add generic container serialization #307
Replies: 43 comments
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If you're going down this route, I think there should be two forms of "generic" save and load procedures, one for "Sequence" containers and one for "MapLike" containers, e.g. ones that have a |
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On 8/4/18 2:42 AM, Hans Dembinski wrote:
I noticed that serialization code is missing for
|boost::container::vector| and |boost::container::string| and probably
more of the Boost containers. Boost should be able to serialize
ideally all or at least most of its own containers.
The straight forward thing would be to implement serialization code
for each individual container, but that's a maintenance burden and it
does not exploit the fact that many containers adhere to a standard
interface.
With a bit of concept checking a templated generic serialization code
for containers could be written that adhere to the vector interface;
similar for string, list, and map. The current code for std::vector,
std::string, std::map, std::list could be turned into such a generic
code. The concept checking would implement a form of duck-typing.
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FYI - the pattern which has been followed from the beginning is:
a) if it's a type in the std:: namespace - included serialization (and a
test for it) in the serialization library
b) otherwise, include serialization in the particular boost library.
I would be loath to change this.
…--
Robert Ramey
www.rrsd.com
(805)569-3793
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So, in other words, you think I should open an issue in boost.containers. Fair enough, but my point was to provide generic serialization for standard containers, so that Boost.Serialization would support any vector implementation, be it std, boost, Qt, ... |
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I agree that serialization of custom classes should be in the respective custom libraries, but "standard stuff" can be part of Boost.Serization, and that includes "concepts". Ultimately, if we (Robert) decide that a concept-based serialization is desirable, I think this will necessarily put the serialization functions in the Unfortunately, it looks like there are only a few (official and unofficial)
I am not sure how specific they are to actually implement both I am sure things like this have been partially implemented unintentionally by many people, it is a matter of collecting a bunch of good implementations, for example, in (proposed) As (standard) concepts become more specific or general more implementations I think this setups a more formal way to discuss this issue. |
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This is an interesting idea. It looks like you've peered into the implementation of std container serialization and seen how i've attempted to factor out the commonality in the implementations. So the idea making this more general using concepts is an interesting one and very much in line with the design and subsequence maintenance of the serialization library. I believe that his approach is probably the main reason it has managed to stay relevant after so many years. So I think the idea is quite interesting but seems to me it would require a lot more investigation: a. concepts isn't yet in C++ and people are still arguing about. I've approached serialization as "solving the problem we have" rather than "this is the next great thing". I'm not sure you have enough to work with yet. b. it's very unclear to me how the "overloading" feature of concepts would play in to this. I might or might not be useful. interesting though. c. If one were to make a module which implemented serialization for any components which fullfill std concepts. I agree that the place to place would be in the serialization library. d. the one's you mention above, sequence container, etc. are already factored out and implemented within the serialization library implementation. These are done in terms of specific containers as type parameters. So much of the implementation is "done" e. this would entail a major, major, major effort in terms of documentation, tutorial, examples, etc. If I were to undertake this, I'd recommend that the serialization transition to boost book or doc book first. f. The library does have a good foundation of concepts in the documentation and there is (ad hock) compile time testing of concept definitions. But this is ad hoc and would likely have to be re-visited to make it consistent with this effort. g. Then there is the question of backward compatibility of already created archives. Always an issue. In this case, it would be stickier than other cases. In short, an interesting idea worthy of exploration. But I can't really know where it would lead. |
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That's right Robert. Programming with concepts requires us to think differently (and think really hard). The old paradigm, based on overloads, specific types and matching templates arguments, and the new paradigm can coexists. I think the weak point is that many concepts still have a long way to go to be specific enough to implement serializaliation (and specially deserialization since "mutability" concepts are less intuitive), let alone efficiently. But slowly getting there wouldn't harm and will help identify the concepts we need. Eventually, concept-based serialization will be equivalent and as efficient to already implemented cases (e.g. std::vector, std::set, std::map) but will be "open" to new containers. This will minimize code and code coupling to an optimal minimum. The hard part is to identify concepts and hope that the standard introduce good basic concepts to build upon. This is a very interesting project. BTW, in SE, there is a intriguing posting by Sean Parent https://stackoverflow.com/a/14000046/225186 where he defined the theoretical 'area' function (and "underlying type") for Regular types. In some sense the 'area' is the "size of serialization" (or an upper bound?) and is something to think in relation to concept-based serialization. (this is a generalization to the |
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On 8/5/18 1:52 PM, Alfredo Correa wrote:
That's right Robert. Programming with concepts requires us to think
differently (and think really hard). The old paradigm, based on
overloads, specific types and matching templates arguments, and the
new paradigm can coexists.
I don't think Its not that much different by the time one gets it
implemented.
I think the weak point is that many concepts still have a long way to go
Right ...
slowly getting there wouldn't harm and will help identify the
concepts we need. Eventually, concept-based serialization will be
equivalent and as efficient to already implemented cases (e.g.
std::vector, std::set, std::map) but will be "open" to new containers.
This will minimize code and code coupling to an optimal minimum.
The hard part is to identify concepts and hope that the standard
introduce good basic concepts to build upon. This is a very
interesting project.
Actually I see a way to work on this without mucking anything else up.
Implement serialization for boost/collections in terms of
std::concepts. The actual implemention code can be stolen from the
current implementation of std::vector, std::map, etc.
BTW, in SE, there is a intriguing posting by Sean Parent
https://stackoverflow.com/a/14000046/225186 where he defined the
theoretical 'area' function (and "underlying type") for Regular types.
In some sense the 'area' is the "size of serialization" (or an upper
bound?) and is something to think in relation to concept-based
serialization. (this is a generalization to the |size()| function of a
container).
I don't find this convincing or anyway helpful.
If you want a nice project implement serialization for boost
collections. But do it in way such that applies to any type with
specificly define type requirements (concepts). Then even if it doesn't
work - you'll still have added serialization into an type which might
benefit from it.
…--
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www.rrsd.com
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On Sun, Aug 5, 2018 at 2:05 PM Robert Ramey <[email protected]>
wrote:
On 8/5/18 1:52 PM, Alfredo Correa wrote:
> Programming with concepts requires us to think
> differently (and think really hard). The old paradigm, based on
> overloads, specific types and matching templates arguments, and the
> new paradigm can coexists.
>
I don't think Its not that much different by the time one gets it
implemented.
Sure, actual code will look like everyday code in the end.
It is just that, given certain requirements one has to think hard exactly
how to implement the optimal or acceptable (de)serialization.
Think, for example, of whether to use `reserve` or not given a concept.
>
> The hard part is to identify concepts and hope that the standard
> introduce good basic concepts to build upon. This is a very
> interesting project.
>
Actually I see a way to work on this without mucking anything else up.
Great, are you thinking in the lines of std::enable_if, general
decltype/detect SFINAE or C++ Concepts-lite?
Implement serialization for boost/collections in terms of
std::concepts. The actual implemention code can be stolen from the
current implementation of std::vector, std::map, etc.
I see, yes, that is called "lifting" the code to get more generic code.
The challenge is to lift the code maintaining an acceptable efficiency
(again, think of the example of `reserve`).
>
> BTW, in SE, there is a intriguing posting by Sean Parent
> https://stackoverflow.com/a/14000046/225186 where he defined the
> theoretical 'area' function (and "underlying type") for Regular types.
> In some sense the 'area' is the "size of serialization" (or an upper
> bound?) and is something to think in relation to concept-based
> serialization. (this is a generalization to the |size()| function of a
> container).
>
I don't find this convincing or anyway helpful.
Yes, there is not a urgent need for this yet. But, I think the idea of
`underlying_type` and `area` are deeply related to bit-based
representations of regular types (aka serialization).
That is why I brought the topic, so we start to think in terms of formal
concepts.
(My other point is that any collection serialization that uses `size()` is
a particular case of the more general idea of `area`).
Also, I can imagine archive types that can benefit from knowing in advance
upper bounds to the size of its elements.
If you want a nice project implement serialization for boost
collections. But do it in way such that applies to any type with
specificly define type requirements (concepts). Then even if it doesn't
work - you'll still have added serialization into an type which might
benefit from it.
Yes, I have some code like this, that is why I have strong opinions.
Having said all that, I would say the hardest part is how informally
defined and how badly scattered is the documentation of concepts in the C++
standard.
I think the first step is to identify the concepts we would like to use and
implement, that is why I put this initial list.
We have to understand that the correspondence between serialization and
deserialization will be broken for some concepts.
For example, a Sequence can be always serialized, but it can be
deserialized only when the target size is the expected one (because no
concept that I know of actually has the requirement of a resize).
That is why I say that deserialization based on concepts probably is still
challenging without further refinements (for example ResizableSequence or ElasticSequence, it doesn't have a name as far as I know).
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I don't see why it is necessary to push the discussion towards concepts. We have SFINAE and |
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On top of the generic version, there can be specialized overloads for specific vector-types if needed. The compiler will use the generic version only as a last resort. The same approach can be used for set-like, map-like etc., I just want to keep the discussion simple and concrete for now, so I focus on vector-like. |
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I would also be interested in generic serialization. To use SFINAE, the default implementation of For instance, with: template <typename Archive, typename T>
void serialize(Archive& ar, T& d, const unsigned int version, std::enable_if_t< is_bus_stop<T>::value >* = nullptr) {}I get: Looking at the // note the usage of function overloading to compensate that C++ does not
// currently support Partial Template Specialization for function templates
// We have declared the version number as "const unsigned long".
// Overriding templates for specific data types should declare the version
// number as "const unsigned int". Template matching will first be applied
// to functions with the same version types - that is the overloads.
// If there is no declared function prototype that matches, the second argument
// will be converted to "const unsigned long" and a match will be made with
// one of the default template functions below.The workaround was removed in 5a94e5e though. |
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We could add some level of reflection to check if a given class has a serialize member function, something along the (c++11) lines of: #include <type_traits>
template <typename Archive, typename C>
struct has_serialize
{
private:
template<typename T>
static constexpr auto check(T*)
-> typename
std::is_same<
decltype(std::declval<T>().serialize(std::declval<Archive&>(), std::declval<unsigned int>())),
void
>::type;
template<typename>
static constexpr std::false_type check(...);
typedef decltype(check<C>(0)) type;
public:
static constexpr bool value = type::value;
};The default implementation would then be easily disabled with: // default implementation - call the member function "serialize"
template <class Archive, class T>
inline void serialize(
Archive & ar, T & t, const unsigned int file_version, std::enable_if_t<has_serialize<Archive, T>::value>* = nullptr
){
access::serialize(ar, t, static_cast<unsigned int>(file_version));
}@HDembinski Is that a path worth exploring? |
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@sdebionne I think we have different things in mind. My point was the following: to serialize a vector-like containter, you don't need to access its internals. Any container that supports the vector interface can be (de)serialized with the same templated code, something along the lines of (pseudo-code): To avoid ambiguities, these three functions may only participate in overload resolution when the class |
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@sdebionne Your |
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@HDembinski My point was that a generic version for Vector-like containers will be ambiguous with the default implementation that comes with Boost.Serialization that needs to be disabled somehow (or have lower priority). Or am I missing something?
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@robertramey This is a nice list of useful new features, but if all these items are put together, it is essentially a rewrite of the library. Rewriting boost.serialization along these lines and raising the c++ standard to c++11 in the process would be a worthwhile project, similar to how mpl was replaced by mp11. The boost.serialization code is pretty complex, but mostly because TMP was very awkward in pre-c++11 times. The rewrite could be simpler and easier to maintain. |
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LOL - I'll leave this up to the person who actually undertakes the work - which is not likely to be me. I should note that the serialization as a project is way underestimated - as it was by me. Years of effort along with two grueling boost reviews and lot's and lot's of "help" I had to respond to. But your observation is a very interesting one which comes up in software development on a regular basis. I would note that it is also very common that such ideas often result in failure in that the new system turns out to be less of a success than the system it's meant to replace - if in fact it ever gets done.
I actually disagree with this view. The above list is pretty much orthogonal in that the implementation of one feature wouldn't affect or be affected by the implementation of another feature.
I can see the attraction of raising the requirement to C++11 minimum. It would simplify adding new features and fixing old ones. Same could be said for replacing mpl with mp11. But of course that would obsolete support for older C++ compilers - and likely previously existing archives as well. If it were me, I wouldn't go there just to implement any (or even all) of the features/extensions above. The list above is really quite modest in its scope. It's just that the serialization library is hard due to things other than the restriction to C++03. Actually a much more interesting idea would be to make a new serialization library (boost.serialization2?) entirely from scratch using the very latest C++ facilities. This new library would have the following features: a) it would not be required to be compatible with the current library. It would not be required to be able to read archives created by the current library. It might do so or it might provide a migration path. Not that this would be hard for users to do. It would only be a case of: b) it would draw inspiration from the current library as well as other efforts such as cereal. c) it would be free to use the experience of the current library to narrow the scope of the new library to make it simpler. For example, cereal doesn't support raw pointers. It just supports serialization of pointers through a couple of special types like boost::smart_pointer. This would make implementation much, much easier. It might also not support of tracking. This would leave this up to the user. Again making implementation much safer. It might not build the concept of versioning into the library, but leave it to uses to serialize their own version numbers. d) It would likely be header only. The current library invests a lot of effort to maintain separate parts - one type specific as header only and none specific as precompiled library. At the time the library was first conceived and implemented, code size and reuse were considered of much greater importance than they are now. So eliminating this feature of the code would make a library much easier to write and would also be popular with the current generation of programmers who find working with separately compiled libraries a burden. e) It might serve as a better candidate for migration into the C++ standard. Traditionally, Boost has served as the incubator for libraries for the C++ standard. More recently, this role of boost has been cast aside and its now more as fixing components that the C++ standards committee has messed up. So it's not at all clear what the future of such new serialization library might be in this regard. It's very clear that the current serialization library would not be model for C++ standard serialization. Years ago I had assumed that with a few years experience with the boost serialization library, someone might a serious proposal for C++ standard serialization. Much to my surprise this has never happened. Not even an unserious proposal has been submitted. These points, along with the likely millions of archives which need to be read with the old code will likely mean that the boost serialization library will be around for a very, very long time if we're willing to tweak it from time to time. It will likely be around as long as C++ macros are around. Sooooo - Now you've got TWO projects worth of investment of effort. Extending the the current library and making a whole new one. Each is worthy and important in it's own way. Good Luck with this. PS - helpful tip: Rather than asking "Do you have proof for your claim?" try phrasing your question as "Hmmm - I can't see why this would be necessary, could you expand your explanation on why this would be the case?". This asks the person for help rather than challenging him to defend his position. People are suckers for other people asking for help. |
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#include <type_traits>
#include <boost/serialization/serialization.hpp>
struct foo { int i; };
//Check if T is a model of the Foo concept
template <typename T> struct is_foo : std::false_type {};
template <> struct is_foo<foo> : std::true_type {};
namespace boost {
namespace serialization {
template <class Archive, class T>
std::enable_if_t<is_foo<T>::value> serialize(Archive& ar, T& t, const unsigned int version) {
ar & t.i;
}
}} //namespace boost::serialization
int main() {
struct archive {} ar;
foo f{ 1 };
// 'boost::serialization::serialize': ambiguous call to overloaded function
boost::serialization::serialize(ar, f, 1);
} |
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@sdebionne Sorry, you were right. Thank you for the nice short proof. |
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@robertramey I like the idea of a serialization2 library for the reasons you pointed out, and although I am tempted to work on it, I will probably resist that temptation. I had a look into the Boost.Serialization and I can imagine what huge amount of development effort this was. Like you said, rewriting is easier than designing from scratch, but for a huge library like serialization it would still be a serious undertaking. |
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LOL - thanks for recognizing this. FWIW - it seems pretty simple when it started out. But adding in all the stuff that was "required" made it very hard. Anyone who undertook this would have the advantage of being able to see boost serialization along with some others to get a better idea of the cost and benefits of every feature. Maybe you want to submit a proposal to the C++ standards committee. Even this would be a substantial undertaking. |
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Hi Robert,
I have a question for you, given your experience.
Do you think *reflection* when (if) available in C++ will make writing such
library easier or is it just hopeful thinking?
…On Mon, Jun 3, 2019 at 13:17 Robert Ramey ***@***.***> wrote:
LOL - thanks for recognizing this. FWIW - it seems pretty simple when it
started out. But adding in all the stuff that was "required" made it very
hard. Anyone who undertook this would have the advantage of being able to
see boost serialization along with some others to get a better idea of the
cost and benefits of every feature. Maybe you want to submit a proposal to
the C++ standards committee. Even this would be a substantial undertaking.
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wishful thinking. Since you're asking the question here, I'm presuming you've got some personal experience with the boost and perhaps other serialization libraries. Suppose you want to serialize some data type. For primitive data types like int, there's nothing to do. For compound data types, you need to specify a list of members which hold the permanent state of the type. Right now we do that by writing the "serialize" function. This often times consists of writing a "serialize" function which has ar & ... for each member. If we had reflection, this could be generated automagically via TMP. So this looks attractive. In fact, I'm willing to predict that within a week of reflection becoming available, some smart guy will do just that. So far so good. But think of your experience in the real world. What about the following? It's my view that reflection won't save very much work. Now if you look into the library, you'll see the whole "archive" concept and code required to support it. It's quite complex. In part because it exploits the refection we already have (via type traits which reveal whether a datum is a pointer, value, const, etc...) to load/save the data in an appropriate manner. Addition of reflection won't help implementation of this at all. So ... I believe that though reflection might have it's uses, the benefit it will give to support serialization will in fact be very small - if any. One will still have to specify the "extra" information required. Then someone will propose and "attribute" attached to data members to specify the extra information. OK that can be made to work. But by the time this is done, we will have come full circle in making the language more elaborate to specify things that we're already doing through library facilities. This will continue the trend in C++ to move things that have been supported quite well via libraries (e.g. contracts) into the core language itself which makes things more complex with no corresponding benefit. Robert Ramey |
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I basically agree with Robert, but in my ideal world, serialization would work like copy constructors and assignment operators. For simple cases, the compiler should automatically generate the serialization operator, but the user can always overload it with a special version. |
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Ok, thanks Robert. I had the same impression. The challenge of
serialization is to identify how a class is a value type, and if it is.
I also agree with Hans. I agree that serialization should be recursive by
default. The problem is to agree on what operator or name to use. We can
reserve save(Ar&)/load(Ar&) or use the binary operators <<,>>,&. The
problem is that none of them can be members of the class.
…On Tue, Jun 4, 2019 at 01:16 Hans Dembinski ***@***.***> wrote:
It's my view that reflection won't save very much work.
I basically agree with Robert, but in my ideal world, serialization would
work like copy constructors and assignment operators. For simple cases, the
compiler should automatically generate the serialization operator, but the
user can always overload it with a special version.
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I like the idea of serialization2 too and I wonder if Cereal, somehow boostified, could be that rewrite. Looking at the "Key Differences From Boost", it sounds like there are very little missing features (e.g. versioning) that could be easily added or features that could be dropped (e.g. support for raw pointers). @robertramey Any chance that you were in touch with the uscilab devs and discussed a boost review of Cereal? |
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I looked at the information on cereal to get a sense of it. I've seen bench marks on it and heard good feedback on it. My sense is that it would be a very worthy basis on which to build the next generation serialization library: a) it excludes raw pointers. It supports specific smart point implementations with custom implementations. It would also exclude things like pointer tracking, serialization of derived types though calls to the a base type etc.. This would result in a much simpler library implementation and many users of serialization wouldn't miss this feature. In the context of a boost library, I'm not sure how that would work out. Often a boost library has to fulfill even the most obscure requirements in order to get a consensus in review. The support of raw pointers is a trade off between complexity and completeness. One reason for thinking in terms of serialization2 would be to "please everyone" by offering a choice between the two options. b) is header only. This results in a number of benefits. First, due to the ability of C++ optimizers to collapse inline code, it results in execution time of about half of the current library. The current library is pretty fast, but of course faster is always better. It also simplifies builds due to the elimination of the linkage step. Users love this because separate/compilation of C++ is getting more complex all the time. Also machines are much, much, much faster, and there is much, much, much more memory available to the build, than when the serialization library was built. So the increase stress of constantly (re) building a header library isn't such a burden. (But that burden has been growing as faster or faster than the machine resources!). But still there is something to be said for the usage of the pimple idiom to implement the library and minimize rebuilding. No one complains about the compile time of serialization library either when it's built or when it's used. The current system supports separate compilation on an applications serialization code. But to many users today - separate compilation is a burden rather than a feature. It's a trade off. A good design could support both. c) versioning not supported intrinsically. Versioning is built into the serialization library. This is blessing and a curse. One one hand it forces users to version stuff - even if only by default, but it adds a little overhead. If I had to do it again, I'd exclude default versioning. I'd just let the user specify his how version member along and maintain it with some boiler plate code (that is the version type would be serializable). d) I don't think serialization through virtual base classes can be supported with a cereal-like system. e) The (more or less) clean separation between the serialization concept and archive concept would be maintained as both boost and cereal do now. A large part of the documentation of the boost serialization library would carry over to a cereal inspired system. Sooooo - a cereal like system would have some very attractive features and I think would complement rather than replace the current boost serialization library. It would be much less complex to implement than the current library. But it would still be a lot of work. I think it would be very popular with boost users - simpler to use, faster and "concept compatible" with the old library. If it implemented or used the current archive classes so it was a "drop-in" replacement it would be a huge hit. If it interoperated with archives created by boost serialization it would be an even bigger hit - this might not be possible. Still, even with the boost serialization and cereal to leverage on - it still would be a lot of work. |
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I agree, serialization code is always "cold". |
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Sorry! I clicked the wrong button. |
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Since there was a bit of activity here, I will take the opportunity to give an update. I will summarize what I understood as the problem and then offer a real-life solution. The problem is that expanding the use of B.Serialization in the wild has a chicken and egg problem, a cycle that needs to be broken somehow. First, it is tough to make B.Serialization work with arbitrary types, and it is almost impossible to make them work efficiently. If B.Serialization were a C++20 library, the situation would be slightly different because concept-based serialization would be more doable. Now, on the third-party library side, the opposite happens. What is the best a fundamental library can still do? Well, it can assume that there is a serialization Archive with a specific simple interface and to depend on that. The problem is that B.Serialization is not as simple as one would like. As a step forward, NVP was moved from B.Serialization to B.Core, perhaps for this reason. The other problem is the serialization of contiguous elements, which requires the use of array_wrapper, another explicit dependency on Boost.Serialization. So, that is the situation: B.Serialization will never implement serialization for all the possible classes out there (not even all the containers out there, not efficiently, at least). Fundamental libraries will not depend on Boost, for practical and essential reasons. Bigger programs or libraries will not either, because they will need to start from scratch from the fundamental types to make the use of B.Serialization worthwhile. As an example, I have this fundamental library to manipulate arrays https://gitlab.com/correaa/boost-multi. I have library users who under no circumstances would accept an indirect dependency on Boost, and why should I force that from a fundamental point of view?. Finally, one day, about a year ago, I found a solution. In this way, I could deduce corresponding internals for both libraries and make the library work with both. https://gitlab.com/correaa/boost-multi#serialization Of course, this was possible because Cereal has followed in part the brilliant interface proposed by B.Serialization, and I just patched details with the traits thing. Finally, since these archive_traits are “lazy”, I realized that I could not only work with both libraries but also that I could work with neither. Someone not using serialization would not need to depend on any serialization library. So, that’s how I stopped worrying about serialization and dependencies and love the chaos. So, nowadays, this is what I would do. In this order, as individual and a community. a) if you want to depend on Boost, no problem. Done. But you may still need to generalize code to work with other serialization frameworks. How can B.Serialization ease the pain:
How the community can help:
To end, I would like to mention, https://gitlab.com/correaa/boost-archive-yml . |
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I noticed that serialization code is missing for
boost::container::vectorandboost::container::stringand probably more of the Boost containers. Boost should be able to serialize ideally all or at least most of its own containers.The straight forward thing would be to implement serialization code for each individual container, but that's a maintenance burden and it does not exploit the fact that many containers adhere to a standard interface.
With a bit of concept checking a templated generic serialization code for containers could be written that adhere to the vector interface; similar for string, list, and map. The current code for std::vector, std::string, std::map, std::list could be turned into such a generic code. The concept checking would implement a form of duck-typing.
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