Using the ## preprocessing operator fails

Using the ## preprocessing operator fails

[Ike Starnes]

The following code builds fine in Microsoft C, but fails with gcc / g++:


#include <iostream>

using namespace std;

typedef struct _THREADDATA
{
   int nConvTiles;        // the number of swap buffers in a tiled bitmap
used for caching the disk tiles (default 1)
}
THREADDATA, *pTHREADDATA;

#define DECLARETHREADDATA() pTHREADDATA pThreadData = NULL;
#define GETTHREADDATA() pThreadData
#define THREADVALUE(v) GETTHREADDATA()->##v

static int GetTilesInfo()
{
   DECLARETHREADDATA();
   int nConvTiles;
   nConvTiles = THREADVALUE(nConvTiles);
   return nConvTiles;
}

int main()
{
    cout << "Hello world!" << endl;
    cout << GetTilesInfo();
    return 0;
}


With gcc, the error is:
d:\temp\GNU\HelloWorld\main.cpp:19:1: error: pasting "->" and "nConvTiles"
does not give a valid preprocessing token

Does anyone know the correct fix?
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Re: Using the ## preprocessing operator fails

[Jonathan Wakely]

On 26 August 2011 15:54, Ike Starnes wrote:
>
> The following code builds fine in Microsoft C, but fails with gcc / g++:
>
>
> #include <iostream>
>
> using namespace std;
>
> typedef struct _THREADDATA
> {
>   int nConvTiles;        // the number of swap buffers in a tiled bitmap
> used for caching the disk tiles (default 1)
> }
> THREADDATA, *pTHREADDATA;
>
> #define DECLARETHREADDATA() pTHREADDATA pThreadData = NULL;
> #define GETTHREADDATA() pThreadData
> #define THREADVALUE(v) GETTHREADDATA()->##v
>
> static int GetTilesInfo()
> {
>   DECLARETHREADDATA();
>   int nConvTiles;
>   nConvTiles = THREADVALUE(nConvTiles);
>   return nConvTiles;
> }
>
> int main()
> {
>    cout << "Hello world!" << endl;
>    cout << GetTilesInfo();
>    return 0;
> }
>
>
> With gcc, the error is:
> d:\temp\GNU\HelloWorld\main.cpp:19:1: error: pasting "->" and "nConvTiles"
> does not give a valid preprocessing token
>
> Does anyone know the correct fix?

Just remove the ## from your macro.

## is for pasting two tokens together into one, but in your macro
there's only one token.  Just get rid of it and it will work:

#define THREADVALUE(v) GETTHREADDATA()->v

Re: Using the ## preprocessing operator fails

[Ike Starnes]

Thanks!


Jonathan Wakely-4 wrote:
> 
> On 26 August 2011 15:54, Ike Starnes wrote:
>>
>> The following code builds fine in Microsoft C, but fails with gcc / g++:
>>
>>
>> #include <iostream>
>>
>> using namespace std;
>>
>> typedef struct _THREADDATA
>> {
>>   int nConvTiles;        // the number of swap buffers in a tiled bitmap
>> used for caching the disk tiles (default 1)
>> }
>> THREADDATA, *pTHREADDATA;
>>
>> #define DECLARETHREADDATA() pTHREADDATA pThreadData = NULL;
>> #define GETTHREADDATA() pThreadData
>> #define THREADVALUE(v) GETTHREADDATA()->##v
>>
>> static int GetTilesInfo()
>> {
>>   DECLARETHREADDATA();
>>   int nConvTiles;
>>   nConvTiles = THREADVALUE(nConvTiles);
>>   return nConvTiles;
>> }
>>
>> int main()
>> {
>>    cout << "Hello world!" << endl;
>>    cout << GetTilesInfo();
>>    return 0;
>> }
>>
>>
>> With gcc, the error is:
>> d:\temp\GNU\HelloWorld\main.cpp:19:1: error: pasting "->" and
>> "nConvTiles"
>> does not give a valid preprocessing token
>>
>> Does anyone know the correct fix?
> 
> Just remove the ## from your macro.
> 
> ## is for pasting two tokens together into one, but in your macro
> there's only one token.  Just get rid of it and it will work:
> 
> #define THREADVALUE(v) GETTHREADDATA()->v
> 
> 

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