* C++ Macros, ##, and ::
@ 2008-03-27 21:00 Harvey Chapman
2008-03-27 22:39 ` Daniel Lohmann
2008-03-31 23:38 ` Philipp Thomas
0 siblings, 2 replies; 4+ messages in thread
From: Harvey Chapman @ 2008-03-27 21:00 UTC (permalink / raw)
To: MSX to GCC
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Why does ...
#define MT(name) { name::Type, name::Create }
work, and ...
#define MT(name) { name##::Type, name##::Create }
does not?
Is that first one portable? I understand the error I get for the
second one about name and :: not constituting a valid pre-processor
token, but the first one seems wrong to me. Is :: used in a macro a
special case? Or is it an operator? Just looking for some insight to
help me remember this.
Here is the original code to give you an idea of intent.
#define MT(name) { name::Type, name::Create }
struct types_t
{
uint32_t type;
BaseModel::CreateFn fn;
} types[] =
{
MT(Model800),
MT(Model900)
};
uint32_t typesLen = sizeof(types)/sizeof(types_t);
#undef MT
Thanks,
Harvey
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^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: C++ Macros, ##, and ::
2008-03-27 21:00 C++ Macros, ##, and :: Harvey Chapman
@ 2008-03-27 22:39 ` Daniel Lohmann
2008-03-30 19:28 ` Nathan Sidwell
2008-03-31 23:38 ` Philipp Thomas
1 sibling, 1 reply; 4+ messages in thread
From: Daniel Lohmann @ 2008-03-27 22:39 UTC (permalink / raw)
To: Harvey Chapman; +Cc: MSX to GCC
On 27.03.2008, at 21:59, Harvey Chapman wrote:
> Why does ...
>
> #define MT(name) { name::Type, name::Create }
>
> work, and ...
>
> #define MT(name) { name##::Type, name##::Create }
>
> does not?
>
> Is that first one portable? I understand the error I get for the
> second one about name and :: not constituting a valid pre-processor
> token, but the first one seems wrong to me. Is :: used in a macro a
> special case? Or is it an operator? Just looking for some insight to
> help me remember this.
AFAIK :: is a delimiter to the preprocessor (similar to ,). Hence,
the first one should be portable; the latter one does not work as the
result of the concatenation has to be a token.
To clarify this in the code , you could even write:
> #define MT(name) { name :: Type, name :: Create }
(Well, just a guess -- I am *never* sure when it comes to the
preprocessor....)
Daniel
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: C++ Macros, ##, and ::
2008-03-27 22:39 ` Daniel Lohmann
@ 2008-03-30 19:28 ` Nathan Sidwell
0 siblings, 0 replies; 4+ messages in thread
From: Nathan Sidwell @ 2008-03-30 19:28 UTC (permalink / raw)
To: Daniel Lohmann; +Cc: Harvey Chapman, MSX to GCC
Daniel Lohmann wrote:
>
> On 27.03.2008, at 21:59, Harvey Chapman wrote:
>> Why does ...
>>
>> #define MT(name) { name::Type, name::Create }
>>
>> work, and ...
>>
>> #define MT(name) { name##::Type, name##::Create }
>>
>> does not?
> AFAIK :: is a delimiter to the preprocessor (similar to ,). Hence, the
> first one should be portable; the latter one does not work as the
> result of the concatenation has to be a token.
>
> To clarify this in the code , you could even write:
>
>> #define MT(name) { name :: Type, name :: Create }
you are correct on all points. '::' is a token in its own right.
nathan.
--
Nathan Sidwell :: http://www.codesourcery.com :: CodeSourcery
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: C++ Macros, ##, and ::
2008-03-27 21:00 C++ Macros, ##, and :: Harvey Chapman
2008-03-27 22:39 ` Daniel Lohmann
@ 2008-03-31 23:38 ` Philipp Thomas
1 sibling, 0 replies; 4+ messages in thread
From: Philipp Thomas @ 2008-03-31 23:38 UTC (permalink / raw)
To: gcc-help
On Thu, 27 Mar 2008 16:59:51 -0400, Harvey Chapman wrote:
>#define MT(name) { name::Type, name::Create }
>
>work, and ...
>
>#define MT(name) { name##::Type, name##::Create }
>
>does not?
Because name, '::' and Type are each distinct tokens, i.e. you don't
need to concat them. The standard says that the result of token pasting
again needs to be a valid preprocessing token and name::Type isn't.
Philipp
--
This is the Gentoo generation: Can't program but wants to compile
everything from source.
^ permalink raw reply [flat|nested] 4+ messages in thread
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