In C++/WinRT, what happens when I treat an IInspectable as or convert one to a bool

Last time, we looked at weirdness in how C++/CX treats hat pointers in a bool context. Fortunately, C++/WinRT is much less weird.

The IInspectable type supports a conversion to bool which tests whether the underlying pointer is null. It also supports comparison against nullptr which tests the same thing. And, unlike C++/CX, C++/WinRT uses this conversion for both explicit and contextual conversions.

IInspectable p = winrt::box_value(false);
IInspectable q = winrt::box_value(false);

if (p)                    std::cout << 1;
if ((bool)p)              std::cout << 2;
if (static_cast<bool>(p)) std::cout << 3;
if (p == q)               std::cout << 4;
if (p == false)           std::cout << 5;
if (!p)                   std::cout << 6;
if ((bool)p == (bool)q)   std::cout << 7;

Condition	What’s happening	Result
`if (p)`	Tests `p` against `nullptr`.	prints `1`
`if ((bool)p)`	Tests `p` against `nullptr`.	prints `2`
`if (static_cast<bool>(p))`	Tests `p` against `nullptr`.	prints `3`
`if (p == q)`	Compares two objects for identity.	does not print
`if (p == false)`	Not allowed (compiler error).
`if (!p)`	Tests `p` against `nullptr`.	does not print
`if ((bool)p == (bool)q)`	Tests `p` and `q` against `nullptr`.	prints 7

Note that the last case prints 7 but not for the reason you think. It’s not doing any unboxing at all. It’s just checking whether both variables are non-null.

IInspectable t = winrt::box_value(true);
if ((bool)p == (bool)t)   std::cout << 8; // prints 8!

Bonus chatter: There is a little quirk in the p == false case. My understanding is that prior to C++11, false was a legal null pointer constant, but the rules in C++11 were tightened so that false is no longer a null pointer context.

Microsoft’s Visual Studio C++ compiler, however, continues to accept false as a null pointer constant, even in non-permissive mode. This means that if you’re using Microsoft’s Visual Studio C++ compiler, the fifth row of the table is slightly different:

Condition	What’s happening	Result
`if (p == false)`	`false` converted to `IInspectable{ nullptr }` and compared with `p`	does not print

Author

Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

4 comments

Thomas Pollehn December 6, 2019

Hello Raymond,
Micro-nitpick: Should you not initialize IInspectable p to true instead of false?

Fantastic blog – thank you so much!!
Thomas

Tim Weis December 7, 2019 0

p is presumably initialized to false to highlight the fact, that operator== compares for identity, not equality.
Raymond Chen Author December 6, 2019 0

p is initialized to false on purpose.