"Derived type is being used before it is defined" 在界面块中

Question

在 Windows 上使用 cygwin64 此程序无法编译：

program test
  implicit none

  !define my type
  type myType
    real::foo
    integer::bar
  end type myType

  !define an operator for this type
  interface operator (>)
      logical function compare(a,b)
        type(myType),intent(in) :: a,b
        compare = a%foo>b%foo
      end function compare
  end interface operator (>)

  !simple example of operator usage
  type(myType) :: tfoo, tbar
  tfoo = card(1.,2); tbar = card(3.,4)
  print*, tfoo>tbar
end program test

gfortran（唯一的参数是 "std=f2008"）告诉我：

type(myType),intent(in) :: a,b
                    1
Error: Derived type ‘mytype’ at (1) is being used before it is defined

这让我感到困惑，因为类型是在运算符之前定义的。我对 Fortran 比较陌生，所以这个示例代码可能有更多错误。

出现了同样的问题here，但是将myType封装在一个单独的模块中并没有解决问题。

Answer 1

您的代码有几个问题，但这个特定错误是因为 myType 在主机范围内，但不在接口块中。解决方案是按照链接线程中的建议将派生类型放在单独的模块中，或者 import 来自主机范围单元的派生类型：

  interface operator (>)
      logical function compare(a,b)
        import myType
        type(myType),intent(in) :: a,b
      end function compare
  end interface operator (>)

这在 Fortran 2008 标准 Cl 中有描述。 12.4.3.3 "IMPORT statement"：

1 The IMPORT statement specifies that the named entities from the host scoping unit are accessible in the interface body by host association. An entity that is imported in this manner and is defined in the host scoping unit shall be explicitly declared prior to the interface body.

接口块可能不包含可执行语句 - 因此您在那里的分配无效。此外， card 未在您的代码中定义。

"Derived type is being used before it is defined" 在界面块中

"Derived type is being used before it is defined" in interface block

cygwin

fortran

types

gfortran

operator-keyword