Ampersand Operator in Mixed Managed/Unmanaged C++/CLI Project

Question

In writing a function within a C++/CLI ref class, I wish to pass a pointer to a pointer to an object when calling a native function.

Native Function

void MyNativeFunc(NativeObject** obj);

Managed C++/CLI Function

ref class ManagedType
{
    static NativeObject* m_NativeObj;
    void MyManagedFunc()
    {
        MyNativeFunc(&m_NativeObj);  // Error here
    }
}

The problem here is that &m_NativeObj in C++/CLI is not a NativeObject**. It is an interior_ptr<NativeObject*> which the native function cannot accept. Is there an syntax such that I can perform this simple reference operation without incurring the managed pointer?

Edit: Apologies. Initially, the m_NativeObj was a function member in the question, not a static member of the class. I have the problem only if it is a static member.

Tried that. C2440: 'type cast': cannot convert from 'cli::interior_ptr<NativeObj*>' to 'NativeObj**' — Russell Trahan
I understand this is the result of the 'NativeObject**' pointer having to be stored on the stack which requires a managed pointer because the reference could change. I'm just looking for the syntax to actually do this. There must be a way. — Russell Trahan

Sebacote Sebacote · Accepted Answer · 2015-12-01T17:23:32

I assume the reason your native function is taking a NativeObject** is because the function will assign a new object to it, in which case you can do something like this:

class NativeObject { };

void MyNativeFunc(NativeObject** obj) {
    *obj = new NativeObject;
}

ref class ManagedObject
{
public:
    void Test()
    {
        NativeObject* p = nullptr;
        MyNativeFunc(&p);
        delete m_NativeObj;
        m_NativeObj = p;
    }

    static NativeObject* m_NativeObj = nullptr;
};

On the other hand, if the native function is an in/out function that may assign a new value to the pointer, you can code something like this:

class NativeObject { };

void MyNativeFunc(NativeObject** obj) {
    if (!(*obj))
        *obj = new NativeObject;
}

ref class ManagedObject
{
public:
    void Test()
    {
        pin_ptr<NativeObject> pinned = m_NativeObj;
        NativeObject* p = pinned;
        MyNativeFunc(&p);
        if (p != m_NativeObj)
        {
            delete m_NativeObj;
            m_NativeObj = p;
        }
    }

    static NativeObject* m_NativeObj = nullptr;
};

Ampersand Operator in Mixed Managed/Unmanaged C++/CLI Project

1 Answers