use c Token-Pasting to access field in struct

Question

I'm trying to use c token pasting (##) to access to struct field as below:

typedef struct
{
   int a;
   int b;
} TMP_T;

#define BUILD_FIELD(field) my_struct.##field

int main()
{
     TMP_T my_struct;
     BUILD_FIELD(a) = 5;
     return 0;
}

But got the following error during compilation: error: pasting "." and "a" does not give a valid preprocessing token

I would like to add additional case to that:

 typedef struct {
 int a;
 int b;
 }TMP_T;

 #define BUILD_FIELD(my_struct,field) my_struct.##field


  void func(char* name)
  {
TMP_T tmp_str;
if((name == "a")  || (name == "b"))
{
    BUILD_FIELD(tmp_str, name) = 7;
    printf("%d \n", BUILD_FIELD(a) );
}


 }

 int main()
 {

       func("a");
       return 1;
 }

How should I use the macro to access the specific struct and field. Is it possible? or because its a pre compiled then it couldn't be defined for various fields (a,b)

Thanks Moti

Answer 1

You don't need token pasting at all:

#define BUILD_FIELD(field) my_struct.field

According to the gcc manual token pasting should result in an identifier or a number after the concatenation. The error is due to the .a not being either.