vector returning const reference, impossible to downcast

Question

I am facing an issue in SWIG.

As explained in the doc here, SWIG does not support downcasts in Java and C#. I followed the advices of the doc, and create the correct typemaps for the factory of objects. I can know create an object A and downcast it to B.

However, I also have a method returning a vector of pointers to object A. I wrote some typemaps in order to use the factory I made. But then, I can't downcast any element of the vector.

Here is an example:

class A {  public:
    string name;
    void someMethod(); 
... }

class B : public A { 
       specialBMethod();
... }

class C : public A { 
       specialCMethod();
... }

std::vector<std::shared_ptr<A>> someRandomMethod();

And now, I want to this in C# like:

A tmp = someFactoryMethod("B") // will return a B class;
(B)tmp.specialBMethod(); // works fine;

A_vector test = someRandomMethod();
if (test[0].name == "B")
  (B)tmp.specialBMethod(); // InvalidCastException

The funniest part, is that if I make a copy of the vector, using CopyTo, and put it in an array for example, it works.

A_vector tmp = someRandomMethod();
A[] test = tmp.ToArray(); // imagine the ToArray method was implemented
if (test[0].name == "B")
  (B)tmp.specialBMethod(); // works fine

I think there is a problem when the vector return a const reference of the object. It losts the inheritance and becomes impossible to downcast.

I am pretty lost in this situation. I also opened an issue on the SWIG repo. Any help would be great;

EDIT : as Flexo asked for, here a minimal, complete example.

example.hpp

class A {  
    string name;
public:
    void someMethod(); 
    string getName() { return name; }
... }

class B : public A { 
       specialBMethod();
... }


class C : public A { 
       specialCMethod();
... }

std::vector<std::shared_ptr<A>> someRandomMethod();

example.i

%{
#include "example.hpp"
%}

%pragma(csharp) imclasscode=%{
public static System.Collections.Generic.Dictionary<string, System.Type> aDictionary;

public static System.Collections.Generic.Dictionary<string, System.Type> createDictionary<T>() where T : class
{
    System.Collections.Generic.Dictionary<string, System.Type> dictionary = new System.Collections.Generic.Dictionary<string, System.Type>();
    foreach (System.Type type in
        System.Reflection.Assembly.GetAssembly(typeof(T)).GetTypes())
    {
        if (type.IsClass && !type.IsAbstract && type.IsSubclassOf(typeof(T)))
        {
            string tmp = type.ToString().Split('.')[type.ToString().Split('.').Length - 1].Substring(0, type.ToString().Split('.')[type.ToString().Split('.').Length - 1].IndexOf(typeof(T).Name));
            dictionary.Add(tmp, type);
        }
    }
    return dictionary;
}

public static A createA(System.IntPtr cPtr, bool owner)
{
    A ret = null;
    if (cPtr == System.IntPtr.Zero) {
      return ret;
    }
    string ct = ($imclassname.A_getName(new System.Runtime.InteropServices.HandleRef(null, cPtr)));
    if (aDictionary == null)
        aDictionary = createDictionary<A>();
    if (aDictionary.ContainsKey(ct))
    {
        System.Reflection.BindingFlags flags = System.Reflection.BindingFlags.CreateInstance | System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance;
        ret = (A)System.Activator.CreateInstance(aDictionary[ct], flags, null, new object[] { cPtr, owner }, null);
    }
    else
    {
        ret = new A(cPtr, owner);
    }
    return ret;
}
%}

%typemap(csout, excode=SWIGEXCODE)
  A*, std::shared_ptr<A> {
    System.IntPtr cPtr = $imcall;
    Chip ret = liblogicalaccess_examplePINVOKE.createA(cPtr, $owner);$excode
    return ret;
}

%include "example.hpp"

test.cs

A tmp = someFactoryMethod("B") // will return a B class;
(B)tmp.specialBMethod(); // works fine;

A_vector test = someRandomMethod();
if (test[0].name == "B")
  (B)tmp.specialBMethod(); // InvalidCastException

A_vector tmp = someRandomMethod();
A[] test = new A[tmp.Count];
tmp.CopyTo(test);
if (test[0].name == "B")
  (B)tmp.specialBMethod(); // works fine

See my answer to an older question at: https://stackoverflow.com/a/27209463/168175 — Flexo, Jun 22 '17 at 16:02
Yes, what you explains on this post I do it already, I made the same typemap using the factory I made in C#. Now the problem stays when my method returns a vector of shared_ptr. Maybe I need to make a csout typemap on "std::vector>", but I don't know yet what to put inside. — Quentin S., Jun 23 '17 at 08:03

mattideluxe · Answer 1 · 2017-06-23T05:47:26.467

1

Edit: As the question has changed, my answer doesn't longer contribute anything.

Try a vector of std::unique_ptr as objects aren't polymorphic.

edited Jun 23 '17 at 05:47

answered Jun 22 '17 at 09:59

mattideluxe

468
2
12

This is exactly right - because there's no pointer the objects going into the vector get sliced. No amount of SWIG magic can fix that C++ problem. Once the vector has pointers of some sort though there's still going to be a casting problem in C# code though. – Flexo Jun 22 '17 at 14:57
Actually, I did not explain my problem well. I forgot an essential part: my vector already contains pointers (in this case, shared_ptr). I'm gonna edit the post. The problem stays the same. – Quentin S. Jun 22 '17 at 15:18

score 1 · Accepted Answer · answered Jul 20 '17 at 13:24

THE SOLUTION:

https://github.com/swig/swig/issues/1007

I found the answer on the SWIG issues list. It seems I simply forgot a typemap... Because the getitem method from the std::vector wrapping to C# return a reference to T, so I needed to add :

%typemap(csout, excode=SWIGEXCODE)
std::shared_ptr<A>&, // <- ANSWER HERE
A*, std::shared_ptr<A> {
    System.IntPtr cPtr = $imcall;
    Chip ret = liblogicalaccess_examplePINVOKE.createA(cPtr, $owner);$excode
    return ret;
}

vector returning const reference, impossible to downcast

2 Answers2