I'm just curious, does Objective-C compile into C code or does the Objective-C runtime work like a layer of abstraction above the program? Sorry in advance if I don't know what I'm talking about!
A little history lesson:
Both C++ and Objective C originally started out as preprocessors for C. So you typed in your ObjC code, and it would effectively run a search-and-replace over the code and translate the Objective-C commands into straight C code that used a little helper library (the stuff in objc/runtime.h and similar files).
As the language started getting more complex, it was changed into a full parser that replaced/extended the parser in a C compiler with/into one specific to Objective-C. So while it would be perfectly possible to compile Objective-C into straight C, current ObjC compilers don't do it that way anymore.
Compiling Objective-C into C doesn't make sense, because then it would need to parse the C code and compile it.
Objective-C compiles into machine code. Remember that the language (Objective-C, C, C++) only defines the rules to correctly write code. The compiler checks to see if your code is correct and compiles it, i.e., translates it into executable code.
Also, don't confuse Objective-C language, and the Objective-C runtime. The language defines the syntax, the runtime allows the compiled code to run (in a way it's like you say, it is a layer, but it doesn't get compiled every time with your program).
EDIT:
The runtime implements the core behavior of a computer language. The runtime contains compiled code of functions in a similar way a library does. In C, for example, when you call printf() your code is compiled into machine code and linked with the library containing the implementation of that function; what this machine code does is passing parameters to the executable code in the library.
Speaking strictly from Xcode, the code is compiled with the LLVM compiler. Here is more information about the LLVM compiler. You'll be able to find more information about how the LLVM compiler works online through simple Google searches.
Related
i am currently developing a simple app on my own.
I want to use a library from some sample app. The library consists of .mm files (C++ code) and runs perfectly on the original project.
However, when i copy the library to my file (Objective C), it seems that my other object files do not see the file. All the classes and protocol defined in this library are not recognised at all.
Errors are such as "No type or protocol name", "Unknown typename "classname" " when i try to use the library classes and protocols.
I have spent quite a lot time searching, but to no veil.
Thanks in advance.
Objective-C simply won't understand the C++ language constructs like class, etc. If you want to use this Objective-C++ code in your app then your app also needs to become Objective-C++ which can be done simply by renaming all your source files from *.m to *.mm.
In the case of Swift, however, you cannot expose C++ to it at all, and can only integrate it with C or Objective-C, so you are therefore forced to create a pure Objective-C wrapper for the C++ code (i.e. implementation in .mm but exposing no C++ types in its header file).
Is Microsofts C++/CLI built on top of the C++ Standard (C++98 or C++11) or is it only "similar" and has deviations?
Or, specifically, is every ISO standard conforming C++ program (either C++98 or C++11), also a conforming C++/CLI program?
Note: I interpret the Wikipedia article above only comparing C++/CLI to MC++, not to ISO Standard C++.
Sure, it is an extension to C++03 and can compile any compliant C++03 program that doesn't conflict with the added keywords. The only thing it doesn't support are some of the Microsoft extensions to C++, the kind that are fundamentally incompatible with managed code execution like __fastcall and __try. MC++ was their first attempt at it, kept compatible by prefixing all added keywords with underscores. The syntax was rather forced and not well received by their customers, C++/CLI dropped the practice and has a much more intuitive syntax. Stanley Lippman of C++ Primer fame was heavily involved btw.
The compiler can be switched between managed and native code generation on-the-fly with #pragma managed, the product is a .NET mixed-mode assembly that contains both MSIL and native machine code. The MSIL produced from native C++ source is not exactly equivalent to the kind produced by, say, the C# or VB.NET compilers. It doesn't magically become verifiable and doesn't get the garbage collector love, you can corrupt the heap or blow the stack just as easily. And no optimizer love either, the MSIL gets translated to machine code at runtime and is optimized just like normal managed code with the time restrictions inherent in a jitter. Getting too much native C++ code translated to MSIL is a very common mistake, the compiler hides it too well.
C++/CLI is notable for introducing syntax that got later adopted into C++11. Like nullptr, override, final and enum class. Bit of a problem, actually, it begat __nullptr to be able to distinguish between a managed and a native null pointer. They never found a great solution for enum class, you have to declare it public to get a managed enum type. Some C++11 extensions work, few beyond the ones it already had, auto is fine but no lambda expressions, quite a loss in .NET programming. The language has been frozen since 2005.
The C++/CX language extension is notable as well, one that makes writing C++ code for Store and Phone apps palatable. The syntax resembles C++/CLI a great deal, including the ref class and hats in the syntax. But with objects allocated with ref new instead of gcnew, the latter would have been too misleading. Otherwise very different from C++/CLI at runtime, you get pure native code out of C++/CX. The language extension hides the COM interop code that's underneath, automatically reference-counting objects, translating error codes into exceptions and mapping generics. The resemblance to C++/CLI syntax is no accident, they basically perform the same role. Mapping C++-like syntax to a foreign type system.
CLI is a set of extensions for standard C++. CLI has full support of standard C++ and adds something more. So every C++ program will compile with enabled CLI, except you are using a CLI reserved word and this is the weakness of the extension, because it does not respect the double underscore rule for extensions (such reserved words has to begin with __).
You can deactivate those extensions in the GUI by:
Configuration Properties -> General -> Common Language Runtime Support
Even Bjarne Stroustrup calls CLI an extension:
On the difficult and controversial question of what the CLI binding/extensions to C++ is to be called, I prefer C++/CLI as a shorthand for "The CLI extensions to ISO C++". Keeping C++ as part of the name reminds people what is the base language and will help keep C++ a proper subset of C++ with the C++/CLI extension
Language extensions could always be called deviations from the standard, because it will not compile with a compiler without CLI support (e.g. the ^ pointer).
As I understand correctly, besides the fact that Objective-C language is a strict superset of a "clean" C, added OOP paradigm is simulated by a set of functions partially described in Objective-C Runtime Reference.
Therefore, I'm expecting a possibility to somehow compile Objective-C code in an intermediate C/C++ file (maybe with some asm inserts).
Is it generally possible ?
You could use the clang rewriter to convert to C++. Not aware of a way to go to C though.
The rewriter is available via the "-rewrite-objc" command line option.
As far as I know, there is no software that preprocesses Objective-C code into intermediate C code.
But you could write your Objective-C program entirely in C by calling directly into the Objective-C runtime. The trouble is just that the code might vary between implementations or even different versions of the same runtime.
The question is, is it actually worth the trouble?
I want to create an Objective C class at runtime from a file. So for example I have an objective c application, I want to then point it at a text file (or an .h .m pair, whatever works) and then have the application parse the text file and create the class at runtime (Class no object). Of course I would write the parser and all of that stuff I just want to know if its possible. I read these two articles:
http://www.mikeash.com/pyblog/friday-qa-2010-11-6-creating-classes-at-runtime-in-objective-c.html
http://www.mikeash.com/pyblog/friday-qa-2010-11-19-creating-classes-at-runtime-for-fun-and-profit.html
Which shows how to make an objective C class at runtime, but its being done using C functions which were defined at compile time. If I can find a way to do the same thing using strings to define the functions that would be perfect, because then I don't have to define them at compile time.
That's what called reflective programming. I guess there's no code evaluation support for Obj-C since it's not a scripting language, so the reflection concept for Obj-C is quietly limited. Plus, at run-time the compiler already translate the code into Obj-C clang code and it's a very time-consuming job just to reverse-translate the bytecode and recompile it again
For Obj-C reflection you can refer to these answers
Build a class :
Create objective-c class instance by name?
Implement a method :
Objective-C, how can i hook up a method in another class
Change class for an object :
Objective-C: How to change the class of an object at runtime?
Sure. Totally possible.
I would suggest starting with the Objective-C support in this as it includes a full-on Objective-C parser and code generator.
see my github project cocoa-interprreter it does part of what you want.
it takes a text file and compiles it at runtime .. and then runs the resulting executable using NSTask. It would be quite easy to change it so the binary is loaded into the own process using NSBundle
https://github.com/Daij-Djan/cocoa-interpreter
Or in more practical terms: If I compile a file containing just Obj-C code or just C++ code under Objective-C++, can I be sure they'll behave exactly as they did before? In what ways might they differ?
More or less, that is true; anything that compiles under either C++ or Objective-C will compile with the Objective-C++ compiler.
However, there are some edges where this doesn't hold true and a handful of behavioral differences.
I'm not aware of any specific at-runtime behavioral differences, though. The differences should generally show up during compilation and mostly as problems caused by C++'s "enhanced" notion of types.
Operator overloading can be exceedingly problematic, however. If a body of code makes extensive use of operator overloading, that can cause issue when mixing Objective-C in for the first time. But, again, typically as compiler warnings and rarely as at-runtime bugs.
Objective-C is a superset of C only. You can mix C++ with Objective-C - that is called Objective-C++ - however there are certain rules that you have to follow.
Here you can see all the rules that have to be followed: http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Articles/ocCPlusPlus.html
EDIT: This is a quote from the developer website: "Apple’s Objective-C compiler allows you to freely mix C++ and Objective-C code in the same source file. This Objective-C/C++ language hybrid is called Objective-C++. With it you can make use of existing C++ libraries from your Objective-C applications.".
So it clearly says, that the mixture of C++ and Objective-C is called Objective-C++, but that is not a superset of C++, but it's two languages that with some rules can be used together. (For instance you can't mix objective-c and c++ classes during inheritance, but there are others as well.)
I guess there's at least one example of valid Objective-C code that's not valid Objective-C++:
void SayHello()
{
char* this = "hello!";
printf("%s", this);
}
(what someone would be doing naming a variable 'this', I don't know)