Concatenate strings in a macro using gfortran

Question

The C preprocessor macro for concatenation (##) does not seem to work on a Mac using gfortran. Using other Fortran compilers on other systems works so I am looking for a workaround for gfortran. I have to use the ## to create many variables so I can't do without them.

Example code:

#define CONCAT(x,y) x##y
program main
   integer, parameter:: CONCAT(ID,2) = 3
   print*,"Hello", ID_2
end program main

Compilation error with gfortran on MAC

gfortran m.F90 -o m
m.F90:5.23:
integer, parameter:: ID##2 = 3
                       1
Error: PARAMETER at (1) is missing an initializer

Answer 1

## doesn't work in gfortran (any OS, not just Mac) because it runs CPP in the traditional mode.

According to this thread the gfortran mailing list the correct operator in the traditional mode is x/**/y, so you must distinguish between different compilers:

#ifdef __GFORTRAN__
#define CONCAT(x,y) x/**/y
#else
#define CONCAT(x,y) x ## y
#endif

Others (http://c-faq.com/cpp/oldpaste.html) use this form, which behaves better when a macro passed to the CONCAT (via Concatenating an expanded macro and a word using the Fortran preprocessor):

#ifdef __GFORTRAN__
#define PASTE(a) a
#define CONCAT(a,b) PASTE(a)b
#else
#define PASTE(a) a ## b
#define CONCAT(a,b) PASTE(a,b)
#endif

The indirect formulation helps to expand the passed macro before the strings are concatenated (it is too late after).