Fortran多态性的指针

问题描述

我想使用指针来创建对象之间的链接。使用Fortran和这里是代码片：

 模块base_pars_module 
 type，abstract，public :: base_pars 
 end type 
结束模块
模块test_parameters_module 
使用base_pars_module 
类型，extends（base_pars）:: test_pars 
包含
 procedure :: whoami 
 end type 
包含
函数whoami（this）result（iostat）
 class（test_pars）:: this 
 write（*，*）' base_pars'
结束类型
结束模块
模块base_mask_module 
使用base_pars模块
 type，abstract，public :: base_mask 
类（base_pars），指针： ：parameters 
 end type 
 end module 
 module test_mask_module 
 use base_mask_module 
 implicit none 
 type，extends（base_mask）:: test_mask 
 end类型
结束模块
程序驱动程序
类型（test_pars），target :: par_Test 
类型（test_mask）:: mask_test 
 iostat = par_test％whoami $ b mask_test％parameters => par_test 
 iostat = mask_test％parameters％whoami（）
结束程序
  

参数在 base_mask_module 是一个指向 base_pars 类。我想使用这个指针引用 par_test 对象 test_pars 类型扩展 base_pars 类型。所以指针和目标具有相同的类。但是当我编译这个它给出一个错误：

  driver.f90：17.37：
 
 iostat = mask_test％parameters％whoami（）
 1 
错误：'whoami'at（1）不是'base_pars'结构的成员
  

这是错误还是我做错了？

解决方案

当你有这样的多态性时，有两个事情要考虑一个对象：它的动态类型和其声明类型。 test_mask （ base_mask ）的参数组件声明为

  class（base_pars），pointer :: parameters 
  

这样的组件因此声明了 base_pars 类型。

指针赋值

  mask_test％parameters => par_test 
  

mask_test％parameters 具有与 par_test 相同的动态类型： test_pars 。它是声明类型 base_pars ，但是，当我们关心它的组件和绑定时，它是重要的声明类型。 base_pars 确实没有 whoami 。

然后，声明类型 par_test 的东西。在不改变派生类型的定义的情况下，您可以使用 select type 构造。

 code> select type（pars => mask_test％parameters）
 class is（par_test）
 iostat = pars％whoami与mask_test％参数关联的声明类型par_test的pars 
 end select 
  

相当快地用这种方法相当乏味。总是使用 select type ，区分大量扩展类型，将是相当绑定。另一种方法是确保声明的类型 base_pars 有一个绑定 whoami 。不是像上面那样改变主程序，而是改变模块 base_pars_module ：

  module base_par_modules 
 implicit none！鼓励良好的做法
 
 type，abstract，public :: base_pars 
包含
过程（whoami_if），deferred :: whoami 
结束类型
 $ b b interface 
 integer function whoami_if（this）
 import base_pars！回到我们在模块的不同范围
 class（base_pars）this 
 end function 
 end interface 
 
结束模块
  

因此，我们在 base_pars 中有一个延迟绑定通过扩展类型 test_pars 中的绑定。 mask_test％parameters％whoami（）在主程序中是一个有效的函数，调用的函数是由动态类型参数的声明类型的绑定问题。

如果你知道你的类型层次结构与基本类型有足够的共同点，那就是，所有将提供 whoami 绑定），那么它是有意义的去第二种方法。使用第一种方法，而不是当你有奇怪的特殊情况，我建议应该是罕见的。

I am trying to use pointers to create links between objects. Using Fortran and here is the code piece:

 module base_pars_module
   type,abstract,public :: base_pars
   end type
 end module
 module test_parameters_module
   use base_pars_module
   type, extends(base_pars) :: test_pars
     contains
     procedure :: whoami
   end type
   contains
   function whoami(this) result(iostat)
     class( test_pars) :: this
     write(*,*) 'i am a derived type child of base_pars'
   end type
 end module
 module base_mask_module
   use base_pars module
     type, abstract , public :: base_mask
     class(base_pars),pointer :: parameters
   end type
end module
module test_mask_module
  use base_mask_module
  implicit none
  type, extends(base_mask) :: test_mask
  end type
end module
program driver
type(test_pars) , target :: par_Test
type(test_mask) :: mask_test
  iostat= par_test%whoami()
  mask_test%parameters=>par_test
  iostat=mask_test%parameters%whoami()
end program

parameters at base_mask_module is a pointer with base_pars class. I would like to use this pointer to refer par_test object which is test_pars type that extends base_pars type. So the pointer and the target has the same class. But when I compile this it gives an error:

 driver.f90:17.37:

iostat=mask_test%parameters%whoami()
                                  1
Error: 'whoami' at (1) is not a member of the 'base_pars' structure

Is it a bug or am i doing something wrong?

解决方案

When you have polymorphism like this there are two things to consider about an object: its dynamic type and its declared type. The parameters component of test_mask (base_mask) is declared as

class(base_pars),pointer :: parameters

Such a component therefore has declared type base_pars.

Come the pointer assignment

mask_test%parameters=>par_test

mask_test%parameters has dynamic type the same as par_test: test_pars. It's of declared type base_pars, though, and it's the declared type that is important when we care about its components and bindings. base_pars indeed has no whoami.

You need, then, something which has declared type par_test. Without changing the definitions of the derived types you can do this with the select type construct.

select type (pars => mask_test%parameters)
class is (par_test)
  iostat=pars%whoami()  ! pars of declared type par_test associated with mask_test%parameters
end select

That said, things get pretty tedious quite quickly with this approach. Always using select type, distinguishing between numerous extending types, will be quite a bind. An alternative would be to ensure that the declared type base_pars has a binding whoami. Instead of changing the main program as above, we alter the module base_pars_module:

module base_par_modules
  implicit none  ! Encourage good practice

  type,abstract,public :: base_pars
   contains
    procedure(whoami_if), deferred :: whoami
  end type

  interface
    integer function whoami_if(this)
      import base_pars    ! Recall we're in a different scope from the module
      class(base_pars) this
    end function
  end interface

end module

So, we've a deferred binding in base_pars that is later over-ridden by a binding in the extending type test_pars. mask_test%parameters%whoami() in the main program is then a valid and the function called is that offered by the dynamic type of parameters.

Both approaches here address the problem with the binding of the declared type of parameters. Which best suits your real-world problem depends on your overall design.

If you know that your hierarchy of types will all have enough in common with the base type (that is, all will offer a whoami binding) then it makes sense to go for this second approach. Use the first approach rather when you have odd special cases, which I'd suggest should be rare.

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