I've been reading about encapsulation and keep seeing comments about how changing the privacy of a class or adding getters and setters where there were none before can 'break the code' of people who use your library. I don't really understand this. I'm very inexperienced in programming, and my understanding is that you download a library onto your computer and it's included in the files of the program you're writing, so if the original author changed something in THEIR COPY of the library, it wouldn't affect your copy. Is this wrong? For example, is a library more like a website that your computer connects to through the internet and the original author can update, so that changes they make to it can affect how your code works?
Software is constantly changing, so we must have a way to keep track of the different versions - hence software versions. When you download a library to use in your own program, you (usually, like with a dependency management tool) end up downloading a very specific version of that library.
If a library author was to change the interface to use it, developers using that library would also have to change how they use it when they download the version with those changes. Otherwise, it would break any code that follows an outdated interface.
As long as a library author follows proper versioning procedures, for instance including breaking changes in a new major version, and the changes improve the clarity of the library's interfaces without sacrificing other properties, then the argument is moot. Developers can either continue using the old version or update their code to be compatible with the new version.
Except for maybe in low resource, embedded systems that can use all optimizations available, like accessing object/structure properties directly rather than through a function.
Libraries:
By definition a collection of non-volatile resources used by computer
programs, often for software development. These may include
configuration data, documentation, help data, message templates,
pre-written code and subroutines, classes, values or type
specifications.
Explanation
Let me spend time on defining in coding aspects: Lets say you have a
create a soccer game, what does that need, field, ball, players,
flags.
All this we encapsulate in Class to make as Object Game which
comprises all above.
Now you start building game and realise you are spending redudant time making repeated player names, shirt design , details filling, etc.
To avoid this you make functions which are business specific like 1. generate tshirts and pass (color, design , cloth type) and it returnes you tshirt object in return.
Similary you get player information by passing country and his ID and all this details are return as Player object which have his name, place, country, contacts ets.
This is how the functions in class behaves.
Your ask
privacy of a class or adding getters and setters, ...an 'break the code' of people who use your library
These are ways how you access the object parameters or set values for them , in some languages the getter and setters are auto generated and not required to explicitly set unless you need custom settings during class object creation.
The best advantage of the getting and setter is it ensures the default class creation can be assigned some values which you dont want to change in defaults and also not allow people to enforce new values to that specific parameter of class.
This is how the control is made in place during defining your class and its functions. getter and setter are functions as well with class variables having factility to get/set values as you define the function logic inside.
You ask
my understanding is that you download a library onto your computer and
it's included in the files of the program you're writing, so if the
original author changed something in THEIR COPY of the library, it
wouldn't affect your copy. Is this wrong?
Yes think it like a CD Copy , I sent you a copy so you can use those info from the copy i made, but once i have new features and things added in CompactDisc(CD) it wouldnt be there in your copy i burnt during that time hence you code uses the old version and may use till there is need to update.
You can only get impacted if you take my new CD copy which is called as upgrading your software with my new library version.
Normally big guys software dont immediately change the library in their systems unless there is thorough analysis done with 1. need, 2. security 3. bugs in old fixed in new. factors to address for a new upgrade.
Happy Coding
Software world is free of your mind to code so dont think what is wrong or right just code.
Take a Maths Library building task in hand use anything python, java, c#, objective C, swift, javascript ...
Create library with modules with Circle, Square, Polygon, Sphere objects
Each object they will have thier respective Classes created with theier paramters (circle sample : radius, center(x,y), etc and functions like setRadius, getCircumference, etc)
Similar way all objects makes thier own classes
Abstrat word you used means some function you make private that only class can internally access but not exposed to outside when you create new Maths Object.
Hope this was helpful, happy coding.
I use this extension for Atom, to design my API, written in RAML.
I think I have a problem here :(I masked title and baseUri, sorry) :
If I follow RAML 1.0 specs, I should put a "!include". Strangely, apiworkbench detects no mistake.
If I do that :
Why didn't this work?
Very good conversations. Indeed the specification should be more clear about that, but the reason why libraries follows a different approach than normal !include is that an include simply adds new nodes to an existing where ever you used the !include keyword. Since it really is a simple "add" operation, it does not cover up for any cyclic dependencies.
Libraries are very much different and the use of namespaces (uses) are very much different. The purpose of libraries is to create a common shareable group of assets/definitions of best practices that people also use to create their own libraries or other definitions on top. Cyclic dependencies are inevitable. For that, the RAML workgroup had to come up with a different mechanism than what you have with !include. Hence, for libraries you should always use:
uses
lib: mylib.raml
Hope that explains the rationality behind it, but please let me know if you have more questions.
No, for libraries you must NOT use the include keyword.
It seems the specification is not very clear about this or at least I could not find it clearly specified anywhere. Thus raising an issue about this would be a good idea.
But if you check the examples in the specification you will see that when using libraries (with the "uses" keyword) the "!include" is omitted.
The question is related to my previous question:
access violation at address in module ntdll.dll - RtlEnterCriticalSection with TCanvas.Lock
Apparently there is a bug in Delphi's code (see QC 64898: Access violation in FreeDeviceContexts). This bug goes all the way until D2010, AFAIK.
The suggested workaround worked fine so far. Now I have a dilemma.
I don't like the idea of using a private copy of Controls.pas in my project - I'm not sure it is safe. The Controls unit is a very low level unit, and I really feel it's a drastic move, considering that my huge application works fine, except for the mentioned problem. I'm also not sure if/how to rebuild all components/units that rely on the Controls unit in my project.
Is it possible to patch TControlCanvas.CreateHandle(), which uses an internal CanvasList and private members?
NOTE: I will be using the patch for this project only (Delphi 5). I don't mind hard-coding the offsets. AFAIK, patching privates always uses hard-coded offsets, based on the compiler version. I might be able to deal with privates myself (without class helpers), but I have no clue how to handle CanvasList and FreeDeviceContext(), which are declared in the implementation section of the Controls unit.
As discussed in the comments, it is possible to access the private and protected members of classes, even in older versions of Delphi without "class helpers".
However, the problem in this case revolves around the details of a particular method implementation, not just being able to access or modify private member variables. Further, the implementation of a particular method which makes use of an implementation variable in the unit involved. Specifically the CanvasList variable that you have noted.
Even with the benefit of class helpers, there is no simple way to access that implementation variable.
Your current solution is the simplest and safest approach: Using a copy of the entire unit with a modification applied to the specific method required to solve the issue.
Rest assured, this is not an uncommon practise. :)
Your only problem with this approach is to be sure to manage the fact that you are relying on this "privatised" copy of the unit when standing up new development environments or upgrading to new versions of the IDE.
In the case of new development environments, careful project configuration should take care of things (and of course, your modified Controls.pas unit is part of your version controlled project).
In the case of upgrading to newer Delphi versions, you simply have to remember to revisit the modified Controls unit in each new version, updating the private copy in your project and re-applying the modifications you have made as appropriate. In most if not all cases this should be straightforward.
But I Really Want to Access the CanvasList Variable
As I say above, there is no simple way to access the implementation variable used in that unit (which will be necessary if you were to somehow contrive to "patch" the code at runtime, rather than replacing it with a modified copy at compile time).
But that implies that there is a **non-**simple way. And there is.
Like any data in your application, that variable resides at some memory address in your process. It's only the compiler scoping rules which prevent you from addressing it directly in source. There is nothing stopping you figuring out how to find that location at runtime and addressing that memory location via a pointer as you would any other "raw" memory address to which you have access.
I don't have a worked up demonstration of how to do that and strongly recommend that trying to implement such a solution is a waste of time and effort, given that an easier solution exists (copying and modifying the unit).
Apart from anything else, depending upon how reliable the method is for determining the memory location involved, direct access to that memory location could prove potentially vulnerable not only to differences between compiler versions but even to changes arising from compiler settings.
In terms of the end result, it is no better than copying the unit but is certainly far harder and far less reliable.
Objective-C has no namespaces; it's much like C, everything is within one global namespace. Common practice is to prefix classes with initials, e.g. if you are working at IBM, you could prefix them with "IBM"; if you work for Microsoft, you could use "MS"; and so on. Sometimes the initials refer to the project, e.g. Adium prefixes classes with "AI" (as there is no company behind it of that you could take the initials). Apple prefixes classes with NS and says this prefix is reserved for Apple only.
So far so well. But appending 2 to 4 letters to a class name in front is a very, very limited namespace. E.g. MS or AI could have an entirely different meanings (AI could be Artificial Intelligence for example) and some other developer might decide to use them and create an equally named class. Bang, namespace collision.
Okay, if this is a collision between one of your own classes and one of an external framework you are using, you can easily change the naming of your class, no big deal. But what if you use two external frameworks, both frameworks that you don't have the source to and that you can't change? Your application links with both of them and you get name conflicts. How would you go about solving these? What is the best way to work around them in such a way that you can still use both classes?
In C you can work around these by not linking directly to the library, instead you load the library at runtime, using dlopen(), then find the symbol you are looking for using dlsym() and assign it to a global symbol (that you can name any way you like) and then access it through this global symbol. E.g. if you have a conflict because some C library has a function named open(), you could define a variable named myOpen and have it point to the open() function of the library, thus when you want to use the system open(), you just use open() and when you want to use the other one, you access it via the myOpen identifier.
Is something similar possible in Objective-C and if not, is there any other clever, tricky solution you can use resolve namespace conflicts? Any ideas?
Update:
Just to clarify this: answers that suggest how to avoid namespace collisions in advance or how to create a better namespace are certainly welcome; however, I will not accept them as the answer since they don't solve my problem. I have two libraries and their class names collide. I can't change them; I don't have the source of either one. The collision is already there and tips on how it could have been avoided in advance won't help anymore. I can forward them to the developers of these frameworks and hope they choose a better namespace in the future, but for the time being I'm searching a solution to work with the frameworks right now within a single application. Any solutions to make this possible?
Prefixing your classes with a unique prefix is fundamentally the only option but there are several ways to make this less onerous and ugly. There is a long discussion of options here. My favorite is the #compatibility_alias Objective-C compiler directive (described here). You can use #compatibility_alias to "rename" a class, allowing you to name your class using FQDN or some such prefix:
#interface COM_WHATEVER_ClassName : NSObject
#end
#compatibility_alias ClassName COM_WHATEVER_ClassName
// now ClassName is an alias for COM_WHATEVER_ClassName
#implementation ClassName //OK
//blah
#end
ClassName *myClass; //OK
As part of a complete strategy, you could prefix all your classes with a unique prefix such as the FQDN and then create a header with all the #compatibility_alias (I would imagine you could auto-generate said header).
The downside of prefixing like this is that you have to enter the true class name (e.g. COM_WHATEVER_ClassName above) in anything that needs the class name from a string besides the compiler. Notably, #compatibility_alias is a compiler directive, not a runtime function so NSClassFromString(ClassName) will fail (return nil)--you'll have to use NSClassFromString(COM_WHATERVER_ClassName). You can use ibtool via build phase to modify class names in an Interface Builder nib/xib so that you don't have to write the full COM_WHATEVER_... in Interface Builder.
Final caveat: because this is a compiler directive (and an obscure one at that), it may not be portable across compilers. In particular, I don't know if it works with the Clang frontend from the LLVM project, though it should work with LLVM-GCC (LLVM using the GCC frontend).
If you do not need to use classes from both frameworks at the same time, and you are targeting platforms which support NSBundle unloading (OS X 10.4 or later, no GNUStep support), and performance really isn't an issue for you, I believe that you could load one framework every time you need to use a class from it, and then unload it and load the other one when you need to use the other framework.
My initial idea was to use NSBundle to load one of the frameworks, then copy or rename the classes inside that framework, and then load the other framework. There are two problems with this. First, I couldn't find a function to copy the data pointed to rename or copy a class, and any other classes in that first framework which reference the renamed class would now reference the class from the other framework.
You wouldn't need to copy or rename a class if there were a way to copy the data pointed to by an IMP. You could create a new class and then copy over ivars, methods, properties and categories. Much more work, but it is possible. However, you would still have a problem with the other classes in the framework referencing the wrong class.
EDIT: The fundamental difference between the C and Objective-C runtimes is, as I understand it, when libraries are loaded, the functions in those libraries contain pointers to any symbols they reference, whereas in Objective-C, they contain string representations of the names of thsoe symbols. Thus, in your example, you can use dlsym to get the symbol's address in memory and attach it to another symbol. The other code in the library still works because you're not changing the address of the original symbol. Objective-C uses a lookup table to map class names to addresses, and it's a 1-1 mapping, so you can't have two classes with the same name. Thus, to load both classes, one of them must have their name changed. However, when other classes need to access one of the classes with that name, they will ask the lookup table for its address, and the lookup table will never return the address of the renamed class given the original class's name.
Several people have already shared some tricky and clever code that might help solve the problem. Some of the suggestions may work, but all of them are less than ideal, and some of them are downright nasty to implement. (Sometimes ugly hacks are unavoidable, but I try to avoid them whenever I can.) From a practical standpoint, here are my suggestions.
In any case, inform the developers of both frameworks of the conflict, and make it clear that their failure to avoid and/or deal with it is causing you real business problems, which could translate into lost business revenue if unresolved. Emphasize that while resolving existing conflicts on a per-class basis is a less intrusive fix, changing their prefix entirely (or using one if they're not currently, and shame on them!) is the best way to ensure that they won't see the same problem again.
If the naming conflicts are limited to a reasonably small set of classes, see if you can work around just those classes, especially if one of the conflicting classes isn't being used by your code, directly or indirectly. If so, see whether the vendor will provide a custom version of the framework that doesn't include the conflicting classes. If not, be frank about the fact that their inflexibility is reducing your ROI from using their framework. Don't feel bad about being pushy within reason — the customer is always right. ;-)
If one framework is more "dispensable", you might consider replacing it with another framework (or combination of code), either third-party or homebrew. (The latter is the undesirable worst-case, since it will certainly incur additional business costs, both for development and maintenance.) If you do, inform the vendor of that framework exactly why you decided to not use their framework.
If both frameworks are deemed equally indispensable to your application, explore ways to factor out usage of one of them to one or more separate processes, perhaps communicating via DO as Louis Gerbarg suggested. Depending on the degree of communication, this may not be as bad as you might expect. Several programs (including QuickTime, I believe) use this approach to provide more granular security provided by using Seatbelt sandbox profiles in Leopard, such that only a specific subset of your code is permitted to perform critical or sensitive operations. Performance will be a tradeoff, but may be your only option
I'm guessing that licensing fees, terms, and durations may prevent instant action on any of these points. Hopefully you'll be able to resolve the conflict as soon as possible. Good luck!
This is gross, but you could use distributed objects in order to keep one of the classes only in a subordinate programs address and RPC to it. That will get messy if you are passing a ton of stuff back and forth (and may not be possible if both class are directly manipulating views, etc).
There are other potential solutions, but a lot of them depend on the exact situation. In particular, are you using the modern or legacy runtimes, are you fat or single architecture, 32 or 64 bit, what OS releases are you targeting, are you dynamically linking, statically linking, or do you have a choice, and is it potentially okay to do something that might require maintenance for new software updates.
If you are really desperate, what you could do is:
Not link against one of the libraries directly
Implement an alternate version of the objc runtime routines that changes the name at load time (checkout the objc4 project, what exactly you need to do depends on a number of the questions I asked above, but it should be possible no matter what the answers are).
Use something like mach_override to inject your new implementation
Load the new library using normal methods, it will go through the patched linker routine and get its className changed
The above is going to be pretty labor intensive, and if you need to implement it against multiple archs and different runtime versions it will be very unpleasant, but it can definitely be made to work.
Have you considered using the runtime functions (/usr/include/objc/runtime.h) to clone one of the conflicting classes to a non-colliding class, and then loading the colliding class framework? (this would require the colliding frameworks to be loaded at different times to work.)
You can inspect the classes ivars, methods (with names and implementation addresses) and names with the runtime, and create your own as well dynamically to have the same ivar layout, methods names/implementation addresses, and only differ by name (to avoid the collision)
Desperate situations call for desperate measures. Have you considered hacking the object code (or library file) of one of the libraries, changing the colliding symbol to an alternative name - of the same length but a different spelling (but, recommendation, the same length of name)? Inherently nasty.
It isn't clear if your code is directly calling the two functions with the same name but different implementations or whether the conflict is indirect (nor is it clear whether it makes any difference). However, there's at least an outside chance that renaming would work. It might be an idea, too, to minimize the difference in the spellings, so that if the symbols are in a sorted order in a table, the renaming doesn't move things out of order. Things like binary search get upset if the array they're searching isn't in sorted order as expected.
#compatibility_alias will be able to solve class namespace conflicts, e.g.
#compatibility_alias NewAliasClass OriginalClass;
However, this will not resolve any of the enums, typedefs, or protocol namespace collisions. Furthermore, it does not play well with #class forward decls of the original class. Since most frameworks will come with these non-class things like typedefs, you would likely not be able to fix the namespacing problem with just compatibility_alias.
I looked at a similar problem to yours, but I had access to source and was building the frameworks.
The best solution I found for this was using #compatibility_alias conditionally with #defines to support the enums/typedefs/protocols/etc. You can do this conditionally on the compile unit for the header in question to minimize risk of expanding stuff in the other colliding framework.
It seems that the issue is that you can't reference headers files from both systems in the same translation unit (source file). If you create objective-c wrappers around the libraries (making them more usable in the process), and only #include the headers for each library in the implementation of the wrapper classes, that would effectively separate name collisions.
I don't have enough experience with this in objective-c (just getting started), but I believe that is what I would do in C.
Prefixing the files is the simplest solution I am aware of.
Cocoadev has a namespace page which is a community effort to avoid namespace collisions.
Feel free to add your own to this list, I believe that is what it is for.
http://www.cocoadev.com/index.pl?ChooseYourOwnPrefix
If you have a collision, I would suggest you think hard about how you might refactor one of the frameworks out of your application. Having a collision suggests that the two are doing similar things as it is, and you likely could get around using an extra framework simply by refactoring your application. Not only would this solve your namespace problem, but it would make your code more robust, easier to maintain, and more efficient.
Over a more technical solution, if I were in your position this would be my choice.
If the collision is only at the static link level then you can choose which library is used to resolve symbols:
cc foo.o -ldog bar.o -lcat
If foo.o and bar.o both reference the symbol rat then libdog will resolve foo.o's rat and libcat will resolve bar.o's rat.
Just a thought.. not tested or proven and could be way of the mark but in have you considered writing an adapter for the class's you use from the simpler of the frameworks.. or at least their interfaces?
If you were to write a wrapper around the simpler of the frameworks (or the one who's interfaces you access the least) would it not be possible to compile that wrapper into a library. Given the library is precompiled and only its headers need be distributed, You'd be effectively hiding the underlying framework and would be free to combine it with the second framework with clashing.
I appreciate of course that there are likely to be times when you need to use class's from both frameworks at the same time however, you could provide factories for further class adapters of that framework. On the back of that point I guess you'd need a bit of refactoring to extract out the interfaces you are using from both frameworks which should provide a nice starting point for you to build your wrapper.
You could build upon the library as you and when you need further functionality from the wrapped library, and simply recompile when you it changes.
Again, in no way proven but felt like adding a perspective. hope it helps :)
If you have two frameworks that have the same function name, you could try dynamically loading the frameworks. It'll be inelegant, but possible. How to do it with Objective-C classes, I don't know. I'm guessing the NSBundle class will have methods that'll load a specific class.
I am playing with scripting for mysql-proxy. What I am trying to accomplish is irrelevant. What I am finding, however, is that there appears to be elements of the lua interface that are undocumented. The big "smoking gun" I have is the disconnect_client() hook. I cannot find it documented anywhere in the official documentation, though it's used in an example on the explanation of the admin interface. It's also used extensively in the example docs. A quick grep shows its use in the following scripts included in the 5.5.8 distribution:
active-queries.lua
active-transactions.lua
load-multi.lua
ro-pooling.lua
rw-splitting.lua
tutorial-keepalive.lua
My other example that has caught my eye is the list of attributes for entries in the proxy.global.backends table. The documentation lists the following attributes:
dst.name
dst.address
dst.port
connected_clients
state
type
However, several of the example scripts reference a fairly complex element called pool. Take for instance tutorial-keepalive.lua:
local s = proxy.global.backends[i]
local pool = s.pool
local cur_idle = pool.users[proxy.connection.client.username].cur_idle_connections
At first I thought it was being added somewhere else in the lua, but I've looked and I can't find any code assigning into proxy.global.backends[i].pool.
So, two questions I suppose:
Am I nuts? Please feel free to demonstrate how I've overlooked the obvious and the documentation is really quite clear.
Assuming I am correct, is there anywhere to find the complete documentation? A nice link would be great (though I couldn't google one up), but even "take a look at this .c file from the mysql-proxy distro that defines the interface". At least that would give me something to poke at.
Thanks
Just opened Proxy/Lua myself. I'm afraid, as with all things Mysql, once you dip under the covers you are on your own. The same goes for LUA.
You will need other addons to LUA to do what you want to do, I would like to suggest:
http://peterodding.com/code/lua/apr/docs/#shared_memory
for the Apache Protable library binding... saved me a shedload of time.
As for the internals of MySql, you will probably need to look in teh C source files, sadly, as we are in unexplored territory here.
One thing I have found is that the LUA structures in Mysql are not Lua tables, but Lua 'user data'. This means that they are effectively shared memory, so are not changeable by Lua, as Mysql uses them too. I am going to try the luaposix library to see if I can get inbto them, as there are elements I would like to change, if possible.
Good luck
peter.colclough#toolstation.com