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Asked: 2026-06-01T05:10:14+00:00

Consider the following source file, which is (at least should be) valid C. void

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Consider the following source file, which is (at least should be) valid C.

void id() {
}

I am trying to compile it with gcc -c test.m, but I get the following error:

test.m:1: error: ‘id’ redeclared as different kind of symbol
<built-in>:0: error: previous declaration of ‘id’ was here

If Obj-C were a strict superset, wouldn’t it mean that all valid C programs also valid Obj-C programs? Note that I am not #importing anything, nor am I linking.

Granted, maybe LLVM(1) is doing something magical by default, or maybe this is a bug in it.

(1): i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00)

EDIT: Let me clarify this question – the question is not about the id part, I know it has special meaning. My point is that it is commonly said that Obj-C is a strict superset of C. However, this can’t be the case if it has keywords that conflict with valid C programs.

So either id is not reserved by the compiler, or Obj-C is not a strict superset. The point of this question is to ask which is the case, or is there something obvious I am missing.

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1 Answer

  1. Editorial Team

    Superset means it adds some extra features too. Like id in this case. It is a defined type in Objective C. And these extra features of the superset always supersede the subset.

    Its just customary that people use the term strict superset. You need to take it with a pinch of salt!

    See these old posts in SO for a similar discussion

    Is objective C 2.0 a proper superset of C?

    Which dialect of C is Objective-C a "strict superset" of?

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