The C++/CX String^ is not an object, even though it wears a hat

Raymond Chen

C++/CX refers to Windows Runtime strings as Platform::String^ with a hat instead of a star. But even though the string wears a hat, it is not an Object^.

The String^ type is a representation of the Windows Runtime HSTRING. And of the rules of HSTRING is that a null pointer is a valid HSTRING, and it represents the empty string, that is, a string with no characters.

This makes String^ a strange sort of beast.

String^ s = L""; // sets s = nullptr

If you assign an empty string to it, you get nullptr back.

void f(String ^s)
    if (s) { /* string is not empty */ }

Testing a String^ against nullptr tests whether the string is empty.

String ^s = nullptr; // represents empty string
auto data = s->Data(); // legal! returns pointer to L""
auto length = s->Length(); // legal! returns 0.
auto equal = s->Equals(L"nope"); // legal! returns false.

That’s right: I dereferenced a null pointer and it felt good.

Calling methods on a null String^ pointer is legal, and the operations are performed on an empty string.

This weird behavior of null String^ pointers has consequences beyond just strings. If you convert a null String^ to Object^ (a boxing operation), the null-ness is preserved:

String^ s = L""; // s is nullptr
Object^ o = s; // o is nullptr!

This differs from the behavior in other projections like C#, JavaScript, and C++/WinRT, where boxing an empty string produces a non-null object (that in turn holds an empty string).

The fact that a String^ is not an Object^ means that you cannot reinterpret between them.

String^ s = /* some value */;
Object^ o = reinterpret_cast<Object^>(s); // crash

The reinterpret_cast will treat a String^ as an Object^. But a String^ is secretly a HSTRING, whereas an Object^ is secretly an IInspectable*. The reinterpret-cast tells the compiler to treat this HSTRING as if it were an IInspectable*, and bad things happen, since the compiler is going to try to call the AddRef method from the IInspectable‘s vtable, but HSTRINGs don’t have a vtable, much less a vtable with AddRef in slot 1.

What you need to do is box the string into an object and unbox the object back into a string.

Object^o = s; // box the string into an object
String^s = static_cast<String^>(o); // unbox the object into a string

Bonus chatter: C++/CX delegates are also not objects, even though they too wear hats.