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

Raymond Chen

Raymond

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;
ConditionWhat’s happeningResult
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:

ConditionWhat’s happeningResult
if (p == false)false converted to IInspectable{ nullptr }
and compared with p
does not print

 

Raymond Chen
Raymond Chen

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