why my interface is sealed in maxon::Result?

[yushang]

Hi guys,
I'm following the maxonsdk.module listed in here. Which declare a SimpleNumberInterface and implement it with SimpleNumberImpl. I mimic the steps as follow

0. projectdefinition.txt

Platform=Win64;OSX;Linux
Type=DLL
APIS=core.framework
stylecheck=false
ModuleId=net.yu.myp

1. myifc.h

#pragma once
#include "maxon/object.h"
class MyIfc : MAXON_INTERFACE_BASES(maxon::ObjectInterface) {
MAXON_INTERFACE(MyIfc,MAXON_REFERENCE_NORMAL,"net.yu.interface.myifc");
public:
  MAXON_METHOD void Hello();
};
#include "myifc1.hxx"
MAXON_DECLARATION(maxon::Class<MyIfcRef>, MyIfcDecl, "net.yu.myifcdecl");
#include "myifc2.hxx"

2. myifc.cpp

#include "myifc.h"
class MyIfcImpl : public maxon::Component<MyIfcImpl,MyIfc> {
   MAXON_COMPONENT();
public:
  MAXON_METHOD void Hello() {
  }
};
MAXON_COMPONENT_CLASS_REGISTER(MyIfcImpl, MyIfcDecl);

3. main.cpp

#include "myifc.h"
extern "C"
__declspec(dllexport)
int c4d_main(int action, void* p1, void* p2, void* p3) {
  MyIfcRef ifc = MyIfcDecl().Create();   // A
  return 1;
}

At line A , I got the following error

error C2440: 'initializing': cannot convert from 'maxon::Result<MyIfcRef>' to 'MyIfcRef'

How does this happen? Many thanks!

[m_adam]

This post is deleted!

[ferdinand]

Hello @yushang,

thank you for reaching out to us. Please have a look at our Error System documentation. You are missing an iferr_return and an error scope in your code.

int c4d_main(int action, void* p1, void* p2, void* p3) 
{
  // Errors imply being propagated upwards in a call chain. When we want to terminate such chain,
  // i.e., have this function return `int` (you should use Int32 as int is ambiguous) instead of
  // `maxon::Result<maxon::Int32>` we can use a custom error handler to handle errors being thrown.
  // When the return type is `maxon::Result<T>` we can just use an `iferr_scope;` instead.
  iferr_scope_handler
  {
      DebugOutput("c4d_main failed with '@'", err);
      return 0;
  };

  // Create methods return a `maxon::Result<T>` object, i.e., either the value or the error. The
  // `iferr_return` keyword will unpack that error for us.
  MyIfcRef ifc = MyIfcDecl().Create() iferr_return;

  // Technically possible, but bad code:
  maxon::Result<MyIfcRef> result = MyIfcDecl().Create();

  // Can make sense in some cases, but should be generally avoided:
  MyIfcRef ifc = MyIfcDecl().Create() iferr_ignore("We don't care about the error here."_s);

  // Finally, there is a special form of branching that allows to handle errors in a more detailed way:
  ifnoerr (MyIfcRef ifc = MyIfcDecl().Create())
  {
    // Runs when the creation was successful.
  }
  
  iferr (MyIfcRef ifc = MyIfcDecl().Create())
  {
    // Would run when the creation failed.
  }

  return 1;
}

Cheers,
Ferdinand

[yushang]

@ferdinand cool. this really helpful!