Is there any way to serialize (or, more generally to save to file) an object from a class that I can't modify?
All of the serialization approaches that I've found so far require some manner of intrusion, at the very least adding methods to the class, adding friends etc.
If the object is from a class in a library that I didn't write myself, I don't want to have to change the classes in that library.
More concretely: I'm developing a module-based application which also involves inter-module communication (which is as simple as: one module, the source, is the owner of some data, and another module, the destination, has a pointer to that data, and there's a simple updating/hand shaking protocol between the modules). I need to be able to save the state of the modules (which data was last communicated between the modules). Now, it would be ideal if this communication could handle any type, including ones from external libraries. So I need to save this data when saving the module state.
In general, there is no native serialization in C++, at least according to the wikipedia article on the topic of serialization.
There's a good FAQ resource on this topic here that might be useful if you haven't found it already. This is just general info on serialization, though; it doesn't address the specific problem you raise.
There are a couple of (very) ugly approaches I can think of. You've likely already thought of and rejected these, but I'm throwing them out there just in case:
Do you have access to all the data members of the class? Worst case, you could read those out into a c-style structure, apply your compiler's version of "packed" to keep the sizes of things predictable, and get a transferable binary blob that way.
You could cast a pointer to the object as a pointer to uint8_t, and treat the whole thing like an array. This will get messy if there are references, pointers, or a vtable in there. This approach might work for POD objects, though.
Related
I'm implementing serialization in my component based game engine, to enable saving and loading in my game. I'm using Cereal to help me with serialization. However, two things are unclear to me:
I have a lot of components, and those components also contain classes, etc. Do I need to write serialization functions for all of them? That would mean that I have to write about 100 serialization functions. Most of them would be the same (just serialize all member variables). Is there a way to reduce the amount of work?
What do I do if I want to serialize a class containing classes from another codebase? For example, I'm using SDL and TinyXml. Would that mean that I have to write serialization functions in those codebases?
I hope I can prevent the grunt work of adding all those serialization functions.
Unfortunately there is no magic. Whatever serialization library you use, be it boost::serialization, s11n or MFC, the problem is that you always have to declare for each individual class how to serialize itself.
This is inherent to the fact that there is no metadata available on the members of classes, that could permit to automate serialization of complex classes on the base their member's type.
The only way round this problem is adopt classes designed on purpose to address dynamic self referencing. But this might be at a cost in terms of performance, or with an overhead in construction instead of archiving. Arpproaches could be a combination of:
archiving aware base classes.
map or containers of properties instead of local hard-coded variables.
eventually use self archiving base types for each serializable member, and at construction of an object register them in a kind of archivwing worklist.
Another way round would be to design a code generator that could run on your headers, and generate mecanically the selialization code. But this is already an ambitious project by itself.
A last thought: all this manual archiving code is for sure an overhead. However, it allows you to handle evlolution of your object structure, for example if a newer version of your code adds or removes some members and has to deserialize a file written with an older version. THis is something that could not easily be achieved with an automated approach.
All the code I write (C++ or AS3) is heavily scripted (JSON or XML). My problem is that parsing can be very slow at times, especially with less powerful devices like mobiles.
Here is an example of a Flash script of mine:
<players class="fanlib.gfx.TSprite" vars="x=0|y=-50|visible=Bool:true">
<player0 class="fanlib.gfx.TSprite" vars="x=131|y=138">
<name class="fanlib.text.TTextField" format="Myriad Pro,18,0xffffff,true,,,,,center,,,,0" alignX="center" alignY="bottom" filters="DropShadow,2" vars="background=Bool:false|backgroundColor=0|embedFonts=Bool:true|multiline=Bool:false|mouseEnabled=Bool:false|autoSize=center|text=fae skata|y=-40"/>
<avatar class="fanlib.gfx.FBitmap" alignX="center" alignY="center" image="userDefault.png"/>
<chip class="fanlib.gfx.FBitmap" alignX="center" alignY="center" image="chip1.png" vars="x=87|y=68"/>
<info class="fanlib.text.TTextField" format="Myriad Pro,18,0xffffff,true,,,,,center,,,,0" alignX="center" alignY="top" filters="DropShadow,2" css=".win {color: #40ff40}" vars="y=40|background=Bool:false|backgroundColor=0|embedFonts=Bool:true|multiline=Bool:false|mouseEnabled=Bool:false|autoSize=center"/>
</player0>
<player1 class="Copy:player0" vars="x=430|y=70">
<chip class="Child:chip" image="chip2.png" vars="x=-82|y=102"/>
</player1>
<player2 class="Copy:player0" vars="x=778|y=70">
<chip class="Child:chip" image="chip3.png" vars="x=88|y=103"/>
</player2>
<player3 class="Copy:player0" vars="x=1088|y=137">
<chip class="Child:chip" image="chip4.png" vars="x=-111|y=65"/>
</player3>
<player4 class="Copy:player0" vars="x=1088|y=533">
<chip class="Child:chip" image="chip5.png" vars="x=-88|y=-23"/>
</player4>
<player5 class="Copy:player0" vars="x=585|y=585">
<chip class="Child:chip" image="chip6.png" vars="x=82|y=-54"/>
</player5>
<player6 class="Copy:player0" vars="x=117|y=533">
<chip class="Child:chip" image="chip7.png" vars="x=85|y=-26"/>
</player6>
</players>
The script above creates "native" (as in "non-dynamic") Flash objects. TSprite is a Sprite descendant, FBitmap inherits from Bitmap etc. At 71KBs, it takes tens of seconds to be parsed on my Sony XPeria.
Instead of optimizing the parser (which wouldn't probably gain too much anyway) I am contemplating converting my scripts to binaries, so that scripts will be used for debugging and the finalized binaries for release-built code.
One question is, how does one handle pointers from one object to another when serializing them? How are pointers translated from memory to disk file-friendly format,then back to memory?
Another question is, what about "nested" objects? In Flash for example, an object can be a graphics container of other objects. Could such a state be serialized? Or must objects be saved separately and, when loaded from disk, added to their parents through the nesting functions (i.e. addChild etc...)?
If possible, I would prefer generic guidelines that could apply to languages as different as C++ or AS3.
As far as I've understood your idea is to save some time by replacing creation of the objects from some fixture script (xml/json) to deserializing (from binary) previously serialized objects. If that's the case I beleive that you've taken wrong approach to solve this issue.
Since you've asked for general guidelines I'll try to explain my reasoning, not delving too deeply into language-specific details. Note that I'll talk about the common case and there may be exceptions from it. There is no silver bullet and you should analyze your scenario to pick the best solution for it.
From one point of view creating set of objects based on fixture/script is not that different from deserializing objects from binary. In the end they both are about turning some "condensed" state into objects that you can use. Yes, it is true that binaries are usually smaller in size but formats like JSON have not that many overhead in commomon case (XML is usually more redundant though). In general you won't save much time/memory on deserializing this state from binary instead of parsing it from script. Here is a real world example from somthing I worked with: Mental Ray is a de-facto standard for rendering 3d scenes\special effects in a movie industry. It uses textual file format to represent scenes that is somewhat similar to JSON in many aspects. Mental Ray is heavy on computations, performance is one of the key issues here, yet it lives perfectly fine without binary scene file format. So, analyzing this aspect you can say that there is no substantial difference between these 2 approaches.
From another point of view, there is a difference that may come into play. While deserializing object implies only creating object and loading state into it's fields, creating object from script may also include some extra initialization on top of that. So, in some cases there may be benefit from using deserialization approach.
However, in the end I would argue that it is not a good idea to simply replase your scripted objects to serialized objects because scripting and serialization are conceptually different things and have different purposes (though they do have something in common). Using serialization approach you'll loose flexibility in modifying your fixtured state (it is usually much harder for humans to edit binaries instead of JSON/XML) as well as ability to do initialization work.
So, think about what you actually need in your scenario and stick with it. That's the best way.
Now, if it happen to be that you actually need your objects to be scripted but this approach is not fast enough I would investigate speeding it up in one of 2 ways:
Analyze whether it is possible to restructure your data the way it will take less time to load. This is not always possible, however, it might be worth trying it.
Analyze what else your scripting engine does to init object except of simply creating them and load state into their fields and try to optimize it. This approach is actually has the most potential since this is the only part that has substantional difference in terms of performance (between scripting and deserialization approach) and does not lead to misuse of concepts. Try to see whether you are able to reduce this amount of work, needed to init object. It may be a good idea to tailor something more specific for your needs if you are currently using some generic scripting engine\framework at the moment.
Now, answering your original questions...
how does one handle pointers from one object to another when serializing them?
References are headache most of serialization implementations does not mess with.
One of the approaches is to use something to identify object during serialization (pointer, for example), serialize object preserving this this identity, and store references from another objects to this object not as primitive type, but as a reference type (basically saving identity). When deserializing - keep track of all deserialized objects and reuse them when deseializing reference typed field.
How are pointers translated from memory to disk file-friendly format,then back to memory?
It is a rarity when serialization deals with raw memory. This approach is only good for primitive types and does not work with poiners/references well. Languages that support reflection\introspection usually use it to inspect values of the fields to serialize objects. If we talk about pure C++, where reflection is poor - there is no other reliable way except to make object itself define means to serialize itself to bytestream and use these methods.
Another question is, what about "nested" objects? In Flash for example, an object can be a graphics container of other objects. Could such a state be serialized? Or must objects be saved separately and, when loaded from disk, added to their parents through the nesting functions (i.e. addChild etc...)?
If we talk about deserialization - these should probably be treated like references (see answer above). Using methods like addChild is not a good idea since it can contain some logic that will mess things.
Hope this answers your questions.
You should realty take a look at Adobe Remote Object.
Typically using serialization could cost you problems, such as:
I have a serialized object from application version 2.3 and now at the new version 2.4 it been modified: property removed / property added. This makes my serialized object unparsable.
While developing a serialization protocol that will support cross platform, you may actually wish to kill yourself while debugging. I remember myself doing this and I spent hours to find out that my flash using Big Indian and C# using Small Indian.
Adobe solved those problems for you, they created a nice binary protocol called AMF - Action Message Format. It have many implementations on various platforms that can communicate with your actions script.
Here you may find some C++ implementations.
i^m currenty searching for a good OO design to serialize a C++/Qt Application.
Imagine the classes of the application organized based on a tree structure, implemented with the Composite-Pattern, like in the following picture.
The two possible principles i thought of:
1.)
Put the save()/load() function in every class which has to be serializable.
If have seen this many times, usually implemented with boost.
Somewhere in the class you will find something like this:
friend class boost::serialization::access;
template<class Archive>
void serialize(Archive & ar, const unsigned int version)
{
ar & m_meber1;
}
You could also seperate this into save() and load().
But the disadvantage of this approach is :
If you wannt to change the serialization two months later (to XML, HTML or something very curious, which boost not supports) you have to adopt all the thousands of classes.
Which in my opinion is not a good OO-Design.
And if you wannt to support different serializations (XML, Binary, ASCII, whatever....) at the same time than 80% of the cpp exists just for serialization functions.
2.)
I know boost also provides a Non intrusive Version of the Serialization
"http://www.boost.org/doc/libs/1_49_0/libs/serialization/doc/tutorial.html"
So another way is to implement an Iterator which iterates over the composite tree structure and serializes every object (and one iterator for the deserialization)
(I think this is what the XmlSerializer-Class of the .NET-Framework does, but i^m not realy familiar with .NET)
This sounds better because seperate save() and load() and only one spot to change if serialization changes.
So this sounds better, BUT:
- You have to provide a setter() and a getter() for every parameter you wannt to serialize. (So, there is no private Data anymore. (Is this good/bad?))
- You could have a long inheritance hirarchy (more than 5 Classes) hanging on the composite tree.
So how do you call the setter()/getter() of the derived classes?
When you can only call a interface function of the base Composite-Component.
Another way is to serialize the objects data into a seperate abstract format.
From which all the possible following serializations (XML, TEXT, whatever is possible) get their data.
One Idea was to serialize it to QDomNode.
But i think the extra abstraction will cost performance.
So my question is:
Does anyone know a good OO-Design for serialization?
Maybe from other programming languages like JAVA, Python, C#, whatever....
Thank you.
Beware of serialization.
Serialization is about taking a snapshot of your in-memory representation and restoring it later on.
This is all great, except that it starts fraying at the seams when you think about loading a previously stored snapshot with a newer version of the software (Backward Compatibility) or (god forbid) a recently stored snapshot with an older version of the software (Forward Compatibility).
Many structures can easily deal with backward compatibility, however forward compatibility requires that your newer format is very close to its previous iteration: basically, just add/remove some fields but keeps the same overall structure.
The problem is that serialization, for performance reasons, tends to tie the on-disk structure to the in-memory representation; changing the in-memory representation then requires either the deprecation of the old archives (and/or a migration utility).
On the other hand, messaging systems (and this is what google protobuf is) are about decoupling the exchanged messages structures from the in-memory representation so that your application remains flexible.
Therefore, you first need to choose whether you will implement serialization or messaging.
Now you are right that you can either write the save/load code within the class or outside it. This is once again a trade-off:
in-class code has immediate access to all-members, usually more efficient and straightforward, but less flexible, so it goes hand in hand with serialization
out-of-class code requires indirect access (getters, visitors hierarchy), less efficient, but more flexible, so it goes hand in hand with messaging
Note that there is no drawback about hidden state. A class has no (truly) hidden state:
caches (mutable values) are just that, they can be lost without worry
hidden types (think FILE* or other handle) are normally recoverable through other ways (serializing the name of the file for example)
...
Personally I use a mix of both.
Caches are written for the current version of the program and use fast (de)serialization in v1. New code is written to work with both v1 and v2, and writes in v1 by default until the previous version disappears; then switches to writing v2 (assuming it's easy). Occasionally, massive refactoring make backward compatibility too painful, we drop it on the floor at this point (and increment the major digit).
On the other hand, exchanges with other applications/services and more durable storage (blobs in database or in files) use messaging because I don't want to tie myself down to a particular code structure for the next 10 years.
Note: I am working on server applications, so my advices reflect the particulars of such an environment. I imagine client-side apps have to support old versions forever...
I am computational scientist that work with large amount of simulation data and often times I find myself saving/loading data into/from the disk. For simple tasks, like a vector, this is usually as simple as dumping bunch of numbers into a file and that's it.
For more complex stuff, life objects and such, I have save/load member functions. Now, I'm not a computer scientist, and thus often times I see terminologies here on SO that I just do not understand (but I love to). One of these that I've came across recently is the subject of serialization and Boost.Serialization library.
From what I understand serialization is the simply the process of converting your objects into something that can be saved/loaded from dist or be transmitted over a network and such. Considering that at most I need to save/load my objects into/from disk, is there any reason I should switch from the simple load/save functions into Boost.Serialization? What would Boost.Serialization give me other than what I'm already doing?
That library takes into accounts many details that could be non very apparent from a purely 'applicative' point of view.
For instance, data portability WRT big/little numeric endianess, pointed data life time, structured containers, versioning, non intrusive extensions, and more. Moreover, it handles the right way the interaction with other std or boost infrastructure, and dictates a way of code structuring that will reward you with easier code maintenance. You will find ready to use serializers for many (all std & boost ?) containers.
And consider if you need to share your data with someone other, there are chances that referring to a published, mantained, and debugged schema will make things much easier.
I have this general problem in design, refactoring or "triage":
I have an existing multi-threaded C++ application which searches for data using a number of plugin libraries. With the current search interface, a given plugin receives a search string and a pointer to a QList object. Running on a different thread, the plugin goes out and searches various data sources (locally and on the web) and adds the objects of interest to the list. When the plugin returns, the main program, still on the separate thread, adds this data to the local data store (with further processing), guarding this insertion point using a mutex. Thus each plugin can return data asynchronously.
The QT-base plugin library is based on message passing. There are a fair number of plugins which are already written and tested for the application and they work fairly well.
I would like to write some more plugins and leverage the existing application.
The problem is that the new plugins will need more information from the application. They will to need intermittent access to the local data store itself as they search. So to get this, they would need direct or indirect access both the hash array storing the data and the mutex which guards multiple access to the store. I assume the access would be encapsulated by adding an extra method in a "catalog" object.
I can see three ways to write these new plugins.
When loading a plugin, pass them
a pointer to my "catalog" at the
start. This becomes an extra,
"invisible" interface for the new
plugins. This seems quick, easy,
completely wrong according to OO but
I can't see what the future problems would be.
Add a method/message to the
existing interface so I have a
second function which could be
called for the new plugin libraries,
the message would pass a pointer to
the catalog to the plugins. This
would be easy for the plugins but it
would complicate my main code and
seems generally bad.
Redesign the plugin interface.
This seems "best" according to OO,
could have other added benefits but
would require all sorts of
rewriting.
So, my questions are
A. Can anyone tell me the concrete dangers of option 1?
B. Is there a known pattern that fits this kind of problem?
Edit1:
A typical function for calling the plugin routines looks like:
elsewhere(spec){
QList<CatItem> results;
plugins->getResult(spec, &results);
use_list(results);
}
...
void PluginHandler::getResults(QString* spec, QList<CatItem>* results)
{
if (id->count() == 0) return;
foreach(PluginInfo info, plugins) {
if (info.loaded)
info.obj->msg(MSG_GET_RESULTS, (void*) spec, (void*) results);
}
}
It's a repeated through-out the code. I'd rather extend it than break it.
Why is it "completely wrong according to OO"? If your plugin needs access to that object, and it doesn't violate any abstraction you want to preserve, it is the correct solution.
To me it seems like you blew your abstractions the moment you decided that your plugin needs access to the list itself. You just blew up your entire application's architecture. Are you sure you need access to the actual list itself? Why? What do you need from it? Can that information be provided in a more sensible way? One which doesn't 1) increase contention over a shared resource (and increase the risk of subtle multithreading bugs like race conditions and deadlocks), and 2) doesn't undermine the architecture of the rest of the app (which specifically preserves a separation between the list and its clients, to allow asynchronicity)
If you think it's bad OO, then it is because of what you're fundamentally trying to do (violate the basic architecture of your application), not how you're doing it.
Well, option 1 is option 3, in the end. You are redesigning your plugin API to receive extra data from the main app.
It's a simple redesign that, as long as the 'catalog' is well implemented and hide every implementation detail of your hash and mutex backing store, is not bad, and can serve the purpose well enough IMO.
Now if the catalog leaks implementation details then you would better use messages to query the store, receiving responses with the needed data.
Sorry, I just re-read your question 3 times and I think my answer may have been too simple.
Is your "Catalog" an independent object? If not, you could wrap it as it's own object. The Catalog should be completely safe (including threadsafe)--or better yet immutable.
With this done, it would be perfectly valid OO to pass your catalog to the new plugins. If you are worried about passing them through many layers, you can create a factory for the catalog.
Sorry if I'm still misunderstanding something, but I don't see anything wrong with this approach. If your catalog is an object outside your control, however, such as a database object or collection then you really HAVE to encapsulate it in something you can control with a nice, clean interface.
If your Catalog is used by many pieces across your program, you might look at a factory (which, at it's simplest degrades to a Singleton). Using a factory you should be able to summon your Catalog with a Catalog.getType("Clothes"); or whatever. That way you are giving out the same object to everyone who wants one without passing it around.
(this is very similar to a singleton, by the way, but coding it as a factory reminds you that there will almost certainly be more than one--also remember to allow a Catalog.setType("Clothes", ...); for testing.