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9581 :: wrapping c++ function - cannot instantiate 'T2'

• mantielero (orginal) [2022-11-06T18:24:43+01:00] view original

  I am trying to wrap a function which is used to cast to another type.

  The C++ example is:

  ++
  Handle(Geom_Point) aPnt1;
  Handle(Geom_CartesianPoint) aPnt2, aPnt3;
  aPnt2 = new Geom_CartesianPoint();
  aPnt1 = aPnt2; // OK, standard assignment
  aPnt3 = Handle(Geom_CartesianPoint)::DownCast (aPnt1);
  // OK, the actual type of aPnt1 is Geom_CartesianPoint, although the static type of the handle is Geom_Point
  Run

  where Geom_CartesianPoint inherits from Geom_Point. And Handle is some form of smart pointer. (Sorry, if the terminology is not correct, since I am not a pro).

  In the header file, there are two functions defined as follows:

  ++
      //! Down casting operator from handle to base type
      template <class T2>
      static typename opencascade::std::enable_if<is_base_but_not_same<T2, T>::value, handle>::type
        DownCast (const handle<T2>& theObject)
      {
        return handle (dynamic_cast<T*>(const_cast<T2*>(theObject.get())));
      }
      
      //! Down casting operator from pointer to base type
      template <class T2>
      static typename opencascade::std::enable_if<is_base_but_not_same<T2, T>::value, handle>::type
        DownCast (const T2* thePtr)
      {
        return handle (dynamic_cast<T*>(const_cast<T2*>(thePtr)));
      }
  Run

  What I tried something as the following in several forms:

  proc downcast*[T; T2](this: Handle[T] ): Handle[T2] {.cdecl,
      importcpp: "Handle('0)::Downcast(@)", header: "Standard_Handle.hxx".}
  Run

  but when I compile I get:

  
  Error: cannot instantiate: 'T2'
  Run

  How can I fix this?

---

• mantielero (orginal) [2022-11-06T19:52:38+01:00] view original

  Well I think my lack of knowledge on OOP is quite deep.

  I was just experimenting with this example, which fails. You cannot convert objects like that.

  type
    Person = object of RootObj
      name: string
    
    Student = object of Person
  
  var a:Person
  var b:Student
  b.name = "joe"
  a = b
  Run

  The following works:

  type
    Person = ref object of RootObj
      name: string
    
    Student = ref object of Person
  
  var a = new Person
  var b = new Student
  b.name = "joe"
  a = b
  b.name = "peter"
  assert b.name == "peter"
  assert a.name == "peter"
  assert (a of Person)
  assert (b of Student)
  Run

  The first issue that I have above, is trying to mimic this behaviour in C++:

  // Geom_CartesianPoint inherits from Geom_Point
  Handle(Geom_Point) aPnt1;
  Handle(Geom_CartesianPoint) aPnt2;
  aPnt2 = new Geom_CartesianPoint();
  aPnt1 = aPnt2; // OK, standard assignment
  Run

  In my bindings, Geom_CartesianPoint inherits from Geom_Point. But their respective handles are unreleated.

  In Nim:

  import occt
  
  var aPnt1: Handle[Geom_Point]
  var aPnt2: Handle[Geom_CartesianPoint]
  aPnt2 = newHandle( cnew newGeomCartesianPoint(1.0,2.0,3.0) )
  #aPnt1 = aPnt2  # This fails
  Run

  fails.

  If I replace the last line with:

  aPnt1 = cast[Handle[Geom_Point]](addr aPnt2)  # This fails as well
  Run

  it also fails:

  
  /home/jose/.cache/nim/tmp01_d/@mtmp01.nim.cpp: In function 'void NimMainModule()':
  /home/jose/.cache/nim/tmp01_d/@mtmp01.nim.cpp:117:92: error: use of deleted function 'NimMainModule()::<unnamed union>::<constructor>()'
    117 |         union { TY__34Wf6HfOat7G9bBA2jRAzyA* source; TY__7y8d4b9bdzRBHHJhUtf9af9ag dest; } LOC1;
        |                                                                                            ^~~~
  /home/jose/.cache/nim/tmp01_d/@mtmp01.nim.cpp:117:15: note: 'NimMainModule()::<unnamed union>::<constructor>()' is implicitly deleted because the default definition would be ill-formed:
    117 |         union { TY__34Wf6HfOat7G9bBA2jRAzyA* source; TY__7y8d4b9bdzRBHHJhUtf9af9ag dest; } LOC1;
        |               ^
  /home/jose/.cache/nim/tmp01_d/@mtmp01.nim.cpp:117:84: error: union member 'NimMainModule()::<unnamed union>::dest' with non-trivial 'opencascade::handle<T>::handle() [with T = Geom_Point]'
    117 |         union { TY__34Wf6HfOat7G9bBA2jRAzyA* source; TY__7y8d4b9bdzRBHHJhUtf9af9ag dest; } LOC1;
        |                                                                                    ^~~~
  /home/jose/.cache/nim/tmp01_d/@mtmp01.nim.cpp:117:92: error: use of deleted function 'NimMainModule()::<unnamed union>::~<constructor>()'
    117 |         union { TY__34Wf6HfOat7G9bBA2jRAzyA* source; TY__7y8d4b9bdzRBHHJhUtf9af9ag dest; } LOC1;
        |                                                                                            ^~~~
  /home/jose/.cache/nim/tmp01_d/@mtmp01.nim.cpp:117:15: note: 'NimMainModule()::<unnamed union>::~<constructor>()' is implicitly deleted because the default definition would be ill-formed:
    117 |         union { TY__34Wf6HfOat7G9bBA2jRAzyA* source; TY__7y8d4b9bdzRBHHJhUtf9af9ag dest; } LOC1;
        |               ^
  compilation terminated due to -fmax-errors=3.
  Error: execution of an external compiler program 'g++ -c -std=gnu++14 -funsigned-char  -w -fmax-errors=3 -fpermissive -I/usr/include/opencascade/   -I/home/jose/.choosenim/toolchains/nim-1.6.6/lib -I/home/jose/src/nimlang/occt.nim/examples -o /home/jose/.cache/nim/tmp01_d/@mtmp01.nim.cpp.o /home/jose/.cache/nim/tmp01_d/@mtmp01.nim.cpp' failed with exit code: 1
  Run

  How can I convert the references from one type to the other?

---

• mantielero (orginal) [2022-11-06T20:50:01+01:00] view original

  Finally a managed to do it with the following:

  aPnt1 = newHandle( cast[ptr Geom_Point](aPnt2.get) )
  Run

  where:

  • aPnt2.get: get is wrapped from the library I am working on. It gives you the pointer from the given handle. So its type is: ptr Geom_CartesianPoint.
  • cast[ptr Geom_Point](aPnt2.get) `: then I get the conversion from `ptr Geom_CartesianPoint to ptr Geom_Point.
  • newHandle( ... ): this creates new handles. It can do it from a pointer as it is being done in this case.

  The downcasting can be done in a similar way.

  So the following works for me:

  import occt
  
  # Geom_CartesianPoint a sub-class of Geom_Point;
  var aPnt1: Handle[Geom_Point]
  var aPnt2, aPnt3: Handle[Geom_CartesianPoint]
  
  assert(`*`(aPnt2) of Geom_Point)
  assert(`*`(aPnt2) of Geom_CartesianPoint)
  aPnt2 = newHandle( cnew newGeomCartesianPoint(1.0,2.0,3.0) )
  aPnt1 = newHandle( cast[ptr Geom_Point](aPnt2.get) )
  
  assert (`*`(aPnt1).x) == (`*`(aPnt2).x)
  
  aPnt3 = newHandle( cast[ptr Geom_CartesianPoint](aPnt1.get) )
  assert (`*`(aPnt1).x) == (`*`(aPnt3).x)
  Run

  I am not sure if there is any better way of doing so.

---

• Araq (orginal) [2022-11-07T10:03:49+01:00] view original

  You can use

  
  *x
  Run

  instead of

  
  `*`(x)
  Run

  And ideally you use some other operator, unary * for pointer deref is not idiomatic.

---

• mantielero (orginal) [2022-11-07T12:30:41+01:00] view original

  Thanks Araq. I will try that this afternoon, but I think I tried and it failed for some reason. In any case, I will probably hide completely needing to do so.

  I continue with the doubt about how to properly wrap:

  template <class T2>
      static typename opencascade::std::enable_if<is_base_but_not_same<T2, T>::value, handle>::type
        DownCast (const handle<T2>& theObject)
      {
        return handle (dynamic_cast<T*>(const_cast<T2*>(theObject.get())));
      }
  Run

  given that later is used like this:

  aPnt3 = Handle(Geom_CartesianPoint)::DownCast (aPnt1);
  Run

  and why the following is producing the cannot instantiate 'T2' error:

  proc downcast*[T; T2](this: Handle[T] ): Handle[T2] {.cdecl, importcpp: "Handle('0)::Downcast(@)".}
  Run

---

• Araq (orginal) [2022-11-07T14:41:30+01:00] view original

  Well T2 cannot be inferred so you have to call it like so downcast[int, byte](this).

---

• mantielero (orginal) [2022-11-07T17:52:16+01:00] view original

  Good point. In fact, this works:

  aPnt3 = downcast[Geom_Point, Geom_CartesianPoint](aPnt1)
  Run

  Is there a way to leave it like the following?

  aPnt3 = downcast[Geom_CartesianPoint](aPnt1) # Given that aPnt1's type is Geom_Point
  Run

  PS: *x worked for me. Thanks.

---

• sls1005 (orginal) [2022-11-09T04:23:57+01:00] view original

  You don't have to give A if it can be inferred from an argument.

  proc downcast*[B](this: Handle[auto] ): Handle[B] {...}
  Run

---

• mantielero (orginal) [2022-11-09T21:19:17+01:00] view original

  @sls1005 thanks a lot for that.

  But for some reason is failing. I got:

  ...
  proc dcast*[T](this: Handle[auto] ):Handle[T] {.cdecl, importcpp: "\'0::DownCast(@)".}
  ...
  Run

  and

  import occt
  
  var aPnt1: Handle[Geom_Point]
  var aPnt2, aPnt3: Handle[Geom_CartesianPoint]
  
  aPnt2 = newHandle( cnew newGeomCartesianPoint(1.0,2.0,3.0) ) #
  aPnt1 = newHandle( cast[ptr Geom_Point](aPnt2.get) )
  
  #echo typeof(aPnt1)  # --> Handle[geom_types.GeomPoint]
  #echo typeof(aPnt3)  # --> Handle[geom_types.GeomCartesianPoint]
  
  aPnt3 = dcast[Geom_CartesianPoint]( aPnt1 )
  Run

  The error I got is:

  Error: type mismatch: got <void, Handle[geom_types.GeomPoint]>
  but expected one of:
  proc `()`(this: BitPredicate; theLink: BVH_EncodedLink): bool
    first type mismatch at position: 1
    required type for this: BitPredicate
    but expression 'dcast[Geom_CartesianPoint]' is of type: void
  ....
  Run

  Why am I getting <void, Handle[geom_types.GeomPoint]> instead of Handle[geom_types.GeomPoint]? Which it is what I get when I echo typeof(aPnt1).

---

• sls1005 (orginal) [2022-11-11T05:50:08+01:00] view original

  but expression 'dcast[Geom_CartesianPoint]' is of type: void

  Maybe you forgot the parentheses?

---

• mantielero (orginal) [2022-11-11T16:33:43+01:00] view original

  I don't think so. It is complaining about line:

  aPnt3 = dcast[Geom_CartesianPoint]( aPnt1 )
  Run

---

• sls1005 (orginal) [2022-11-11T16:46:50+01:00] view original

  But this compiles:

  proc static_cast[T](self: auto): T {.importcpp: "static_cast<'0>(#)".}
  var x: int64 = 1
  echo static_cast[bool](x)
  Run

  Maybe use auto or Handle (without brackets) instead of Handle[auto]?

---

• mantielero (orginal) [2022-11-11T17:21:28+01:00] view original

  It continues failing:

  # nim cpp -r tmp01
  import occt
  
  proc mydowncast*[A; B](this: Handle[A] ): Handle[B] {.cdecl, importcpp: "\'0::DownCast(@)".}
  
  proc dcast1*[T](this: auto ):T {.cdecl, importcpp: "\'0::DownCast(@)".}
  
  proc dcast2*[T](this: Handle[auto] ): Handle[T] {.cdecl, importcpp: "\'0::DownCast(@)".}
  
  proc dcast3*[T](this: Handle ): Handle[T] {.cdecl, importcpp: "\'0::DownCast(@)".}
  
  var aPnt1: Handle[Geom_Point]
  var aPnt2, aPnt3: Handle[Geom_CartesianPoint]
  
  aPnt2 = newHandle( cnew newGeomCartesianPoint(1.0,2.0,3.0) ) #
  aPnt1 = newHandle( cast[ptr Geom_Point](aPnt2.get) )
  
  
  #aPnt3 = mydowncast[Geom_Point,Geom_CartesianPoint](aPnt1)  # <-- This works
  #aPnt3 = dcast1[Handle[Geom_CartesianPoint]](aPnt1)         # <-- This doesn't
  #aPnt3 = dcast2[Geom_CartesianPoint](aPnt1)                 # <-- This doesn't
  #aPnt3 = dcast3[Geom_CartesianPoint](aPnt1)                 # <-- This doesn't
  Run

  One thing that puzzles me is the error report:

  
  Error: type mismatch: got <void, Handle[geom_types.GeomPoint]>
  but expected one of:
  proc `()`(this: BitPredicate; theLink: BVH_EncodedLink): bool
    first type mismatch at position: 1
    required type for this: BitPredicate
    but expression 'dcast3[Geom_CartesianPoint]' is of type: void
  proc `()`(this: BlendFuncTensor; row: cint; col: cint; mat: cint): cfloat
    first type mismatch at position: 1
    required type for this: BlendFuncTensor
    but expression 'dcast3[Geom_CartesianPoint]' is of type: void
  proc `()`(this: ChFiDS_Map; i: cint): TopToolsListOfShape
    first type mismatch at position: 1
    required type for this: ChFiDS_Map
    but expression 'dcast3[Geom_CartesianPoint]' is of type: void
  ....
  Run

  We can see that is complaining about ` 'dcast3[Geom_CartesianPoint]' is of type: void`. But it is trying to much the call against the operator (). I am using that experimental feature in some places across the library:

  {.experimental: "callOperator".}
  Run

  Could it be a bug in Nim?

---

• DeletedUser (orginal) [2022-11-12T01:55:28+01:00] view original

  Might be related to https://github.com/nim-lang/Nim/issues/9831

---