如何指向fortran中的类方法? [英] How to point to a class method in fortran?

问题描述

我有一个模块，该模块定义具有某些属性的类和一个作为函数的方法( proc1 ).另一个模块定义了一个函数( proc2 )，该函数使用一个参数评估函数，并给出指向该函数和参数的指针.在主程序中，我想定义一个指向类的方法( proc1 )的函数指针( fun )，以便评估 fun(x)等同于评估 obj1％proc1(x).那可能吗?

I have a module that defines a class with some attributes and a method (proc1) which is a function. Another module defines a function (proc2) that evaluates a function with one argument, given a pointer to that function and the argument. In the main program, I would like to define a function pointer (fun) which points to the class' method (proc1) such that evaluating fun(x) would be equivalent to evaluating obj1 % proc1(x). Is that possible?

我的编译器gfortran 7.3抱怨

My compiler, gfortran 7.3, complains that

预期的参数列表位于(1)

Expected argument list at (1)

，其语法如下.

module objmod
   implicit none
   ! object with some data and a method
   type obj 
      real                  :: a, b, c
   contains
      procedure, pass(self) :: proc1
   end type obj

contains
   ! method which uses object and a variable
   pure real function proc1(self, var) 
      class(obj), intent(in) :: self
      real, intent(in)       :: var
      proc1 = var * (self % a + self % b + self % c)
   end function proc1

end module objmod

module funmod
   implicit none

contains
   ! function that evaluates fun(a), or does more complicated stuff
   real function proc2(funptr, a)
      procedure(real), pointer :: funptr
      real, intent(in)         :: a
      proc2 = funptr(a)
   end function proc2

end module funmod

program funPtr

   use objmod
   use funmod

   implicit none

   type(obj)                :: obj1
   real                     :: a, x
   procedure(real), pointer :: fun

   obj1 % a =  1.0
   obj1 % b =  2.0
   obj1 % c =  3.0
   a        = -1.5
   fun => obj1 % proc1 ! function pointer to fun(x) = proc1(obj1, x)

   x = proc2(fun, a)

   print *, x
end program funPtr

推荐答案

不，您不能这样做.该标准说，对于指针分配的proc-target:

No, you can't do this. The standard says, for a proc-target of pointer assignment:

C1030(R1040)过程名称应为内部名称，模块或虚拟过程，过程指针，特定的内部函数表16.2中列出的函数或外部程序通过使用或主机关联进行访问，在范围界定单元中引用为一个过程或具有EXTERNAL属性的过程.

C1030 (R1040) A procedure-name shall be the name of an internal, module, or dummy procedure, a procedure pointer, a specific intrinsic function listed in Table 16.2, or an external procedure that is accessed by use or host association, referenced in the scoping unit as a procedure, or that has the EXTERNAL attribute.

在您的示例中， obj1％proc1 都不是这些.如果为 fun 提供了显式接口，则可以将指针分配给 proc1 (即实际过程，而不是类型组件).当然，这样做会丢失传递的对象方面.

In your example, obj1%proc1 is none of these. You COULD pointer assign to proc1 (that is, the actual procedure and not the type component) if you gave fun an explicit interface. Of course, you lose the passed object aspect if you do this.

问题在于函数指针没有类型绑定过程的上下文.

The issue is that a function pointer doesn't have the context of the type-bound procedure.

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