问题
As stated in the title, I want to directly modify data that I access through a pointer retrieved from a function. Having a reference returned by a function appearing on the l.h.s. of an assignment(=) is no issue in C++ but the following minimal example in fortran errors out:
module test_mod
implicit none
integer, target :: a=1, b=2, c=3 ! some member variables
contains
function get(i)
integer, pointer :: get
integer, intent(in) :: i
select case (i)
case (1)
get => a
case (2)
get => b
case (3)
get => c
end select
end function get
end module test_mod
program test
use test_mod
implicit none
integer, pointer :: i_p
!> prints out 1 2 3
print*, get(1), get(2), get(3)
!> this is what I want but I get the error
!> Error: 'get' at (1) is not a variable
get(2) = 5
!> this works but is not what I want
i_p => get(2)
i_p = 5
end program test
Is there any way to accomplish this behaviour; maybe I'm missing some attributes? I would like to bypass writing any setter routines such as
set(i,value)
since it should mimic the appearance of an array.
In my application, the member variables a,b,c are actually arrays of different size
a = [a1, a2, a3]
b = [b1, b2]
c = [c1]
and I want the getter get(i,j) to mimic a matrix of pointers
j = 1 2 3
i = 1: [[a1, a2, a3],
i = 2: [b1, b2, XX],
i = 3: [c1, XX, XX]]
wehre XX would be referencing to null().
Update: I am using gfortran (version 5.2.0) and the deployment machines would have only versions starting from 4.6.x and upwards. Therefore, the suggested fortran 2008 standard features are unfortunately not available to me. Is it possible to mimic the behaviour described above without having a compiler supporting it out of the box?
Update 2: So I ended up implementing a structure as follows
type Vec_t
integer, allocatable, dimension(:) :: vec
end type Vec_t
type(Vec_t), allocatable, dimension(:), target :: data
which I initialise like this (my triangular matrix application I mention at the end)
allocate(data(max))
do i=1,max
allocate(data(i)%vec(i))
end do
and I access & write to it through
print*, data(2)%vec(1)
data(2)%vec(1) = 5
which is not precisely what I was after but good enough for my application.
回答1:
Let's look at what you want to do:
get(2)=5
and the error message
Error: 'get' at (1) is not a variable
That looks pretty comprehensive: you can't do what you want. Or, perhaps...
get(2) is indeed, under the rules of Fortran 2003, not a variable. In Fortran 2003 a variable is given by the rules R601 and R603, which is a list of designators.
The left-hand side of an assignment must be a variable.
But look at Fortran 2008 and its definition of a variable. Now a variable is either one of those same designators (or ones related to coarrays or complex parts), but it could also (C602 to R602) be a function reference which
shall have a data pointer result.
This is summarized in the introduction of Fortran 2008, detailing the extensions over Fortran 2003, as
A pointer function reference can denote a variable in any variable definition context.
get(2) is a reference to a function that has a data pointer result. get(2) then may appear on the left-hand side of an assignment statement under the rules of Fortran 2008.
Alas, this interpretation of Fortran is not widely supported by current compilers: at the time of answering just the Cray compiler.
This means that this answer is really saying that you have two options: switch compiler or wait until this feature is more widespread. As both of these are likely impractical, you probably want another answer which gives something slightly more portable as a workaround.
I prefer my link to that given by innoSPG, as although this latter is based on the former, the description of the appropriate field "Pointer functions - pointer function ref is a variable" is slightly more clear. This is, though, a more accessible document and a viable alternative.
来源：https://stackoverflow.com/questions/31570274/have-a-function-in-fortran-return-a-reference-that-can-be-placed-on-the-left-han