Manual Reference Pages - transfer (3)
transfer(3f) - [FORTRAN:INTRINSIC:BIT MANIPULATION] Transfer bit patterns
CONTENTS
Syntax
Description
Arguments
Return Value
Example
Comments
Standard
Class
SYNTAX
result = transfer(source, mold[, size])
DESCRIPTION
Interprets the bitwise representation of SOURCE in memory as if it is the representation of a variable or array of the same type and type parameters as MOLD.
This is approximately equivalent to the C concept of *casting* one type to another.
ARGUMENTS
SOURCE - Shall be a scalar or an array of any type. MOLD - Shall be a scalar or an array of any type. SIZE - (Optional) shall be a scalar of type INTEGER.
RETURN VALUE
The result has the same type as MOLD, with the bit level representation of SOURCE. If SIZE is present, the result is a one-dimensional array of length SIZE. If SIZE is absent but MOLD is an array (of any size or shape), the result is a one-dimensional array of the minimum length needed to contain the entirety of the bitwise representation of SOURCE. If SIZE is absent and MOLD is a scalar, the result is a scalar.
If the bitwise representation of the result is longer than that of SOURCE, then the leading bits of the result correspond to those of SOURCE and any trailing bits are filled arbitrarily.
When the resulting bit representation does not correspond to a valid representation of a variable of the same type as MOLD, the results are undefined, and subsequent operations on the result cannot be guaranteed to produce sensible behavior. For example, it is possible to create LOGICAL variables for which VAR and
EXAMPLE
Sample program:
program demo_transfer integer :: x = 2143289344 print *, transfer(x, 1.0) ! prints "nan" on i686 end program demo_transfer
COMMENTS
[[Joe Krahn]]: Fortran uses **molding** rather than **casting**.
Casting, as in C, is an in-place reinterpretation. A cast is a device that is built around an object to change it’s shape.
Fortran TRANSFER reinterprets data out-of-place. It can be considered **molding** rather than casting. A **mold** is a device that confers a shape onto an object placed into it.
The advantage of molding is that data is always valid in the context of the variable that holds it. For many cases, a decent compiler should optimize TRANSFER into a simple assignment.
There are disadvantages of this approach. It is problematic to define a union of data types because you must know the largest data object, which can vary by compiler or compile options. In many cases, an EQUIVALENCE would be far more effective, but Fortran Standards committees seem oblivious to the benefits of EQUIVALENCEs when used sparingly.
STANDARD
[[Fortran 90]] and later
CLASS
Transformational function
| transfer (3) | March 19, 2019 |
