enum and macro collision

[David Brown]

On 17/01/17 12:12, vijay nag wrote:
> On Tue, Jan 17, 2017 at 4:36 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>> On 17 January 2017 at 11:03, vijay nag wrote:
>>> Hello Gcc,
>>>
>>> I came across this nasty C code of an enumerator and a macro having
>>> the same name. Although this is a buggy code, I found it surprising
>>> that GCC compiles in one of the cases(snippet 1) while in the other
>>> compiler flags an error(snippet 2). Would be interested to know  why
>>> the order of definitions/declarations matter and what C standards the
>>> compiler is adhering to ?
>>
>> Just look at the preprocessed code, it should be obvious.
>>
>> Macros aren't defined until they're defined.
>>
>> When the macro is defined after the enumerator it can't change it and
>> you get a valid enumerator definition, FOO = 1.
>>
>> When the macro is defined earlier it produces an invalid enumerator,
>> 0xDEADBEEF = 1
>>
>> What isn't clear?
> 
> Thanks for the reply, it is clear now. So, can enum have same name as
> that of a macro and Is it allowed in C ?
> 

You can have macros with names that clash with all sorts of things in C,
including identifiers and keywords.  Usually it's a bad idea, but it's
up to you.  Macros are just textual substitution - after the #define FOO
line, any time the token FOO is found the compiler (pre-processor)
pastes in the definition of FOO.  And previous uses of the word FOO are
lost (unless you get them via other macros).