum with some if statements in the respective functions, etc.

I don't know the Serenity codebase and use-case very well, and perhaps this kind of thing has a place in large libraries, but honestly even then I'd lean towards just making everything simple & public and use a prefix when needed, or describe appropriate usage in documentation. Seems to work in the python world (minus a few outlier codebases that go OOP-crazy).

picacho2• 7 years ago

yeah, python LOC > 5000 try to debugging it while some dude just modified your type instance's guts.

eps3• 7 years ago

OP's approach works if all dudes involved read the code before chaning it and stay off private parts even if they aren't guarded by the language constructs.

xleviator3• 7 years ago

In larger codebases with many developers, attention to the API is required in order to be able to maintain code without breaking clients.

The main downside of Badge<T> is that it comes with a performance penalty and ABI change for something that should be statically deducible (empty struct is one byte in C++).

dearrifling4• 7 years ago

You can make register_device() an inline function that just calls a do_register_device() private function without the badge. That way you don't have to pay for the empty class.

maxxxxx• 7 years ago

Don't you have to have pretty high performance needs for this class to make a performance difference? Unless it's called multiple thousand times per second I don't see it making much of a difference.

vnorilo8• 7 years ago

There is no performance penalty in any modern compiler that I am aware of (effectively no code will be emitted).

You can have a look here: https://godbolt.org/z/DmeYL-

keyle• 7 years ago

I have to ask... why 'm_'?

nxpnsv• 7 years ago

I think it is supposed to mean member, but i don’t remember, common enough practice though

bronxbomber92• 7 years ago

It's a simplified form of Hungarian notation: https://en.wikipedia.org/wiki/Hungarian_notation

oAlbe2• 7 years ago

I don't understand why you have been downvoted for asking "why m_?". Is everybody on here supposed to know everything about everything, else you get downvoted into grey unreadability?

Personally, I think that bashing/penalizing ignorance (and I mean the word ignorance literally, as being uninformed about a topic, no offense here whatsoever) is one of the lowest forms of abuse. It says much more about the downvoter than it does about the downvoted. And this not even considering the fact that searching online for "m_" isn't going to help you answer your own question.

If everybody just downvoted the questions they deem unworthy of an answer because "you are supposed to know this", there would be no questions answered.

Sorry for the OT.

enriquto2• 7 years ago

OOP also; I've seen so many crazy class architectures that could just be a handful of plain-old-functions,

also, tons of "generic" numerical code with templates everywhere, where the only two possible instantiations are "float" and "double". Some people do really like to add useless complexity everywhere.

_pmf_• 7 years ago

Not only is it needlessly complex, but untested generic code is a minefield of subtle future bugs ... "oh, the function is there and tested, so it should work" ... well, not in this particular combination of the completely unrestricted generic parameter set.

edflsafoiewq• 7 years ago

What's wrong with that? It seems like the easiest way to write code that works with either floats or doubles.

geezerjay3• 7 years ago

I started doing everything all-public ala python (struct instead of class by default) with occasionally an m_ prefix for "intended to be private", and I've never looked back.

This is the sort of accidental complexity that access specifiers eliminated, but somehow you managed to reintroduce by trying to reinvent the wheel.

The 'm_' prefix only means "private member variable, don'access it" to you and you alone, and anyone who will stumble on your code will not be aware or obligated to follow your personal style. That would not be the case.if you simply used a basic feature of C++ that's been available since the time of "C with classes".

To me, you're the epitome of the main source of C++'s problems: programmers who failed to learn the basics of a language and instead decided to develop convoluted hacks to circumvent proper use of the language because they believe their new discovery is clever although in reality they only managed to needlessly add complexity to a code base while adding brittle and bug-prone code.

What else? Do away with memory deallocation because there's nothing of the sort in Python and your C++ code is packed with segfaults?

And by the way, the pimpl pattern is used to provide libraries a stable ABI in spite of what change might be done to the implementation. It is not a fad or a clever-looking trick.

QuadrupleA6• 7 years ago

Python has a well-established convention of _underscore for intended-to-be-private members, with no enforcement, and it works fine. Just document your conventions. The horrors of some other programmer using it wrong seem overblown to me.

In my IDE in c++ I can quickly refactor m_ to non-m to make it "semantically public" without having to edit header files, change struct packing order / ABI, etc. And most of the time I don't even bother with m_. It's one less thing to think about. I honestly don't think access specifiers are worth wasting mental cpu cycles on in most projects.

Besides, in c++ people can always cast your intricately-protected object to a void pointer and start manipulating it nefarioisly anyway. Just document and communicate, and write simple straightforward code.

I used pimpl at the time for faster recompiles when changing "private" stuff. Better build tools got me around that problem personally, but I'm sure it has it's uses.

gjm11• 7 years ago

you're the epitome of the main source of C++'s problems

This seems unnecessarily personal. See https://news.ycombinator.com/newsguidelines.html for the, um, best practices around these parts, which you are not following here.

barrkel2• 7 years ago

Everything public versions poorly when you can't update all clients of your code after you change your implementation. It increases coupling in ways you can't control.

It also doesn't work very well with code completion - implementation methods get mixed up with surface API - and usability by third parties suffers.

If you're on a team of one, and you own the project for its whole lifetime, then you're fine.

adrianN• 7 years ago

The implicit contract is that anybody who uses m_ "intended to be private" members is responsible for fixing breakage.

QuadrupleA6• 7 years ago

Admittedly my use case is generally projects that are under my control.

But in the case of say an open source library, how likely are clients going to update their code with each new release of the library? Typical case is to do it from time to time, especially with compiled code / shared libs, and expect a few incompatibilities to sort out and fix when you do.

In a fast evolving large team project, seems it depends on how well the boundaries of the subsystems are worked out. Misusing supposed-to-be-private members is one minor possibility amidst many challenges.

zvrba2• 7 years ago

but a few years ago I started doing everything all-public

This is a recipe for disaster as soon as inheritance is in the picture.

QuadrupleA6• 7 years ago

Everything is a recipe for disaster one inheritance is in the picture :)

akling9• 7 years ago

This style of development obviously will not scale to large, long-term C++ projects with many developers, as anyone who has worked on such projects knows all too well.

No disrespect to you, QuadrupleA, do whatever makes you the most productive, but I wouldn't want to work together with you in this way :)

mirsadm2• 7 years ago

I hear this argument a lot, why would it not scale?

QuadrupleA6• 7 years ago

It's more of a C style I guess, that I prefer, or a very selective use of C++. Certainly C has supported some big projects.

Anyway language rants aside, your project looks very cool - I remember seeing it posted here a while back.

scoutt• 7 years ago

I come from an embedded development environment, and for me simplicity is everything. But even that, I think the approach of an "all public" class is a little bit risky, and can be a pain to debug if accidentally you change an internal state.

If a class is a black box, when you put your hand inside it, it becomes the "pain box" from the Bene Gesserit.

henning• 7 years ago

This is a typical example of a solution to an artificial, self-imposed problem created by C++'s assumptions.

As a programmer, I would feel frustrated to have to figure this out instead of spending time on something a user would actually care about.

jandrewrogers3• 7 years ago

Which popular programming languages provide proper ACLs (or equivalent functionality) for class members?

inetknght• 7 years ago

I'm aware of C++ and Xojo for sure.

jmpeax• 7 years ago

Considering the whole problem could easily be solved with a comment: // should only be called by Device

akling9• 7 years ago

I’m sure you’re very familiar with the patterns used in the codebase(s) you spend your time in.

In this case, once you know what a Badge<T> is, it’s obvious what it communicates about a function that takes one.

cgrealy• 7 years ago

While this is a clever little pattern, I'd argue that if you have a method of class X that can only be called from class Y, that is a code smell.

ksherlock• 7 years ago

protected and friend are both smelly keywords?

dearrifling4• 7 years ago

Some people argue exactly that. I don't necessarily agree.

Toine• 7 years ago

Sometimes, clever little patterns like that are useful to get stuff done in time. However, it that case it definitely smells a lot. Also, the global singleton stinks.

To me this is a violation of the Interface Segregation Principle. I think the author went the right way to fix it, but not far enough.

He added a parameter to segregate the device registration operations from other operations. It works, but the segregation is artificial : you still need a Device object (which seems to be used by a lot of classes, another smell, god object ?)

Instead of asking for an entire Device object, register_device should ask for a different type, specific to that operation, like « NewDevice », « UnregisteredDevice », etc. This object would depend on very specific information only available at the device creation. Now, if another developer tried to call register_device, he would need to create a weird object that he never heard of, with parameters he cannot even provide.

Thoughts ?

dkersten2• 7 years ago

Is an object really a god object if lots of people use it? I thought it was the opposite way around: a large object that contains too much functionality, but this device could be a tiny single purpose object for all we know, that a lot of clients need to use.

jupp0r• 7 years ago

This is especially helpful in conjunction with private constructors and make_shared or make_unique. You cannot even friend make_shared because of namespace aliasing in standard libraries, but shared_from_this can make improperly allocated objects error at runtime. Private badges/tokens solve that problem by allowing public constructors requiring a parameter value of the badge/token type that has limited visibility, but can be constructed inside a factory and passed to make_shared.

mindfulplay• 7 years ago

I often wonder how this would map to other languages (as C++ often entails complexity and clever work arounds such as this article).

In Java or C# (and of course in C++), you could have a token system where the token can only be provided by Device :

VFS: void register(DeviceToken):

DeviceToken: empty interface (marker)

Device: private DeviceToken provideDeviceToken();

This hides both VFS and Device classes from accessing each others' private members.

You can also constrain how gets to provide DeviceToken by adding sufficient comments in code.

choppaface• 7 years ago

Isn’t this exactly the problem that categories in Objective C solve? Why not adopt that pattern versus this “Badge” thing?

akling9• 7 years ago

Hello friends! Author here, nice to see so much discussion :)

I've spent most of my adult life working on large C++ codebases with public API's (most notably Qt and WebKit), which has led me to cultivate a defensive programming mindset. Someone mentioned Hyrum's law, which is 100% accurate in my experience.

I should be honest and admit that Badge<T> is not yet diligently applied everywhere in the Serenity codebase. I wanted to "feel it out" for a while first, but now I've decided I like it, so I'm applying it more and more going forward.

Oh and while I have your attention, I recently posted a May 2019 update video[1] on the Serenity OS project if you're curious how things are going :)

[1] https://www.youtube.com/watch?v=KHpGvwBTRxM

w0utert3• 7 years ago

I like this idea, except for the fact that the extra parametrer pollutes the method signature and method calls, for something that really should only be a compile-time directive.

I'm actually curious why something like this has never been included as a language feature, it seems trivial to come up with a syntax for fine-grained access control, that is fully backwards-compatible and a true zero-cost abstraction.

Most obvious first shot:

  class A
  {
    private:
      void badgedFunction() const friend Device, SomeOtherClass;
  };

Did you ever check with the C++ working group if there are any proposals for language extensions for this purpose?

akling9• 7 years ago

Yeah, polluting the signatures is a shame.

I've only just decided that I enjoy using this pattern so my internal standards committee hadn't progressed to the proposal stage yet. :)

Your syntax makes perfect sense to me, and I think it would make a great addition to the language. I wonder what the real working group would say. (I tried quickly googling for something that might sound like an existing proposal but came up empty.)

nercht12• 7 years ago

Seems more of a people problem than a software problem. That's normal. Why not just use a comment that says "Don't use this. It will break."? If people can't obey simple instructions, you have a different problem entirely.

barrkel2• 7 years ago

Code completion doesn't necessarily read comments. Why would people read the headers before using?

If people would only obey simple rules, C++ would be a memory safe language. Alas, they don't.

skocznymroczny• 7 years ago

In D, you can access private members of a class within a single module. I like it, because it allows to move methods outside of the class, but doesn't require explicit friend declarations. And you still have the data protection because other modules can't access the private fields.

nikbackm• 7 years ago

Maybe C++ could add this feature at some point now that it too will get modules.

AstralStorm• 7 years ago

This is essentially a capability system enforced by the compiler, which means your code is not actually in control for any third party caller.

A glaring security hole. Any old hacker can forge or clone a data structure. This "badge" (AKA token) has to be explicitly unpredictably replay-proof generated and hard to forge, and also automatically verified.

mkettn• 7 years ago

whats the advantage over declaring register_device as a functor object from an anon class? for example:

  class Device;
  class VFS {
  public:
   class {
   friend Device;
   void operator()(int y) {/* do stuff with y here*/}}     register_device;
  };
  class Device {
  public:
   void foo(VFS fs) {
    fs.register_device(1);
   }
  };
  
  int main(int argc, char *argv[]) {
  // fails:
  // VFS().register_device(2);
  // works:
   Device().foo(VFS());
   return 0;
  }

and no need for a badge class (erm... well but an anon class).

my c++ skills are a bit rusty, because most of the time python works just fine.

akling9• 7 years ago

Hmm, IMO Badge has two main advantages over your approach:

1. Aesthetics. (This is down to personal taste of course, but I much prefer how Badge gets the job done without needing multiple lines of code at every declaration.)

2. What if I need to put the function implementation out-of-line?

C++ Patterns: The Badge

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