I find the concept of metaclass fascinating, it treats classes as first class objects. You can assign a class to a variable, pass it to a method, and even create new classes at runtime. It seems that, every programming language that supports metaclasses are either dynamic typed languages(Smalltalk, Python, Ruby) or gradually typed languages(Objective C and Groovy both support static and dynamic typing). I have not seen a statically typed language that supports metaclasses.
Are static typing and metaclasses incompatible with each other? It does seem to me that, metaclass' functionalities require a certain degree of dynamism. I still wonder if it is technically possible for a static typed language like Java, C# and Kotlin to support metaclass and have classes as first class objects, or is it theoretically impossible?
Given that Python itself is implemented in C, and its objects can be "seen" and used in C through its API, the answer is "yes".
What may be harder in some languages is that objects belonging to a class hyerarchy were they have proper metaclasses, introspection, and runtime class creation may be a little tougher - requiring the code that implements such metaclasses to replicate part of the language runtime itself, or to call, at runtime, different code generation functions - at very least, one class could generate the source code with the declaration for the dynamically created classes, and iterate with the runtime to compile and load that code into the current process.
With C++, for example, where one can have full control of in-memory layout of data, maybe it is possible to emulate compiled classes with a lighter approach, that just fill in data-structures and redo the name-mangling part to attach class members.
If that would be something convenient, however, it is another question altogether. Maybe it is just better to leave metaclasses either for languages that support it out of the box, or, for languages were you are implementing a class system from scratch (like the Python runtime does using C).
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Every web development framework I've come across, including the best designed ones (Web2Py, Kohana) make use of some form of global variables to represent objects that are global within the application domain- for example, objects representing 'request' and 'response'.
On the other hand, the idea that global variables are bad is one of the fundamental axioms of programming. Indeed the now common disparagements of the singleton pattern usually point to the fact that it's nothing more than globals in disguise, as if that were explanation enough.
I'm trying to understand once and for all how globals can be so condemnable and at the same time be a seemingly indispensable part of all our web frameworks?
What is a global? Taking your text I assume you mean a variable that's declared at global scope. Such variable could be overridden by any assignment and break existing functionality.
However, in OO languages, everything is inside a class and assignment can be wrapped in gettors and settors for properties, or completely hidden behind methods. This gives a safe way of dealing with globals. Note that in proper OO languages (Java, C#, VB.NET etc) it is not possible to have global variables (sometimes a language construct suggests otherwise, but static fields in C# or modules in VB, mixins in Ruby are all wrapped in classes and thus not truly global).
A singleton, you mention it, is a special kind of global. As a designer you can control how many instances run of it. A car only needs one engine, a country only one government (or war breaks loose) and a program needs only one main thread. Globals are a necessity to programming, the real discussion should not be, do we need them, but how to solidly create and use them.
You say that request and response objects are globals in web development. They are not. They are (usually, depending on your toolset) convenience variables set in scope before your code is run. Since a web application can have multiple request objects at any given time, I think these are a poor example of a global variable. They are not (but they are usually local and a singleton to your current thread).
One important feature that you cannot cover in traditional procedural languages (like Basic, Pascal, C) is access control and thus concurrency and thread safety for global variables. In .NET for instance, any static method or property in the BCL (one could say that any static variable is global by definition) is thread-safe by design. Guidelines for user-defined static methods or properties suggest you do the same.
EDIT: the danger is with languages that allow global variables but at the same time propagate themselves as truly OO. While these are wonderful languages, it is indeed dangerous to step out of the protection of OO and create globals in for instance Perl, Python, Ruby, PHP.
I don't know exactly in what context those globals are used in web frameworks, but anything global starts to create problems as soon as you need to have solid access control. If you start to use such a global in concurrently executing program, it's quite hard to say who and when accessed and changed it. It creates so-called shared state. This makes debugging even more difficult.
Anyway, I am not really in favour of such statements. This only leads to oversimplifications. You have to weight you requirements and then decide if this or that pattern brings more positive or negative effects...
Languages such as Java and PHP support reflection, which allows objects to provide metadata about themselves. Are there any legitimate use cases where you would need to be able to do something like ask an object what methods it has outside of the realm of reverse engineering? Are any of those use cases actually implemented today?
Reflection is used extensively in Java by frameworks which are leveraged at runtime to operate with other code dynamically. Without reflection, all links between code must be done at compile time (statically).
So, for example, any useful plug-in framework (OSGi, JSPF, JPF), leverages Reflection. Any injection framework (Spring, Guice, etc) leverages Reflection.
Any time you want to write a piece of code that will interact with another piece of code without having that piece of code available when compiling, Reflection is the way forward in Java.
However, this is best left to frameworks and should be encapsulated.
There certainly are good use cases. For example, obtaining developer-provided metadata. Java APIs are increasingly using annotations to provide info about methods/fields/classes and their use. Like input validation, binding to data representations... You could use these at compile-time to generate metadata descriptors and use those, but to do it at runtime would require reflection. Even if you used the metadata descriptors, they'd end up containing things like class, method and field names that'd need to be accessed via reflection.
Another use case: dynamic languages. Take Ruby... It allows you to check up-front whether an object would respond to a method name before trying to call that method. Something like that requires reflection.
Or how about when a class or method name must be provided from outside compiled code, like when selecting an implementation of some API. That's just gonna be a bit of text. Looking up what it resolves to comes down to reflection.
Frameworks like Spring or Hibernate make extensive use of reflection to inspect a class and see the annotations.
Frameworks for debugging, serialization, logging, testing...
From what I gather, Gosu is simply C# for the JVM (which is a good thing). Is it true? What are some difference between Gosu and C# (Except the class library and the fact it runs on JVM) ?
We didn't build Gosu to be one language or the other for the JVM. Rather we built Gosu to be a useful language for the JVM. In addition, we recognized that Gosu needed to be familiar to the multitudes of existing programmers who are most comfortable with the imperative, object-oriented model. To achieve that we borrowed heavily from several languages e.g., Java, C#, EcmaScript, Ruby, and some others. The result, we think, is a language that is uniquely positioned on the JVM.
What is entirely unique about Gosu, however, is it's open type system.
Gosu's type system consists of a configurable number of type loaders. A type loader's primary responsibility is to resolve a type name in its domain and return an implementation of Gosu's IType interface. This is what is most unique about Gosu -- its type system is open to other domains to participate with first-class representation. I frequently use the term, DST (Domain Specific Type), to get across the idea. For instance, Gosu does not discriminate between a Gosu Class a Java Class or XML Type or what have you; they're all just types to Gosu's compiler. Check out the DynamicType example in the download to get a glimpse of the power and breadth the open type system provides. Essentially, the example demonstrates that C#'s "dynamic types where required" can simply be a new type loader domain in Gosu. Or check out the Ronin framework to see how easily the web and database domains can map seamlessly into Gosu.
It is important to understand that not all type loader domains in Gosu are required to produce bytecode. Those that do implement an interface for getting at the corresponding Java class. Those that don't provide call handlers and property accessors for reflective MethodInfo and PropertyInfo evaluation, respectively. Note all types provide TypeInfo, see IType.getTypeInfo(). For instance, the parser works against the TypeInfo, MethodInfo, etc. abstractions as the means for a level playing field between disparate types. At runtime, however, unless a type provides a Java bytecode class the MethodInfos and PropertyInfos also are responsible for handling calls.
No. If you look at the "notable differences" page (differences between Java and Gosu) you'll see a lot of things which are like C#, but also things which aren't in C# such as case-insensitivity and making semi-colons optional. There are also things which certainly aren't mentioned but which are part of C#:
Custom value types
Operator overloading
LINQ
Dynamic typing where required
I think it would be a mistake to regard Gosu as "C# for the JVM" rather than "a JVM language which mixes bits of Java, bits of C# and some bits from other languages too".
Qt has a build-in supprt for creating objects with integrated reference counting via QSharedData and QSharedDataPointer. All works great, but for each such object I need to write a lot of code: QSharedData-based implementation class with constructor and copy constructor, object class itsef with accessor methods for each filed.
For a simple structures with 5-10 fields this requires really lot of near same code. Is it some ways to automate such classes generation? Maybe it's some generators exists that take a short description and automatically generates implementation class and object class with all accessors?
You usually don't have to implement copy ctor or operator= when using QSharedData/Pointer. The default impls copy/assign the QSharedData-derived member, which usually does the Right Thing (TM).
For the public class, you need to implement the ctor creating the private object, and if the private class is not declared in the header but in the implementation (which is better), a dtor (doing nothing, the only point is that is not inlined and defined in the .cpp, after the private declaration).
For the private class, no method/ctor/dtor implementations are necessary.
For simple value-based classes, writing setters is of course tedious, but the same is true if you use plain private member variables. The overhead in LOC doesn't grow with the number of members.
And no, there is no standard generator solution for that I know of, although writing a script or emacs macro etc. doing it is not that hard. Probably would make sense to add such things to a publicly available toolbox, or QtCreator...
I don't think generators would exist for these things, but I suggest two things:
(ab)use existing shared containers (QVector, QList...)
read the documentation (with examples) on QSharedData, QSharedDataPointer, and QExplicitelySharedDataPointer
The two subclasses have simple examples that show how to implement the shared-ness it seems. I can't help you further though, because I've never had the need to create my own.
On second thought, why not make all data fields public, and use the QSharedData derivative as a struct-like class with reference counting? Maybe not nice on encapsulation, but if you're careful, nothing wrong should happen.
Recently I was talking to a co-worker about C++ and lamented that there was no way to take a string with the name of a class field and extract the field with that name; in other words, it lacks reflection. He gave me a baffled look and asked when anyone would ever need to do such a thing.
Off the top of my head I didn't have a good answer for him, other than "hey, I need to do it right now". So I sat down and came up with a list of some of the things I've actually done with reflection in various languages. Unfortunately, most of my examples come from my web programming in Python, and I was hoping that the people here would have more examples. Here's the list I came up with:
Given a config file with lines like
x = "Hello World!"
y = 5.0
dynamically set the fields of some config object equal to the values in that file. (This was what I wished I could do in C++, but actually couldn't do.)
When sorting a list of objects, sort based on an arbitrary attribute given that attribute's name from a config file or web request.
When writing software that uses a network protocol, reflection lets you call methods based on string values from that protocol. For example, I wrote an IRC bot that would translate
!some_command arg1 arg2
into a method call actions.some_command(arg1, arg2) and print whatever that function returned back to the IRC channel.
When using Python's __getattr__ function (which is sort of like method_missing in Ruby/Smalltalk) I was working with a class with a whole lot of statistics, such as late_total. For every statistic, I wanted to be able to add _percent to get that statistic as a percentage of the total things I was counting (for example, stats.late_total_percent). Reflection made this very easy.
So can anyone here give any examples from their own programming experiences of times when reflection has been helpful? The next time a co-worker asks me why I'd "ever want to do something like that" I'd like to be more prepared.
I can list following usage for reflection:
Late binding
Security (introspect code for security reasons)
Code analysis
Dynamic typing (duck typing is not possible without reflection)
Metaprogramming
Some real-world usages of reflection from my personal experience:
Developed plugin system based on reflection
Used aspect-oriented programming model
Performed static code analysis
Used various Dependency Injection frameworks
...
Reflection is good thing :)
I've used reflection to get current method information for exceptions, logging, etc.
string src = MethodInfo.GetCurrentMethod().ToString();
string msg = "Big Mistake";
Exception newEx = new Exception(msg, ex);
newEx.Source = src;
instead of
string src = "MyMethod";
string msg = "Big MistakeA";
Exception newEx = new Exception(msg, ex);
newEx.Source = src;
It's just easier for copy/paste inheritance and code generation.
I'm in a situation now where I have a stream of XML coming in over the wire and I need to instantiate an Entity object that will populate itself from elements in the stream. It's easier to use reflection to figure out which Entity object can handle which XML element than to write a gigantic, maintenance-nightmare conditional statement. There's clearly a dependency between the XML schema and how I structure and name my objects, but I control both so it's not a big problem.
There are lot's of times you want to dynamically instantiate and work with objects where the type isn't known until runtime. For example with OR-mappers or in a plugin architecture. Mocking frameworks use it, if you want to write a logging-library and dynamically want to examine type and properties of exceptions.
If I think a bit longer I can probably come up with more examples.
I find reflection very useful if the input data (like xml) has a complex structure which is easily mapped to object-instances or i need some kind of "is a" relationship between the instances.
As reflection is relatively easy in java, I sometimes use it for simple data (key-value maps) where I have a small fixed set of keys. One one hand it's simple to determine if a key is valid (if the class has a setter setKey(String data)), on the other hand i can change the type of the (textual) input data and hide the transformation (e.g simple cast to int in getKey()), so the rest of the application can rely on correctly typed data.
If the type of some key-value-pair changes for one object (e.g. form int to float), i only have to change it in the data-object and its users but don't have to keep in mind to check the parser too. This might not be a sensible approach, if performance is an issue...
Writing dispatchers. Twisted uses python's reflective capabilities to dispatch XML-RPC and SOAP calls. RMI uses Java's reflection api for dispatch.
Command line parsing. Building up a config object based on the command line parameters that are passed in.
When writing unit tests, it can be helpful to use reflection, though mostly I've used this to bypass access modifiers (Java).
I've used reflection in C# when there was some internal or private method in the framework or a third party library that I wanted to access.
(Disclaimer: It's not necessarily a best-practice because private and internal methods may be changed in later versions. But it worked for what I needed.)
Well, in statically-typed languages, you'd want to use reflection any time you need to do something "dynamic". It comes in handy for tooling purposes (scanning the members of an object). In Java it's used in JMX and dynamic proxies quite a bit. And there are tons of one-off cases where it's really the only way to go (pretty much anytime you need to do something the compiler won't let you do).
I generally use reflection for debugging. Reflection can more easily and more accurately display the objects within the system than an assortment of print statements. In many languages that have first-class functions, you can even invoke the functions of the object without writing special code.
There is, however, a way to do what you want(ed). Use a hashtable. Store the fields keyed against the field name.
If you really wanted to, you could then create standard Get/Set functions, or create macros that do it on the fly. #define GetX() Get("X") sort of thing.
You could even implement your own imperfect reflection that way.
For the advanced user, if you can compile the code, it may be possible to enable debug output generation and use that to perform reflection.