Overloading the REAL intrinsic

I was reading about Fortran’s intrinsic function REAL recently and discovered that it behaved inconsistently, so I fixed it by overloading the function in a module that I generally use everywhere.

I use REAL to extract the real part of a complex number. The KIND of the return value is equal to that of it’s argument, but only if the argument is COMPLEX. If the argument itself is of type REAL, then it returns a value using the compiler’s default REAL KIND. For me, since I program in double precision, this can cause me to accidentally lose precision, if I’m not careful.

I see three ways to fix this:

  1. Always include the KIND when calling REAL, for example REAL(foo,8). I’m likely to forget, and it clutters my code without adding information to the reader.
  2. Change the compiler default KIND. In gfortran, I just need a command-line argument: gfortran -fdefault-real-8. However, this option can change from compiler to compiler.
  3. Overload the intrinsic function to return a REAL value with the same KIND as the argument.

So, I went with the last option. To get this to work, I put the function in my module prec_def, which is where I set the precision for the entire code. I used an interface for the overloading, so that I only overrode the intrinsic for the argument names and types that I specify.

Here’s the result:

module prec_def
use iso_fortran_env, only : REAL64 !to get the KIND parameter for 64-bit precision
implicit none

integer, parameter :: long=REAL64

interface real
module procedure real_real
end interface


real(Long) function real_real(num)
real(Long), intent(in) :: num

!call the intrinsic function, specifying KIND
real_real = real(num,Long)

end function real_real

end module prec_def

Let me know what you think!

2 thoughts on “Overloading the REAL intrinsic

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