Objective-C Preprocessor Definition, Dynamic C-String to NSString Declaration

Question

I'm trying to create a macro definition that can emit C++ or Objective-C depending on context but can't seem to construct an NSString inside a macro easily. The C++ version is simple because it uses regular strings, but making one that emits NSString is proving tricky:

#define FOO(x) bar(@##x)

The intended result is to convert a string argument to an NSString argument by prefixing with @:

FOO("x")
// => bar(@"x")

What I get instead is an error that prevents compilation:

Pasting formed '@"x"', an invalid preprocessing token

Oleg Trakhman · Accepted Answer · 2012-06-06 20:01:26Z

5

NSString *x = @"text";

Equals:

NSString *x = CFSTR("text");

PS NSString* and CFStringRef and __CFString* and also NSCFStringRef are all the same: Toll-Free Bridged Types

edited Jun 6, 2012 at 20:01

answered Jun 6, 2012 at 19:48

Oleg Trakhman

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5 Comments

tadman Over a year ago

CFSTR is promising but it's a CFStringRef and not an NSString*.

Richard J. Ross III Over a year ago

@tadman they are one and the same, and are toll-free-bridged.

tadman Over a year ago

I'm getting "Incompatible pointer types passing 'CFStringRef' (aka const struct __CFString *) to parameter of type NSString*" when trying this.

Sergey Kalinichenko Over a year ago

@tadman You can cast CFSTR to NSString*, it's toll-free bridged.

tadman Over a year ago

The definition that worked: #define AS_NSSTRING(x) ((NSString*)CFSTR(x))

Sergey Kalinichenko · Accepted Answer · 2012-06-06 20:07:32Z

3

You cannot use ## to concatenate elements unless they form a valid preprocessing token together, but you can call NSString's constructor that takes a C string, like this:

#define FOO(x) [NSString stringWithUTF8String:(x)]

edited Jun 6, 2012 at 20:07

answered Jun 6, 2012 at 19:56

Sergey Kalinichenko

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3 Comments

tadman Over a year ago

Is that going to be something that has to be de-allocated later, unlike @"..." is static, right?

Sergey Kalinichenko Over a year ago

@tadman This produces an autoreleased string, you do not own it unless you retain it.

Richard J. Ross III Over a year ago

Should be stringWithUTF8String:, instead.

Ken Thomases · Accepted Answer · 2012-06-06 20:32:00Z

3

Um, why not this:

#define FOO(x) bar(@x)

?

There's no need to do token pasting or stringifying or anything weird. You just want what's in the argument list at the substitution point to be preceded by an @ sign. So just do that.

answered Jun 6, 2012 at 20:32

Ken Thomases

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3 Comments

Olie Over a year ago

only works if x is in quotes. Sometimes that is not desirable. For example: #define CASE_TO_NAME(x) case x: return (NSString*) CFSTR(x); break then a switch statement switch (val) { CASE_TO_NAME(foo); CASE_TO_NAME(bar); // etc. }

Ken Thomases Over a year ago

Sure, but that's not what the question was about. And your example will break because it doesn't stringify x in CFSTR(x).

Olie Over a year ago

My example is fixed when you change to CFSTR(#x), which should be obvious. (Why can't I edit my comment?!)

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