I need to generate function name and then call it.
Is it possible to do like in php
<?php call_user_func_array(array($object, $method));?>?
There are four options:
Make the methods you want to call like this signals. Signals can be emited by name GLib.Signal.emit_by_name (g_signal_emit_by_name). The call is from GLib mode, but other modes with signal support are likely to have similar method.
Create a static table/hash table of delegate objects manually in code. The main advantage is that it is type-safe. Disadvantage is that you have to add each method in two places. It will also work in all vala modes.
Another option is to tell vala compiler to build the "gir" binding and use the GObject Introspection library to call the functions. That is much more complicated, but the compiler will maintain the list of available methods for you. This method is specific to the GLib mode.
The last option is to use the GLib.Module.symbol (g_module_symbol) function of GLib to find the symbol. You'll need to know the "mangled" C name of the symbol and it will not be type-safe. You will have to match argument types exactly and mind where the invocant should go. It avoids the overhead of GIR, but unlike GIR it can't tell you which methods exist, only get you a specific one. This method is used when connecting signals in GtkBuilder. I mentioned the function from GLib, but POSIX.dlsym can be used the same way.
Related
I see some usages of Extension functions in Kotlin I don't personally think that makes sense, but it seems that there are some guidelines that "apparently" support it (a matter of interpretation).
Specifically: defining an extension function outside a class (but in the same file):
data class AddressDTO(val state: State,
val zipCode: String,
val city: String,
val streetAddress: String
)
fun AddressDTO.asXyzFormat() = "${streetAddress}\n${city}\n${state.name} $zipCode"
Where the asXyzFormat() is widely used, and cannot be defined as private/internal (but also for the cases it may be).
In my common sense, if you own the code (AddressDTO) and the usage is not local to some class / module (hence behing private/internal) - there is no reason to define an extension function - just define it as a member function of that class.
Edge case: if you want to avoid serialization of the function starting with get - annotate the class to get the desired behavior (e.g. #JsonIgnore on the function). This IMHO still doesn't justify an extension function.
The counter-response I got to this is that the approach of having an extension function of this fashion is supported by the Official Kotlin Coding Conventions. Specifically:
Use extension functions liberally. Every time you have a function that works primarily on an object, consider making it an extension function accepting that object as a receiver.
Source
And:
In particular, when defining extension functions for a class which are relevant for all clients of this class, put them in the same file where the class itself is defined. When defining extension functions that make sense only for a specific client, put them next to the code of that client. Do not create files just to hold "all extensions of Foo".
Source
I'll appreciate any commonly accepted source/reference explaining why it makes more sense to move the function to be a member of the class and/or pragmatic arguments support this separation.
That quote about using extension functions liberally, I'm pretty sure means use them liberally as opposed to top level non-extension functions (not as opposed to making it a member function). It's saying that if a top-level function conceptually works on a target object, prefer the extension function form.
I've searched before for the answer to why you might choose to make a function an extension function instead of a member function when working on a class you own the source code for, and have never found a canonical answer from JetBrains. Here are some reasons I think you might, but some are highly subject to opinion.
Sometimes you want a function that operates on a class with a specific generic type. Think of List<Int>.sum(), which is only available to a subset of Lists, but not a subtype of List.
Interfaces can be thought of as contracts. Functions that do something to an interface may make more sense conceptually since they are not part of the contract. I think this is the rationale for most of the standard library extension functions for Iterable and Sequence. A similar rationale might apply to a data class, if you think of a data class almost like a passive struct.
Extension functions afford the possibility of allowing users to pseudo-override them, but forcing them to do it in an independent way. Suppose your asXyzFormat() were an open member function. In some other module, you receive AddressDTO instances and want to get the XYZ format of them, exactly in the format you expect. But the AddressDTO you receive might have overridden asXyzFormat() and provide you something unexpected, so now you can't trust the function. If you use an extension function, than you allow users to replace asXyzFormat() in their own packages with something applicable to that space, but you can always trust the function asXyzFormat() in the source package.
Similarly for interfaces, a member function with default implementation invites users to override it. As the author of the interface, you may want a reliable function you can use on that interface with expected behavior. Although the end-user can hide your extension in their own module by overloading it, that will have no effect on your own uses of the function.
For what it's worth, I think it would be very rare to choose to make an extension function for a class (not an interface) when you own the source code for it. And I can't think of any examples of that in the standard library. Which leads me to believe that the Coding Conventions document is using the word "class" in a liberal sense that includes interfaces.
Here's a reverse argument…
One of the main reasons for adding extension functions to the language is being able to add functionality to classes from the standard library, and from third-party libraries and other dependencies where you don't control the code and can't add member functions (AKA methods). I suspect it's mainly those cases that that section of the coding conventions is talking about.
In Java, the only option in this cases is utility methods: static methods, usually in a utility class gathering together lots of such methods, each taking the relevant object as its first parameter:
public static String[] splitOnChar(String str, char separator)
public static boolean isAllDigits(String str)
…and so on, interminably.
The main problem there is that such methods are hard to find (no help from the IDE unless you already know about all the various utility classes). Also, calling them is long-winded (though it improved a bit once static imports were available).
Kotlin's extension methods are implemented exactly the same way down at the bytecode level, but their syntax is much simpler and exactly like member functions: they're written the same way (with this &c), calling them looks just like calling a member function, and your IDE will suggest them.
(Of course, they have drawbacks, too: no dynamic dispatch, no inheritance or overriding, scoping/import issues, name clashes, references to them are awkward, accessing them from Java or reflection is awkward, and so on.)
So: if the main purpose of extension functions is to substitute for member functions when member functions aren't possible, why would you use them when member functions are possible?!
(To be fair, there are a few reasons why you might want them. For example, you can make the receiver nullable, which isn't possible with member functions. But in most cases, they're greatly outweighed by the benefits of a proper member function.)
This means that the vast majority of extension functions are likely to be written for classes that you don't control the source code for, and so you don't have the option of putting them next to the class.
I prefer working with files that are less than 1000 lines long, so am thinking of breaking up some Erlang modules into more bite-sized pieces.
Is there a way of doing this without expanding the public API of my library?
What I mean is, any time there is a module, any user can do module:func_exported_from_the_module. The only way to really have something be private that I know of is to not export it from any module (and even then holes can be poked).
So if there is technically no way to accomplish what I'm looking for, is there a convention?
For example, there are no private methods in Python classes, but the convention is to use a leading _ in _my_private_method to mark it as private.
I accept that the answer may be, "no, you must have 4K LOC files."
The closest thing to a convention is to use edoc tags, like #private and #hidden.
From the docs:
#hidden
Marks the function so that it will not appear in the
documentation (even if "private" documentation is generated). Useful
for debug/test functions, etc. The content can be used as a comment;
it is ignored by EDoc.
#private
Marks the function as private (i.e., not part of the public
interface), so that it will not appear in the normal documentation.
(If "private" documentation is generated, the function will be
included.) Only useful for exported functions, e.g. entry points for
spawn. (Non-exported functions are always "private".) The content can
be used as a comment; it is ignored by EDoc.
Please note that this answer started as a comment to #legoscia's answer
Different visibilities for different methods is not currently supported.
The current convention, if you want to call it that way, is to have one (or several) 'facade' my_lib.erl module(s) that export the public API of your library/application. Calling any internal module of the library is playing with fire and should be avoided (call them at your own risk).
There are some very nice features in the BEAM VM that rely on being able to call exported functions from any module, such as
Callbacks (funs/anonymous funs), MFA, erlang:apply/3: The calling code does not need to know anything about the library, just that it's something that needs to be called
Behaviours such as gen_server need the previous point to work
Hot reloading: You can upgrade the bytecode of any module without stopping the VM. The code server inside the VM maintains at most two versions of the bytecode for any module, redirecting external calls (those with the Module:) to the most recent version and the internal calls to the current version. That's why you may see some ?MODULE: calls in long-running servers, to be able to upgrade the code
You'd be able to argue that these points'd be available with more fine-grained BEAM-oriented visibility levels, true. But I don't think it would solve anything that's not solved with the facade modules, and it'd complicate other parts of the VM/code a great deal.
Bonus
Something similar applies to records and opaque types, records only exist at compile time, and opaque types only at dialyzer time. Nothing stops you from accessing their internals anywhere, but you'll only find problems if you go that way:
You insert a new field in the record, suddenly, all your {record_name,...} = break
You use a library that returns an opaque_adt(), you know that it's a list and use like so. The library is upgraded to include the size of the list, so now opaque_adt() is a tuple() and chaos ensues
Only those functions that are specified in the -export attribute are visible to other modules i.e "public" functions. All other functions are private. If you have specified -compile(export_all) only then all functions in module are visible outside. It is not recommended to use -compile(export_all).
I don't know of any existing convention for Erlang, but why not adopt the Python convention? Let's say that "library-private" functions are prefixed with an underscore. You'll need to quote function names with single quotes for that to work:
-module(bar).
-export(['_my_private_function'/0]).
'_my_private_function'() ->
foo.
Then you can call it as:
> bar:'_my_private_function'().
foo
To me, that communicates clearly that I shouldn't be calling that function unless I know what I'm doing. (and probably not even then)
Assume I have a Cocoa-based Mac or iOS app. I'd like to run a static analyzer on my app's source code or my app's binary to retrieve a list of all Objective-C methods called therein. Is there a tool that can do this?
A few points:
I am looking for a static solution. I am not looking for a dynamic solution.
Something which can be run against either a binary or source code is acceptable.
Ideally the output would just be a massive de-duped list of Objective-C methods like:
…
-[MyClass foo]
…
+[NSMutableString stringWithCapacity:]
…
-[NSString length]
…
(If it's not de-duped that's cool)
If other types of symbols (C functions, static vars, etc) are present, that is fine.
I'm familiar with class-dump, but AFAIK, it dumps the declared Classes in your binary, not the called methods in your binary. That's not what I'm looking for. If I am wrong, and you can do this with class-dump, please correct me.
I'm not entirely sure this is feasible. So if it's not, that's a good answer too. :)
The closest I'm aware of is otx, which is a wrapper around otool and can reconstruct the selectors at objc_msgSend() call sites.
http://otx.osxninja.com/
If you are asking for finding a COMPLETE list of all methods called then this is impossible, both statically and dynamically. The reason is that methods may be called in a variety of ways and even be dynamically and programmatically assembled.
In addition to regular method invocations using the Objective-C messages like [Object message] you can also dispatch messages using the C-API functions from objc/message.h, e.g. objc_msgSend(str, del). Or you can dispatch them using the NSInvocation API or with performSelector:withObject: (and similar methods), see the examples here. The selectors used in all these cases can be static strings or they can even be constructed programmatically from strings, using things like NSSelectorFromString.
To make matters worse Objective-C even supports dynamic message resolution which allows an object to respond to messages that do not correspond to methods at all!
If you are satisfied with only specific method invocations then parsing the source code for the patterns listed above will give you a minimal list of methods that may be called during execution. But the list may be both incomplete (i.e., not contain methods that may be called) as well as overcomplete (i.e., may contain methods that are not called in practice).
Another great tool is class-dump which was always my first choices for static analysis.
otool -oV /path to executable/ | grep name | awk '{print $3}'
This question already has answers here:
What is reflection and why is it useful?
(23 answers)
Closed 6 years ago.
I was just curious, why should we use reflection in the first place?
// Without reflection
Foo foo = new Foo();
foo.hello();
// With reflection
Class cls = Class.forName("Foo");
Object foo = cls.newInstance();
Method method = cls.getMethod("hello", null);
method.invoke(foo, null);
We can simply create an object and call the class's method, but why do the same using forName, newInstance and getMthod functions?
To make everything dynamic?
Simply put: because sometimes you don't know either the "Foo" or "hello" parts at compile time.
The vast majority of the time you do know this, so it's not worth using reflection. Just occasionally, however, you don't - and at that point, reflection is all you can turn to.
As an example, protocol buffers allows you to generate code which either contains full statically-typed code for reading and writing messages, or it generates just enough so that the rest can be done by reflection: in the reflection case, the load/save code has to get and set properties via reflection - it knows the names of the properties involved due to the message descriptor. This is much (much) slower but results in considerably less code being generated.
Another example would be dependency injection, where the names of the types used for the dependencies are often provided in configuration files: the DI framework then has to use reflection to construct all the components involved, finding constructors and/or properties along the way.
It is used whenever you (=your method/your class) doesn't know at compile time the type should instantiate or the method it should invoke.
Also, many frameworks use reflection to analyze and use your objects. For example:
hibernate/nhibernate (and any object-relational mapper) use reflection to inspect all the properties of your classes so that it is able to update them or use them when executing database operations
you may want to make it configurable which method of a user-defined class is executed by default by your application. The configured value is String, and you can get the target class, get the method that has the configured name, and invoke it, without knowing it at compile time.
parsing annotations is done by reflection
A typical usage is a plug-in mechanism, which supports classes (usually implementations of interfaces) that are unknown at compile time.
You can use reflection for automating any process that could usefully use a list of the object's methods and/or properties. If you've ever spent time writing code that does roughly the same thing on each of an object's fields in turn -- the obvious way of saving and loading data often works like that -- then that's something reflection could do for you automatically.
The most common applications are probably these three:
Serialization (see, e.g., .NET's XmlSerializer)
Generation of widgets for editing objects' properties (e.g., Xcode's Interface Builder, .NET's dialog designer)
Factories that create objects with arbitrary dependencies by examining the classes for constructors and supplying suitable objects on creation (e.g., any dependency injection framework)
Using reflection, you can very easily write configurations that detail methods/fields in text, and the framework using these can read a text description of the field and find the real corresponding field.
e.g. JXPath allows you to navigate objects like this:
//company[#name='Sun']/address
so JXPath will look for a method getCompany() (corresponding to company), a field in that called name etc.
You'll find this in lots of frameworks in Java e.g. JavaBeans, Spring etc.
It's useful for things like serialization and object-relational mapping. You can write a generic function to serialize an object by using reflection to get all of an object's properties. In C++, you'd have to write a separate function for every class.
I have used it in some validation classes before, where I passed a large, complex data structure in the constructor and then ran a zillion (couple hundred really) methods to check the validity of the data. All of my validation methods were private and returned booleans so I made one "validate" method you could call which used reflection to invoke all the private methods in the class than returned booleans.
This made the validate method more concise (didn't need to enumerate each little method) and garuanteed all the methods were being run (e.g. someone writes a new validation rule and forgets to call it in the main method).
After changing to use reflection I didn't notice any meaningful loss in performance, and the code was easier to maintain.
in addition to Jons answer, another usage is to be able to "dip your toe in the water" to test if a given facility is present in the JVM.
Under OS X a java application looks nicer if some Apple-provided classes are called. The easiest way to test if these classes are present, is to test with reflection first
some times you need to create a object of class on fly or from some other place not a java code (e.g jsp). at that time reflection is useful.
I have a Util module in my VB.NET program that has project-wide methods such as logging and property parsing. The general practice where I work seems to be to call these methods directly without prefixing them with Util. When I was new to VB, it took me a while to figure out where these methods/functions were coming from. As I use my own Util methods now, I can't help thinking that it's a lot clearer and more understandable to add Util. before each method call (you know immediately that it's user-defined but not within the current class, and where to find it), and is hardly even longer. What's the general practice when calling procedures/functions of VB modules? Should we prefix them with the module name or not?
Intellisense (and "Goto Definition") should make it trivial to find where things are located, but I always preface the calls with a better namespace, just for clarity of reading. Then it's clear that it's a custom function, and not something built in or local to the class you're working with.
Maybe there's a subtle difference I'm missing, but I tend to use shared classes instead of modules for any code that's common and self-contained - it just seems easier to keep track of for me, and it would also enforce your rule of prefacing it, since you can't just call it from everywhere without giving a namespace to call it from.
I usually put the complete namespace for a shared function, for readibility.
Call MyNameSpace.Utils.MySharedFunction()
Util is such a generic name.
Example from the .Net framework. You have System.Web.HttpUtility.UrlEncode(...). Usually you refer to this as HttpUtility.UrlEncode since you have an import statement at the top.
The name of the class which has the static utility methods should be readable and explainable. That is good practice. If you have good class names they might just as well reside in a Utils namespace, but the class name should not be Utils.
Put all your logging in a Logger class. All your string handing in a StringUtils class etc. And try to keep the class names as specific as possible, and I'd rather have more classes with fewer functions than the other way around.