Passing by refference from C++\CLI to C++

Question

I have a C++\CLI wrapper to a C++ API of a commercial program. A certain wrapper function looks like this:

int ReleaseObject(WDataObject^ object) {
    return (m_api->ReleaseObject(object->m_object));
};

On the previous version of the API, the C++ side was:

m_api->ReleaseObject(const api:DataObject *object)

And in the current API version the function has changed to:

m_api->ReleaseObject(const api:DataObject *&object)

My knowledge on C++\CLI is none and my C++ knowledge is very basic, and I have no clue on how to modify the wrapper function to make it compile.

I'd like to know how to modify the wrapper c++\CLI function to pass the object by reference according to what the C++ API expects.

EDIT:

Error when compiling:

Error   3   error C2664: 'int api::Api::ReleaseObject(api::DataObject *&)' : cannot convert argument 1 from 'api::DataObject *' to 'api::DataObject *&' ...\pf_api_test - 15.2.2\libs\digapiwrapper15.1_64bit\digApiWrapper.h   851 1   digApiWrapper

EDIT 2:

    [SerializableAttribute]
        public ref class WDataObject {
            public:

            api::DataObject* m_object;

            WApi^ t_api;

            String^  Name;
            int  Class;
            String^ key;

            WDataObject(api::DataObject* obj) { m_object = obj; };

            ~WDataObject();

 /*HERE GOES ALL THE FUNCTIONS WHICH I'M NOT SURE IF THEY ARE RELEVANT*/
    }

Does it *not* compile now, or are you just guessing? If so, specify the *exact* error. — crashmstr, May 26 '15 at 13:20
@MoKaT http://stackoverflow.com/questions/500580/in-c-cli-what-does-the-hat-character-do — Drax, May 26 '15 at 13:48
@SantiPeñate-Vera can you show you declaration of `WDataObject`? I don't know that we have enough information without it. — crashmstr, May 26 '15 at 13:54
added the beginning of the definition in an edit, if more is needed I'll add it. — Santi Peñate-Vera, May 26 '15 at 13:59

score 1 · Accepted Answer · answered May 26 '15 at 14:00

You have to pass a pointer to your m_object variable. You did not quote the compiler error message, but it is somewhat guessable, it will not allow you to pass a pointer to a member of a managed class. That tends to end very poorly when the garbage collector moves the object while the native code is executing.

You must create a stable pointer reference, the simplest way to do so is by copying the pointer to a local variable. Like this:

int ReleaseObject(WDataObject^ object) {
    api:DataObject* arg = object->m_object;
    int retval = m_api->ReleaseObject(&arg);
    object->m_object = arg;
    return retval;
}

Which also fixes a likely bug in your original code, you probably should have set m_object to NULL yourself so you don't release it again. Be sure to check for NULL in your destructor and finalizer and don't call ReleaseObject() again.

Thanks for the reply, but it doesn't work yet. on [api:DataObject* arg = object->m_object;] the error is [ an ordinary pointer to a C++/CLI ref class or interface class is not allowed] and in [int retval = m_api->ReleaseObject(&arg);] the error is [initial value of reference to non-const must be an lvalue]. I believe it has to do with the fact that * is for C++ and not for C++\CLI. — Santi Peñate-Vera, May 27 '15 at 08:18

Santi Peñate-Vera · Answer 2 · 2015-05-27T14:39:17.690

0

After a lot of trial and error, I got the answer of Hans Passant to work:

int ReleaseObject(WDataObject^ object) {
    //Before: return (m_api->ReleaseObject(object->m_object));
    api::DataObject* obj = object->m_object;
    int val = m_api->ReleaseObject(obj);
    object->m_object = obj;
    return val;
};

I'll give his answer as correct.

edited May 27 '15 at 14:39

answered May 27 '15 at 14:30

Santi Peñate-Vera

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Passing by refference from C++\CLI to C++

2 Answers2