What are the duck-typing requirements of MFC IPTR?

Raymond Chen

We continue our survey of duck-typing requirements of various C++ COM smart pointer libraries by looking at MFC’s IPTR, running it through our standard tests.

// Dummy implementations of AddRef and Release for
// testing purposes only. In real code, they would
// manage the object reference count.
struct Test
{
    void AddRef() {}
    void Release() {}
    Test* AddressOf() { return this; }
};

struct Other
{
    void AddRef() {}
    void Release() {}
};

// Pull in the smart pointer library
// (this changes based on library)
#include <afxole.h>
 
extern const GUID IID_Test; 
extern const GUID IID_Other;
using TestPtr = IPTR(Test); 
using OtherPtr = IPTR(Other)

void test()
{
    Test test;

    // Default construction
    TestPtr ptr;

    // Construction from raw pointer
    TestPtr ptr2(&test);

    // Copy construction
    TestPtr ptr3(ptr2);

    // Attaching and detaching
    auto p = ptr3.GetInterfacePtr();
    ptr3.Detach();                  
    ptr.Attach(p);

    // Assignment from same-type raw pointer
    ptr3 = &test;

    // Assignment from same-type smart pointer
    ptr3 = ptr;

    // Accessing the wrapped object
    // (this changes based on library)
    if (ptr.GetInterfacePtr() != &test) {
        std::terminate(); // oops
    }
    if (ptr->AddressOf() != &test) {
        std::terminate(); // oops
    }

    // Returning to empty state
    ptr3.Release(); // requires ptr3 be non-empty
    ptr3 = static_cast<Test*>(nullptr);
    ptr3.Attach(nullptr);

    // Receiving a new pointer
    // (this changes based on library)
    Test** out = &ptr3;

    // Bonus: Comparison.
    if (ptr == ptr2) {}
    if (ptr != ptr2) {}
    if (ptr < ptr2) {}

    // Litmus test: Accidentally bypassing the wrapper
    ptr->AddRef();
    ptr->Release();

    // Litmus test: Construction from other-type raw pointer
    Other other;
    TestPtr ptr4(&other);

    // Litmus test: Construction from other-type smart pointer
    OtherPtr optr;
    TestPtr ptr5(optr);

    // Litmus test: Assignment from other-type raw pointer
    ptr = &other;

    // Litmus test: Assignment from other-type smart pointer
    ptr = optr;

    // Destruction
}

The IPTR macro assumes that the IID that corresponds to the alleged interface is named IID_Blah. We give it a dummy ID interface in the form of a reference to a variable that is never defined. This ensures a linker error if the code ever tries to use that fake interface ID.

The IPTR.Detach method does not return the detached pointer. You have to fetch it yourself via Get­Interface­Ptr() if you want to save it.

Returning the smart pointer to an empty state can be done in multiple ways.

  • The Release() method releases the wrapped pointer, but there must be a non-null wrapped pointer in the first place. It is an error to call ptr3.Release() on an empty ptr3.
  • You can use the assignment operator to assign a nullptr. However, there are multiple assignment operators,¹ so you have to cast the nullptr to Test* to say, “I am assigning a new pointer to be wrapped.”
  • A sneaky way to avoid the extra typing is to use the Attach() method to attach a null pointer. The first parameter to Attach() is always a Test*, so there is no ambiguity in passing just nullptr.

The only way to receive a new pointer is to use the & operator. This operator releases any previous wrapped pointer before receiving the new one.

The IPTR fails the bonus comparison test, but not through any fault of the Test class. The IPTR simply doesn’t support comparison at all, not even for COM interfaces.

The IPTR macro fails the “accidental bypass” litmus test, in the same way that _com_ptr_t did. As with _com_ptr_t, there are two ways of releasing the object that are dangerously similar:

ptr2.Release(); // good
ptr2->Release(); // bad

The same cautions apply.

The IPTR passes the other litmus tests: All of the attempts to convert or assign from another type of smart pointer or raw pointer generate errors.

Okay, so here’s the scorecard for IPTR.

MFC IPTR scorecard
Default construction Pass
Construct from raw pointer Pass
Copy construction Pass
Destruction Pass
Attach and detach Pass
Assign to same-type raw pointer Pass
Assign to same-type smart pointer Pass
Fetch the wrapped pointer GetInterfacePtr()
Access the wrapped object ->
Receive pointer via & release old
Release and receive pointer &
Preserve and receive pointer N/A
Return to empty state Pass
Comparison Not supported
Accidental bypass Fail
Construct from other-type raw pointer Pass
Construct from other-type smart pointer Pass
Assign from other-type raw pointer Pass
Assign from other-type smart pointer Pass

Next time, we’ll look at ATL’s CComPtr.

¹ The other assignment operators accept CLSID and PCWSTR and create a new object identifed by the CLSID or ProgId. Fortunately, it’s unlikely that you will assign one of these things to a a COM smart pointer by mistake.