On the overloading of the address-of operator & in smart pointer classes
source link: https://devblogs.microsoft.com/oldnewthing/20221010-00/?p=107269
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On the overloading of the address-of operator & in smart pointer classes
Raymond Chen
Many smart pointer classes overload the address-of operator & to give you access to the inner raw pointer.
Unfortunately, they disagree on what happens to the object being managed by the smart pointer before you get its raw address.
| Library | Existing contents |
|---|---|
| _com_ptr_t | Released |
| ATL (CComPtr) | Must be empty (will assert in Debug) |
| MFC (IPTR) | Released |
| WRL (ComPtr) | Released |
| wil (com_ptr) | Released |
| C++/WinRT (com_ptr) | N/A |
C++/WinRT avoids the confusion by simply not having an overloaded operator& at all! Not having an overloaded operator& also makes it easier to take the address of the smart pointer itself. The put() method Releases any managed COM pointer and then returns the address of the raw pointer.
So let’s finish the table. Let’s say that sp is the name of a variable of the corresponding smart pointer type.
| Library | Release | Don’t release | Assumes empty |
|---|---|---|---|
| _com_ptr_t | &sp |
&sp.GetInterfacePtr() |
|
| ATL (CComPtr) | &sp.p |
&sp |
|
| MFC (IPTR) | &sp |
||
| WRL (ComPtr) | &spp.ReleaseAndGetAddressOf() |
p.GetAddressOf() |
|
| wil (com_ptr) | &spsp.put() |
sp.addressof() |
|
| C++/WinRT (com_ptr) | sp.put() |
Bonus chatter: The possibility of an overloaded operator& is one of those special cases you tend to forget about when writing template library code.¹ In general, it’s not safe to use the & operator to get the address of an object of unknown type, because the operator might be overloaded. You have to use std::addressof.
¹ Hey, at least it’s not an overloaded comma operator. That thing is nasty.
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