Objective-C macro to detect if variable is primitive

Question

I'm looking a macro to detect if a variable is an object or a primitive in Objective-C.

In this context, I know the parameter must be a variable and will never be an expression.

Here is the best thing I've come up with:

#define IS_OBJECT(x)    ( @encode(__typeof__(x))[0] == '@' )
#define IS_PRIMITIVE(x) ( !IS_OBJECT(x) )

Usage:

NSString *testString = @"test";
NSString *nilString = nil;
NSInteger testInteger = 1;

STAssertTrue(IS_OBJECT(testString), @"IS_OBJECT(testString) must be YES");
STAssertTrue(IS_OBJECT(nilString), @"IS_OBJECT(nilString) must be YES");
STAssertFalse(IS_OBJECT(testInteger), @"IS_OBJECT(testInteger) must be NO");

There must be a better way.

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Update

Considering @ChrisDevereux comment, I updated the IS_OBJECT macro.

#define IS_OBJECT(x) ( strchr("@#", @encode(__typeof__(x))[0]) != NULL )

It now passes:

NSString *testString = @"test";
NSString *nilString = nil;
NSInteger testInteger = 1;
Class classTest = [NSString class];

STAssertTrue(IS_OBJECT(testString), @"IS_OBJECT(testString) must be YES");
STAssertTrue(IS_OBJECT(nilString), @"IS_OBJECT(nilString) must be YES");
STAssertFalse(IS_OBJECT(testInteger), @"IS_OBJECT(testInteger) must be NO");
STAssertTrue(IS_OBJECT(classTest), @"IS_OBJECT(classTest) must be YES");

I still don't like this answer, and hope there is something slicker. Is there something in the runtime library which does this?

Answer 1

Here's another way using C11's generic selection mechanism. _Generic is standard (modern) C and supported in clang for a while.

#define IS_OBJECT(T) _Generic( (T), id: YES, default: NO)

It feels a bit less runtime-ish to me so I prefer it over the @encode way. But to be honest I just used it for this answer because I love the power _Generic gives to macros and think more people should start using it. If you don't know it you should read Robert Gamble's article linked above.