What Is Objective C++

What is Objective C++?

Objective-C++ is simply source code that mixes Objective-C classes and C++ classes (two entirely unrelated entities). Your C++ code will work, just as before, and the resulting executable will be linked with the Objective-C runtime, so your Objective-C classes will work as well. You can definitely use it in Xcode -- name your files with the .mm extension.

Also, you might want to read Apple's (sadly deleted, but archived) documentation on Objective-C++.

What does @() mean in Objective-C?

It's a pointer to an NSNumber object. It's called a boxed literal, because the mental picture is of putting a primitive value of expression inside into a "box", that is, an object.

See official documentation if in doubt. Note that pointer can be to a "real" NSNumber object or it can (theoretically, don't know whether this will work in practice) be a tagged pointer (see, e.g., my question).

Note that you can also do things like @"string" and @5, which will create constants in compile time. But you need parentheses to use something which is not a literal, e.g. @(2 + 3). Parentheses form can be used for any expression, even those that compiler cannot compute at compile-time (although if it can, it will just put an expression result into code).

what is the use of @[ ] in objective c

NSArray *objectsToShare = @[objects];

is the same as

NSArray *objectsToShare = [NSArray arrayWithObjects:objects count:count];

What does Objective-C is a superset of C more strictly than C++ mean exactly?

I prepared a simple diagram; it is not very pretty, but hopefully gets the point across:

• Red: the set of all programs valid in C, C++, and Objective-C (relatively small)
• Green: the set of all programs valid in C and Objective-C, but invalid in C++ (even smaller)
• Gray: the set of all programs valid in Objective C and C++, but invalid in C (empty, as far as I know)
• Blue: the set of all programs valid only in Objective C (relatively large)
• Yellow: the set of all programs valid only in C++ (largest)

The set of valid C programs (in red and green) is an strict subset of the set of valid Objective C programs (blue)

What is a receiver in Objective C?

In Objective-C, a message is sent to a receiver.

The message is the method you are calling. The receiver is the what the message is called on.

Example. Let's say you have an NSString:

NSString *str = @"Hello";

Now you want to get the length of the string. You send the length message to str. str is the receiver of the message:

NSInteger len = [str length];

Basically, the receiver is the part on the left inside the square brackets and the message is the part on the right in the square brackets.

What do the plus and minus signs mean in Objective-C next to a method?

+ is for a class method and - is for an instance method.

E.g.

// Not actually Apple's code.
@interface NSArray : NSObject {
}
+ (NSArray *)array;
- (id)objectAtIndex:(NSUInteger)index;
@end

// somewhere else:

id myArray = [NSArray array];         // see how the message is sent to NSArray?
id obj = [myArray objectAtIndex:4];   // here the message is sent to myArray

// Btw, in production code one uses "NSArray *myArray" instead of only "id".

There's another question dealing with the difference between class and instance methods.

What’s the difference between Xcode, Objective-C and Cocoa?

Objective-C is a programming language. It could be said that it’s just a description of what valid Objective-C programs look like and what they mean. If you have a source code listing written in Objective-C, you need an interpreter or a compiler to put the listing to work. Languages like Objective-C are usually compiled, so most people use a compiler (like LLVM). Objective-C is almost exclusively used to develop for iOS and OS X, but there are other uses, too – as an example, some people write Objective-C for Linux.

You can use a text editor to write the sources and a compiler to turn them into an actual programs, but with modern technologies there’s much more to take care of, so that there is another program to make your job easier. These are called Integrated Development Environments, or IDEs. An IDE offers you a convenient way to edit the sources, compile them, debug the resulting programs, read the documentation, and many other things. Xcode is one such IDE. An important observation here is that Xcode does not compile your sources itself, it just calls the standalone compiler (LLVM). And Xcode is not the only IDE you can use to develop Objective-C apps – there’s AppCode, for example.

Writing iOS or OS X apps from scratch each time would be very time-consuming. That’s why Apple provides the developers with a good set of libraries. The libraries are simply a huge amount of source code written by Apple, and this source code takes care of most things that apps have in common. These libraries are called Cocoa.

Now, if you can’t figure out how to extend a class, you are most probably talking about Objective-C. It doesn’t have anything to do with Xcode or Cocoa, you could be very well writing some GNUstep code for Linux using Vim as an IDE and GCC as a compiler. On the other hand, if your Xcode build process fails because of some mysterious setting, or if you’re trying to build a static library in Xcode, that’s clearly an Xcode issue. And if you can’t figure out how to use some NSObject facility or the NSFileManager class, that’s Cocoa. (But it doesn’t have to be Xcode-related, as you could use AppCode or TextMate as your IDE!)

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Originally available on my blog. Feel free to link to the blog post or this question when retagging or explaining the difference.

Meaning of symbol ^ in Objective C

That symbol is used to declare block.

For more information read here Blocks Programming Topics

Some more info:

Block objects are a C-level syntactic and runtime feature. They are
similar to standard C functions, but in addition to executable code
they may also contain variable bindings to automatic (stack) or
managed (heap) memory. A block can therefore maintain a set of state
(data) that it can use to impact behavior when executed.

You can use blocks to compose function expressions that can be passed
to API, optionally stored, and used by multiple threads. Blocks are
particular useful as a callback because the block carries both the
code to be executed on callback and the data needed during that
execution.

How different is Objective-C from C++?

Short list of some of the major differences:

• C++ allows multiple inheritance, Objective-C doesn't.
• Unlike C++, Objective-C allows method parameters to be named and the method signature includes only the names and types of the parameters and return type (see bbum's and Chuck's comments below). In comparison, a C++ member function signature contains the function name as well as just the types of the parameters/return (without their names).
• C++ uses bool, true and false, Objective-C uses BOOL, YES and NO.
• C++ uses void* and nullptr, Objective-C prefers id and nil.
• Objective-C uses "selectors" (which have type SEL) as an approximate equivalent to function pointers.
• Objective-C uses a messaging paradigm (a la Smalltalk) where you can send "messages" to objects through methods/selectors.
• Objective-C will happily let you send a message to nil, unlike C++ which will crash if you try to call a member function of nullptr
• Objective-C allows for dynamic dispatch, allowing the class responding to a message to be determined at runtime, unlike C++ where the object a method is invoked upon must be known at compile time (see wilhelmtell's comment below). This is related to the previous point.
• Objective-C allows autogeneration of accessors for member variables using "properties".
• Objective-C allows assigning to self, and allows class initialisers (similar to constructors) to return a completely different class if desired. Contrast to C++, where if you create a new instance of a class (either implicitly on the stack, or explicitly through new) it is guaranteed to be of the type you originally specified.
• Similarly, in Objective-C other classes may also dynamically alter a target class at runtime to intercept method calls.
• Objective-C lacks the namespace feature of C++.
• Objective-C lacks an equivalent to C++ references.
• Objective-C lacks templates, preferring (for example) to instead allow weak typing in containers.
• Objective-C doesn't allow implicit method overloading, but C++ does. That is, in C++ int foo (void) and int foo (int) define an implicit overload of the method foo, but to achieve the same in Objective-C requires the explicit overloads - (int) foo and - (int) foo:(int) intParam. This is due to Objective-C's named parameters being functionally equivalent to C++'s name mangling.
• Objective-C will happily allow a method and a variable to share the same name, unlike C++ which will typically have fits. I imagine this is something to do with Objective-C using selectors instead of function pointers, and thus method names not actually having a "value".
• Objective-C doesn't allow objects to be created on the stack - all objects must be allocated from the heap (either explicitly with an alloc message, or implicitly in an appropriate factory method).
• Like C++, Objective-C has both structs and classes. However, where in C++ they are treated as almost exactly the same, in Objective-C they are treated wildly differently - you can create structs on the stack, for instance.

In my opinion, probably the biggest difference is the syntax. You can achieve essentially the same things in either language, but in my opinion the C++ syntax is simpler while some of Objective-C's features make certain tasks (such as GUI design) easier thanks to dynamic dispatch.

Probably plenty of other things too that I've missed, I'll update with any other things I think of. Other than that, can highly recommend the guide LiraNuna pointed you to. Incidentally, another site of interest might be this.

I should also point out that I'm just starting learning Objective-C myself, and as such a lot of the above may not quite be correct or complete - I apologise if that's the case, and welcome suggestions for improvement.

EDIT: updated to address the points raised in the following comments, added a few more items to the list.

What does the @ symbol represent in objective-c?

The @ character isn't used in C or C++ identifiers, so it's used to introduce Objective-C language keywords in a way that won't conflict with the other languages' keywords. This enables the "Objective" part of the language to freely intermix with the C or C++ part.

Thus with very few exceptions, any time you see @ in some Objective-C code, you're looking at Objective-C constructs rather than C or C++ constructs.

The major exceptions are id, Class, nil, and Nil, which are generally treated as language keywords even though they may also have a typedef or #define behind them. For example, the compiler actually does treat id specially in terms of the pointer type conversion rules it applies to declarations, as well as to the decision of whether to generate GC write barriers.

Other exceptions are in, out, inout, oneway, byref, and bycopy; these are used as storage class annotations on method parameter and return types to make Distributed Objects more efficient. (They become part of the method signature available from the runtime, which DO can look at to determine how to best serialize a transaction.) There are also the attributes within @property declarations, copy, retain, assign, readonly, readwrite, nonatomic, getter, and setter; those are only valid within the attribute section of a @property declaration.

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