Is it possible to create custom directives in Objective-C?

Question

Objective-C has directives like:

• @interface
• @implementation
• @end
• @protocol
• @property
• @synthesize

I think of these things like sophisticated marco or code-generators. Is it possible to create custom directives for code-generation purposes? One possible use is generating methods for CoreData.

I'm thinking not, because I've never seen anything about it, but my world isn't the world.

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Followup Question:

Jonathan mentioned below that it is possible to write your own preprocessor and this begs the question of how. Currently, #define SYMBOLIC_CONSTANT 102 will replace all instances of the characters SYMBOLIC_CONSTANT with the characters 102 in the file before the files moves on to the compiler.

I know it XCode you can add a "Run Script Phase" to a Targets build process. So I could write a script to find my custom preprocess directives like '$coredata' and then have the script generate a new file that with the characters $coredata replaced with some characters of code. But from what I understand of XCode's build process you can't feed altered files into the Compiler Sources phase. The files are specified and locked by the IDE.

Has anyone done something similar? I know it's possible with external build system, but to be honest I'm not at that level of understanding. I don't know the technical details of what the Build and Run button does.

In the meantime, I'll start reading Apple's XCode Documentation...

Thanks for the responses!

Answer 1

While accepted answer is right, there is a partial hacky solution to this kind of a problem, which libextobjc library adopts. Consider this code, you will find the definitions like the following there:

#define weakify(...) \
    try {} @finally {} \
    metamacro_foreach_cxt(ext_weakify_,, __weak, __VA_ARGS__)

Such definition allows using weakify keyword in the following form:

id foo = [[NSObject alloc] init];
id bar = [[NSObject alloc] init];

@weakify(foo, bar);

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The author of library explains it here:

Since the macros are intended to be used with an @ preceding them (like @strongify(self);), the try {} soaks up the symbol so it doesn't cause syntax errors.

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Updated later

From now on libextobjc uses @autoreleasepool to "soak up the symbol".

Answer 2

Your thinking is correct: it is impossible to do this in your code. The only way to add more @-directives is via the compiler itself. Even if you went to all that trouble, I can almost guarantee that the syntax highlighting support for them is hard-coded into an Xcode configuration file somewhere.

Oh, and if you were considering the use a pre-processor macro, it is my understanding that the @ character is illegal in pre-processor macros.

Edit: I ran a test, and I am correct. Using the @ character in a C preprocessor macro is illegal. They follow the same rule as variable names.