如何判断Fortran过程指针是否关联特定子程序

你遇到的associated函数报错问题，本质是旧版Fortran标准（比如Fortran 95）不允许把具体子程序作为associated的第二个参数——这个特性是Fortran 2003才引入的。下面分两种场景给你解决办法：

场景一：用支持Fortran 2003+的编译器（推荐） #

如果能升级你的gfortran到5.x及以上版本，或者使用Intel Fortran等对新标准支持较好的编译器，就可以直接用标准的associated函数来判断过程指针关联的具体子程序。

比如你可以这样写代码：

module boundary_mod
    implicit none
    
    ! 定义抽象接口
    abstract interface
        subroutine boundary_interface()
        end subroutine boundary_interface
    end interface
    
    ! 声明过程指针
    procedure(boundary_interface), pointer :: bc => null()
    
contains

    subroutine boundaryA()
        print *, "This is boundaryA"
    end subroutine boundaryA
    
    subroutine boundaryB()
        print *, "This is boundaryB"
    end subroutine boundaryB

end module boundary_mod

program test_proc_ptr
    use boundary_mod
    implicit none
    
    ! 关联到boundaryA
    bc => boundaryA
    
    ! 直接用associated判断关联的具体子程序
    if (associated(bc, boundaryA)) then
        print *, "bc is associated with boundaryA"
    else if (associated(bc, boundaryB)) then
        print *, "bc is associated with boundaryB"
    else
        print *, "bc is not associated"
    end if
    
    ! 切换到boundaryB
    bc => boundaryB
    
    if (associated(bc, boundaryA)) then
        print *, "bc is associated with boundaryA"
    else if (associated(bc, boundaryB)) then
        print *, "bc is associated with boundaryB"
    else
        print *, "bc is not associated"
    end if

end program test_proc_ptr

编译的时候记得启用Fortran 2003及以上标准，比如用gfortran的话：

gfortran -std=f2003 test.f90 -o test

场景二：适配旧版编译器（比如你的gfortran 4.8.2） #

如果暂时没法升级编译器，那就得用替代方案：给每个子程序加一个唯一的“标签”，通过额外的指针或者变量来记录当前关联的是哪个子程序。比如下面的写法：

module boundary_mod
    implicit none
    
    ! 定义两个抽象接口：一个是子程序接口，一个是标签函数接口
    abstract interface
        subroutine boundary_interface()
        end subroutine boundary_interface
        
        integer function tag_interface()
        end function tag_interface
    end interface
    
    ! 过程指针和对应的标签指针
    procedure(boundary_interface), pointer :: bc => null()
    procedure(tag_interface), pointer :: bc_tag => null()
    
contains

    subroutine boundaryA()
        print *, "This is boundaryA"
    end subroutine boundaryA
    
    ! boundaryA的唯一标签函数
    integer function tag_boundaryA()
        tag_boundaryA = 1
    end function tag_boundaryA
    
    subroutine boundaryB()
        print *, "This is boundaryB"
    end subroutine boundaryB
    
    ! boundaryB的唯一标签函数
    integer function tag_boundaryB()
        tag_boundaryB = 2
    end function tag_boundaryB

end module boundary_mod

program test_proc_ptr
    use boundary_mod
    implicit none
    
    ! 关联过程指针的同时，关联对应的标签指针
    bc => boundaryA
    bc_tag => tag_boundaryA
    
    ! 通过调用标签函数返回的唯一值来判断
    if (bc_tag() == 1) then
        print *, "bc is associated with boundaryA"
    else if (bc_tag() == 2) then
        print *, "bc is associated with boundaryB"
    else
        print *, "bc is not associated"
    end if
    
    ! 切换关联对象
    bc => boundaryB
    bc_tag => tag_boundaryB
    
    if (bc_tag() == 1) then
        print *, "bc is associated with boundaryA"
    else if (bc_tag() == 2) then
        print *, "bc is associated with boundaryB"
    else
        print *, "bc is not associated"
    end if

end program test_proc_ptr

这个方法的核心是：每次绑定过程指针bc的时候，同时绑定对应的标签指针bc_tag，通过标签返回的唯一整数来区分不同的子程序，完全兼容旧版Fortran标准。

另外提一句：gfortran 4.8.2是2013年的老版本了，对Fortran 2003的支持并不完善，要是条件允许，升级到新一点的版本（比如10.x及以上）会省很多麻烦~

内容的提问来源于stack exchange，提问作者zljt3216