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9.8 Miscellaneous Fortran 2003 Features

9.8.1 Abstract interfaces and the PROCEDURE statement [5.1]

Abstract interfaces have been added, together with the procedure declaration statement. An abstract interface is defined in an interface block that has the ABSTRACT keyword, i.e. 
  ABSTRACT INTERFACE
Each interface body in an abstract interface block defines an abstract interface instead of declaring a procedure. The name of an abstract interface can be used in the procedure declaration statement to declare a specifc procedure with that interface, e.g. 
  PROCEDURE(aname) :: spec1, spec2
declares SPEC1 and SPEC2 to be procedures with the interface (i.e. type, arguments, etc.) defined by the abstract interface ANAME.

The procedure declaration statement can also be used with the name of any procedure that has an explicit interface, e.g. 

  PROCEDURE(x) y
declares Y to have the same interface as X. Also, procedures with implicit interfaces can be declared by using PROCEDURE with a type specification instead of a name, or by omitting the name altogether.

The following attributes can be declared at the same time on the procedure declaration statement: BIND(C...), INTENT(intent), OPTIONAL, POINTER, PRIVATE, PUBLIC, SAVE. For example, 

  PROCEDURE(aname),PRIVATE :: spec3

Note that POINTER declares a procedure pointer (see next section), and that INTENT and SAVE are only allowed for procedure pointers not for ordinary procedures. The NAG Fortran Compiler also allows the PROTECTED attribute to be specified on the procedure declaration statement: this is an extension to the published Fortran 2003 standard.

9.8.2 Named procedure pointers [5.2]

A procedure pointer is a procedure with the POINTER attribute; it may be a named pointer or a structure component (the latter are described elsewhere). The usual way of declaring a procedure pointer is with the procedure declaration statement, by including the POINTER clause in that statement: for example, 
  PROCEDURE(aname),POINTER :: p => NULL()
declares P to be a procedure pointer with the interface ANAME, and initialises it to be a disassociated pointer.

A named procedure pointer may also be declared by specifying the POINTER attribute in addition to its normal procedure declaration: for example, a function declared by a type declaration statement will be a function pointer if the POINTER attribute is included in the type declaration: 

  REAL, EXTERNAL, POINTER :: funptr
The POINTER statement can also be used to declare a procedure pointer, either in conjunction with an interface block, an EXTERNAL statement, or a type declaration statement, for example: 
  INTERFACE
    SUBROUTINE sub(a,b)
      REAL,INTENT(INOUT) :: a,b
    END SUBROUTINE
  END INTERFACE
  POINTER sub

Procedure pointers may also be stored in derived types as procedure pointer components. The syntax and effects are slightly different, making them act like “object-bound procedures”, and as such are described in the object-oriented programming section.

9.8.3 Intrinsic modules [4.x]

The Fortran 2003 standard classifies modules as either intrinsic or non-intrinsic. A non-intrinsic module is the normal kind of module (i.e. user-defined); an intrinsic module is one that is provided as an intrinsic part of the Fortran compiler.

There are five standard modules in Fortran 2003: IEEE_ARITHMETIC, IEEE_EXCEPTIONS, IEEE_FEATURES, ISO_C_BINDING and ISO_FORTRAN_ENV.

A program is permitted to have a non-intrinsic module with the same name as that of an intrinsic module: to this end, the USE statement has been extended: ‘USE,INTRINSIC ::’ specifies that an intrinsic module is required, whereas ‘USE,NON_INTRINSIC ::’ specifies that a non-intrinsic module is required. If these are not used, the compiler will select an intrinsic module only if no user-defined module is found. For example, 

  USE,INTRINSIC :: iso_fortran_env
uses the standard intrinsic module ISO_FORTRAN_ENV, whereas 
  USE,NON_INTRINSIC :: iso_fortran_env
uses a user-defined module with that name. Note that the double-colon ‘::’ is required if either specifier is used.

9.8.4 Renaming user-defined operators on the USE statement [5.2]

It is now possible to rename a user-defined operator on the USE statement, similarly to how named entities can be renamed. For example, 
  USE my_module, OPERATOR(.localid.)=>OPERATOR(.remotename.)
would import everything from MY_MODULE, but the .REMOTENAME. operator would have its name changed to .LOCALID..

Note that this is only available for user-defined operator names; the intrinsic operators .AND. et al cannot have their names changed in this way, nor can ASSIGNMENT(=) be renamed. The local name must be an operator if and only if the remote (module entity) name is an operator: that is, both of 

  USE my_module, something=>OPERATOR(.anything.)
  USE my_module, OPERATOR(.something.)=>anything
are invalid (a syntax error will be produced).

9.8.5 The ISO_FORTRAN_ENV module [5.1]

The standard intrinsic module ISO_FORTRAN_ENV is now available. It contains the following default INTEGER named constants.
CHARACTER_STORAGE_SIZE
size of a character storage unit in bits.
ERROR_UNIT
logical unit number for error reporting (“stderr”).
FILE_STORAGE_SIZE
size of the file storage unit used by RECL= in bits.
INPUT_UNIT
default (‘*’) unit number for READ.
IOSTAT_END
IOSTAT= return value for end-of-file.
IOSTAT_EOR
IOSTAT= return value for end-of-record.
NUMERIC_STORAGE_SIZE
size of a numeric storage unit in bits.
OUTPUT_UNIT
unit used by PRINT, the same as the ‘*’ unit for WRITE.

9.8.6 The IMPORT statement [5.1]

The IMPORT statement has been added. This has the syntax

IMPORT [ [ :: ] name [ , name ]... ]

and is only allowed in an interface body, where it imports the named entities from the host scoping unit (normally, these entities cannot be accessed from an interface body). If no names are specified, normal host association rules are in effect for this interface body.

The IMPORT statement must follow any USE statements and precede all other declarations, in particular, IMPLICIT and PARAMETER statements. Anything imported with IMPORT must have been declared prior to the interface body.

9.8.7 Length of names and statements

Names are now ([4.x]) permitted to be 63 characters long (instead of 31), and statements are now ([5.2]) permitted to have 255 continuation lines (instead of 39).

9.8.8 Array constructor syntax enhancements

Square brackets ([ ]) can now ([5.1]) be used in place of the parenthesis-slash pairs ((/ /)) for array constructors. This allows expressions to be more readable when array constructors are being mixed with ordinary parentheses. 
    RESHAPE((/(i/2.0,i=1,100)/),(/2,3/))  ! Old way
    RESHAPE([(i/2.0,i=1,100)],[2,3])      ! New way

Array constructors may now ([5.2]) begin with a type specification followed by a double colon (::); this makes zero-sized constructors easy (and eliminates potential ambiguity with character length), and also provides assignment conversions thus eliminating the need to pad all character strings to the same length. 

   [ Logical :: ]                             ! Zero-sized logical array
   [ Double Precision :: 17.5, 0, 0.1d0 ]     ! Conversions
   [ Character(200) :: 'Alf', 'Bernadette' ]  ! Padded to length 200

9.8.9 Structure constructor syntax enhancements [5.3]

There are three enhancements that have been made to structure constructors in Fortran 2003:
  1. component names can be used as keywords, the same way that dummy argument names can be used as argument keywords;
  2. values can be omitted for components that have default initialisation; and
  3. type names can be the same as generic function names, and references are resolved by choosing a suitable function (if the syntax matches the function's argument list) and treating as a structure constructor only if no function matches the actual arguments.

A fourth enhancement is made in the Fortran 2008 standard: a value can be omitted for a component that is allocatable.

This makes structure constructors more like built-in generic functions that can be overridden when necessary. Here is an example showing all three enhancements. 

  TYPE quaternion
    REAL x=0,ix=0,jx=0,kx=0
  END TYPE
  ...
  INTERFACE quaternion
    MODULE PROCEDURE quat_from_complex
  END INTERFACE
  ...
  TYPE(quaternion) FUNCTION quat_from_complex(c) RESULT(r)
    COMPLEX c
    r%x = REAL(c)
    r%y = AIMAG(c)
    r%z = 0
    r%a = 0
  END FUNCTION
  ...
  COMPLEX c
  TYPE(quaternion) q
  q = quaternion(3.14159265)  ! Structure constructor, value (~pi,0,0,0).
  q = quaternion(jx=1)        ! Structure constructor, value (0,0,1,0).
  q = quaternion(c)           ! "Constructor" function quat_from_complex.

Also, if the type is an extended type an ancestor component name can be used to provide a value for all those inherited components at once.

These extensions mean that even if a type has a private component, you can use the structure constructor if

9.8.10 Deferred character length [5.2]

The length of a character pointer or allocatable variable can now be declared to be deferred, by specifying the length as a colon: for example, 
  CHARACTER(LEN=:),POINTER :: ch
The length of a deferred-length pointer (or allocatable variable) is determined when it is allocated (see next section) or pointer-associated; for example 
  CHARACTER,TARGET :: t1*3,t2*27
  CHARACTER(:),POINTER :: p
  p => t1
  PRINT *,LEN(p)
  p => t2
  PRINT *,LEN(p)
will first print 3 and then 27. It is not permitted to ask for the LEN of a disassociated pointer that has deferred length.

Note that deferred length is most useful in conjunction with the new features of typed allocation, sourced allocation, scalar allocatables and automatic reallocation.

9.8.11 The ERRMSG= specifier [5.1]

The ALLOCATE and DEALLOCATE statements now accept the ERRMSG= specifier. This specifier takes a scalar default character variable, which in the event of an allocation or deallocation error being detected will be assigned an explanatory message. If no error occurs the variable is left unchanged. Note that this is useless unless the STAT= specifier is also used, as otherwise the program will be terminated on error anyway.

For example, 

  ALLOCATE(w(n),STAT=ierror,ERRMSG=message)
  IF (ierror/=0) THEN
    PRINT *,'Error allocating W: ',TRIM(message)
    RETURN
  END IF

9.8.12 Intrinsic functions in constant expressions [5.2 partial; 5.3 complete]

It is now allowed to use any intrinsic function with constant arguments in a constant expression. (In Fortran 95 real and complex intrinsic functions were not allowed.) For example, 
MODULE m
  REAL,PARAMETER :: e = EXP(1.0)
END

9.8.13 Specification functions can be recursive [6.2]

A function that is used in a specification expression is now permitted to be recursive (defined with the RECURSIVE attribute). For example 
       PURE INTEGER FUNCTION factorial(n) RESULT(r)
         INTEGER,INTENT(IN) :: n
         IF (n>1) THEN
           r = n*factorial(n-1)
         ELSE
           r = 1
         END IF
       END FUNCTION
can now be used in a specification expression. Note that a specification function must not invoke the procedure that invoked it.

9.8.14 Access to the command line [5.1]

The intrinsic procedures COMMAND_ARGUMENT_COUNT, GET_COMMAND and GET_COMMAND_ARGUMENT have been added. These duplicate functionality previously only available via the procedures IARGC and GETARG from the F90_UNIX_ENV module. 

INTEGER FUNCTION command_argument_count()
Returns the number of command-line arguments. Unlike IARGC in the F90_UNIX_ENV module, this returns 0 even if the command name cannot be retrieved. 

SUBROUTINE get_command(command,length,status)
  CHARACTER(*),INTENT(OUT),OPTIONAL :: command
  INTEGER,INTENT(OUT),OPTIONAL :: length,status
Accesses the command line which invoked the program. This is formed by concatenating the command name and the arguments separated by blanks. This might differ from the command the user actually typed, and should be avoided (use GET_COMMAND_ARGUMENT instead).

If COMMAND is present, it receives the command (blank-padded or truncated as appropriate). If LENGTH is present, it receives the length of the command. If STATUS is present, it is set to −1 if COMMAND is too short to hold the whole command, a positive number if the command cannot be retrieved, and zero otherwise. 

SUBROUTINE get_command_argument(number,value,length,status)
  INTEGER,INTENT(IN) :: number
  CHARACTER(*),INTENT(OUT),OPTIONAL :: value
  INTEGER,INTENT(OUT),OPTIONAL :: length,status
Accesses command-line argument number NUMBER, where argument zero is the program name. If VALUE is present, it receives the argument text (blank-padded or truncated as appropriate if the length of the argument differs from that of VALUE). If LENGTH is present, it receives the length of the argument. If STATUS is present, it is set to zero for success, −1 if VALUE is too short, and a positive number if an error occurs.

Note that it is an error for NUMBER to be less than zero or greater than the number of arguments (returned by COMMAND_ARGUMENT_COUNT).

9.8.15 Access to environment variables [5.1]

The intrinsic procedure GET_ENVIRONMENT_VARIABLE has been added. This duplicates the functionality previously only available via the procedure GETENV in the F90_UNIX_ENV module. 
  SUBROUTINE get_environment_variable(name,value,length,status,trim_name)
    CHARACTER(*),INTENT(IN) :: name
    CHARACTER(*),INTENT(OUT),OPTIONAL :: value
    INTEGER,INTENT(OUT),OPTIONAL :: length,status
    LOGICAL,INTENT(IN),OPTIONAL :: trim_name
  END
Accesses the environment variable named by NAME; trailing blanks in NAME are ignored unless TRIM_NAME is present with the value .FALSE.. If VALUE is present, it receives the text value of the variable (blank-padded or truncated as appropriate if the length of the value differs from that of VALUE). If LENGTH is present, it receives the length of the value. If STATUS is present, it is assigned the value 1 if the environment variable does not exist, −1 if VALUE is too short, and zero for success. Other positive values might be assigned for unusual error conditions.

9.8.16 Character kind selection [5.1]

The intrinsic function SELECTED_CHAR_KIND has been added. At this time the only character set supported is 'ASCII'.

9.8.17 Argument passing relaxation [5.1]

A CHARACTER scalar actual argument may now be passed to a routine which expects to receive a CHARACTER array, provided the array is explicit-shape or assumed-size (i.e. not assumed-shape, allocatable, or pointer). This is useful for C interoperability.

9.8.18 The MAXLOC and MINLOC intrinsic functions [5.1]

The MAXLOC and MINLOC intrinsic functions now return zeroes for empty set locations, as required by Fortran 2003 (Fortran 95 left this result processor-dependent).

9.8.19 The VALUE attribute [4.x]

The VALUE attribute specifies that an argument should be passed by value.

9.8.19.1 Syntax

The VALUE attribute may be specified by the VALUE statement or with the VALUE keyword in a type declaration statement.

The syntax of the VALUE statement is:

VALUE [ :: ] name [ , name ] ...

The VALUE attribute may only be specified for a scalar dummy argument; if the dummy argument is of type CHARACTER, its character length must be constant and equal to one.

Procedures with a VALUE dummy argument must have an explicit interface.

9.8.19.2 Semantics

A dummy argument with the VALUE attribute is “passed by value”; this means that a local copy is made of the argument on entry to the routine and so modifications to the dummy argument do not affect the associated actual argument and vice versa.

A VALUE dummy argument may be INTENT(IN) but cannot be INTENT(INOUT) or INTENT(OUT).

9.8.19.3 Example

PROGRAM value_example
  INTEGER :: i = 3
  CALL s(i)
  PRINT *,i  ! This will print the value 3
CONTAINS
  SUBROUTINE s(j)
    INTEGER,VALUE :: j
    j = j + 1  ! This changes the local J without affecting the actual argument
    PRINT *,j  ! This will print the value 4
  END SUBROUTINE
END
This example is not intended to be particularly useful, just to illustrate the functionality.

9.8.20 The VOLATILE attribute [5.0]

This is a horrible attribute which specifies that a variable can be modified by means outside of Fortran. Its semantics are basically the same as that of the C ‘volatile’ type qualifier; essentially it disables optimisation for access to that variable.

9.8.21 Enhanced complex constants [5.2]

The real or imaginary part may now be a named constant, it is not limited to being a literal constant. For example: 
  REAL,PARAMETER :: minusone = -1.0
  COMPLEX,PARAMETER :: c = (0,minusone)
This is not particularly useful, since the same effect can be achieved by using the CMPLX intrinsic function.

9.8.22 The ASSOCIATE construct [5.2]

The ASSOCIATE construct establishes a temporary association between the “associate names” and the specified variables or values, during execution of a block. Its syntax is

ASSOCIATE ( association [ , association ]... )
block
END ASSOCIATE

where block is a sequence of executable statements and constructs, and association is one of

name => expression
name => variable
name

The last of those is short for ‘name => name’. The scope of each “associate name” is the block of the ASSOCIATE construct. An associate name is never allocatable or a pointer, but otherwise has the same attributes as the variable or expression (and it has the TARGET attribute if the variable or expression is a pointer). If it is being associated with an expression, the expression is evaluated on execution of the ASSOCIATE statement and its value does not change during execution of the block — in this case, the associate name is not permitted to appear on the left-hand-side of an assignment or any other context which might change its value. If it is being associated with a variable, the associate name can be treated as a variable.

The type of the associate name is that of the expression or variable with which it is associated. For example, in 

ASSOCIATE(zoom=>NINT(SQRT(a+b)), alt=>state%mapval(:,i)%altitude)
  alt%x = alt%x*zoom
  alt%y = alt%y*zoom
END ASSOCIATE
ALT is associated with a variable and therefore can be modified whereas ZOOM cannot. The expression for ZOOM is of type INTEGER and therefore ZOOM is also of type INTEGER.

9.8.23 Binary, octal and hexadecimal constants [5.2]

In Fortran 95 these were restricted to DATA statements, but in Fortran 2003 these are now allowed to be arguments of the intrinsic functions CMPLX, DBLE, INT and REAL. The interpretation is processor-dependent, but the intent is that this specifies the internal representation of the complex or real value. The NAG Fortran compiler requires these constants to have the correct length for the specified kind of complex or real, viz 32 or 64 bits as appropriate.

For example, on a machine where default REAL is IEEE single precision, 

  REAL(z"41280000")
has the value 10.5.

9.8.24 Character sets [5.1; 5.3]

The support for multiple character sets, especially for Unicode (ISO 10646) has been improved.

The default character set is now required to include lowercase letters and all the 7-bit ASCII printable characters.

The ENCODING= specifer for the OPEN and INQUIRE statements is described in the input/output section.

A new intrinsic function SELECTED_CHAR_KIND(NAME) has been added: this returns the character kind for the named character set, or −1 if there is no kind for that character set. Standard character set names are 'DEFAULT' for the default character kind, 'ASCII' for the 7-bit ASCII character set and 'ISO_10646' for the UCS-4 (32-bit Unicode) character set. The name is not case-sensitive. Note that although the method of requesting UCS-4 characters is standardised, the compiler is not required to support them (in which case −1 will be returned); the NAG Fortran Compiler supports UCS-4 in release 5.3 (as well as UCS-2 and JIS X 0213).

Assignment of a character value of one kind to a character value of a different kind is permitted if each kind is one of default character, ASCII character, or UCS-4 character. Assignment to and from a UCS-4 character variable preserves the original value.

Internal file input/output to variables of UCS-4 character kind is allowed (if the kind exists), including numeric conversions (e.g. the E edit descriptor), and conversions from/to default character and ASCII character. Similarly, writing default character, ASCII character and UCS-4 character values to a UTF-8 file and reading them back is permitted and preserves the value.

Finally, the intrinsic function IACHAR (for converting characters to the ASCII character set) accepts characters of any kind (in Fortran 95 it only accepted default kind).

9.8.25 Intrinsic function changes for 64-bit machines [5.2]

Especially to support machines with greater than 32-bit address spaces, but with 32-bit default integers, several intrinsic functions now all have an optional KIND argument at the end of the argument list, to specify the kind of integer they return. The functions are: COUNT, INDEX, LBOUND, LEN, LEN_TRIM, SCAN, SHAPE, SIZE, UBOUND and VERIFY.

9.8.26 Miscellaneous intrinsic procedure changes [5.2]

The intrinsic subroutine DATE_AND_TIME no longer requires the three character arguments (DATE, TIME and ZONE) to have a minimum length: if the actual argument is too small, it merely truncates the value assigned.

The intrinsic functions IACHAR and ICHAR now accept an optional KIND argument to specify the kind of integer to which to convert the character value. This serves no useful purpose since there are no character sets with characters bigger than 32 bits.

The intrinsic functions MAX, MAXLOC, MAXVAL, MIN, MINLOC and MINVAL all now accept character values; the comparison used is the native (.LT.) one, not the ASCII (LLT) one.

The intrinsic subroutine SYSTEM_CLOCK now accepts a COUNT_RATE argument of type real; this is to handle systems whose clock ticks are not an integral divisor of 1 second.