Skip to content

implicit none

Last edited: 2023-12-19

implicit none (external | type) (Fortran 2018)

implicit none (type)

  • Is the same as the existing IMPLICIT NONE .

  • If an IMPLICIT NONE or IMPLICIT NONE (TYPE) appears, there must be no other IMPLICIT statements in the scoping unit.

  • No more than one IMPLICIT NONE statement shall appear in a scoping unit.

  • IMPLICIT INTEGER (I-N), REAL (A-H, O-Z) : represents the default typing as specified by the Fortran Standard for names when they are not explicitly typed.

implicit none (external)

  • Means that any references to an external procedure must be to a name that is explicitly declared to have the EXTERNAL attribute. In other words, no implicit interfaces.
  • If an IMPLICIT NONE (EXTERNAL) statement appears in a scoping unit, all dummy procedures and external procedures in that scope or a contained scope of BLOCK must have an accessible explicit interface or be declared EXTERNAL.
  • As long as they are explicitly declared as procedures, procedures can have an implicit interface.
  • External procedure or module must be explicity declared.
  • Helps avoid hard-to-detect errors.
  • Requires that the names of external and dummy procedures are explicitly declared as having the external attribute.
  • The declaration can be made by:
    • external statement
    • external attribute in a type declaration statement
    • procedure declaration statement
    • interface block

gfortran -fimplicit-none -Werror=implicit-interface

gfortran has the equivalent flags -fimplicit-none and -Werror=implicit-interface which allows not to use implicit none (external | type) directly in each code written, allowing for simpler and cleaner code. Perhaps the only restriction on the use of flags is when there is a need to use or merge some legacy code.

%%writefile a.f90

subroutine a(x)
    integer :: x
    x = 345
end

program main
    ! implicit none (type, external)  ! a() must be explicitly declared

    ! external a     ! or:
    ! procedure() a  ! or:
    ! procedure() :: a

    integer :: x

    call a(x)
    print*, x
end

Overwriting a.f90

! gfortran a.f90 && ./a.out

         345

! gfortran -fimplicit-none -Werror=implicit-interface -fno-diagnostics-show-option a.f90 && ./a.out

a.f90:16:13:

   16 |     call a(x)
      |             1
Error: Procedure ‘a’ called with an implicit interface at (1)
f951: some warnings being treated as errors

%%writefile a.f90

subroutine F(x)
    real :: x
    x = 123
end

subroutine G(x)
    real :: x
    x = 456
end

real function H(x)
    real :: x
    x = 789
end

program foo
    implicit none (external)
    implicit real(i)  ! this is okay
    external :: G  ! or: 'procedure() G'
    real :: x
    i = 0.0
    call F(i)  ! invalid: F lacks the EXTERNAL attribute.
    call G(i)  ! valid: G has the EXTERNAL attribute.
    x = H(i)   ! function is not affected
end

Overwriting a.f90

! gfortran -Wall -Wextra -std=f2018 a.f90 && ./a.out

a.f90:23:57:

   23 |     call F(i)  ! invalid: F lacks the EXTERNAL attribute.
      |                                                         1
Error: Procedure ‘f’ called at (1) is not explicitly declared

%%writefile a.f90
program main
    implicit none (type)  ! equal to 'implicit none'
    print*, "Hello, world!"
end

Overwriting a.f90

!gfortran -Wall -Wextra -std=f2018 a.f90 && ./a.out

 Hello, world!

implicit none as compiler flag (not 2018 specific)

%%writefile a.f90
program main
    print*, a()
contains
    integer function a()
        a = 123
    end
end

Overwriting a.f90

!gfortran -fimplicit-none -Wall -Wextra -std=f2018 a.f90 && ./a.out

         123

References