Is Objective C really compiled ? Isn't it more like Visual Basic / .NET runtime ? Then what prevents it to be portable to other platforms?

Question

Syntactically it's superset of C. But since messages are sent and processed at runtime this means it cannot be a pure compiled language like c but it needs a runtime like Visual Basic or .Net runtime.

Then what prevents it to be portable to other platforms by transforming this runtime to something like .NET Framework or Java JVM ?

Note: when I say VB it's of course last version 6 which compiles to Bytecode so why do you pretend that Java or .Net are different fundamentally from VB6 except for portability it's the same principle: see Similar to Java, Visual Basic is compiled into an intermediate language called "bytecode." The bytecode is translated into x86 machine language by the Visual Basic runtime module.

Read more: http://www.answers.com/topic/visual-basic#ixzz19iJd3wjA

Similar to Java, Visual Basic is compiled into an intermediate language called "bytecode." The bytecode is translated into x86 machine language by the Visual Basic runtime module.

Answer 1

You appear to be confused between the need for runtime support for language features, and the difference between a native-compiled and VM-compiled (or interpreted) language.

Most high-level languages need some sort of run-time support. Even C has a run-time library, although on many platforms you can choose not to use some or all of it. Modern operating systems provide even more runtime support, and many languages provide OS-specific extensions that integrate these - consider dynamic loading of libraries...

Furthermore, there's nothing stopping you from building a message-passing system on top of a language that doesn't support it intrinsically. Since this is extremely common (especially for programs written for event-driven platforms), integrating it into the language can be looked at as simply factoring some error-prone busywork out of the high-level code and into the language and runtime.

Finally, both Java and most .NET languages are actually compiled - they just compile down to a bytecode that no machine implements natively, requiring the use of a Virtual Machine to actually execute them. The most performant VMs compile the code again prior to executing it - this can be such an effective technique that it has been used to build fast and efficient compiler tool-chains!

Visual Basic is a red herring here - some versions are interpreted, others are compiled, and VB.NET - like other .NET languages - is compiled to bytecode and then JIT-compiled again during execution (while superficially similar, this tends to be implemented very differently from the earlier VB VMs). If nothing else, this should tell you that a language and the means by which programs written in it are executed are not as tightly coupled as is commonly-believed...

In answer to your last question: Objective-C has never not been portable, at least in the sense that C is portable (that is to say, source-code portability). Apple uses the GCC compiler, which has been ported to a dizzying array of platforms... However, once you start taking advantage of platform-specific APIs (probably one of the best reasons for using Obj-C on Apple platforms to begin with...) you're limited to platforms that implement those APIs. Binary portability is possible in theory, but I know of no implementations.

Answer 2

It does not follow at all that message passing cannot be compiled. It merely implies that the message must be resolved to a method call at runtime. Objective-C is certainly a compiled language.

The process is described here. It is true that the initial call to a method is slower than a C++ method call, but the resolution is cached so subsequent calls are faster. Such non deterministic behaviour would make Objective-C unsuited to some applications.

Objective-C is provided as a front-end for GCC, so is highly portable. However only OSX and its predecessor NextSTEP, and iOS have an OS API for which Objective-C is the preferred language (i.e. the language the OS API is written in/for). So while it can be used on any OS that supports GCC, its usefulness on such platforms is limited.