The empty Windows Runtime string is not just a pretty face
source link: https://devblogs.microsoft.com/oldnewthing/20220706-00/?p=106836
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The empty Windows Runtime string is not just a pretty face
Raymond Chen
July 6th, 20225
As I noted some time ago, the empty Windows Runtime string is represented by a null pointer. This has natural but perhaps surprising consequences: Even though it is a null pointer, the empty Windows Runtime string is a real string, with hopes and dreams. Or at least a length and data.
At the ABI level, WindowsGetStringLen reports that a null pointer string has a length of zero, and WindowsGetStringRawBuffer gives you a buffer that consists of a single null terminator.
Since an empty string and a null pointer are indistinguishable at the ABI layer, if you operate at the ABI layer (using raw HSTRINGs) or at a thin projection layer (such as C++/CX and C++/WinRT), you can take advantage of this equivalence.
For starters, you don’t need to check for a null pointer before trying to use the string, because a null pointer is a perfectly valid HSTRING.
| ABI | if (s != nullptr && |
if (s != nullptr && |
| C++/CX | if (s != nullptr && |
if (s != nullptr && |
| C++/WinRT | if (s != hstring{} && |
if (s != hstring{} && |
If you are checking for a nonempty string, you can just check for null. C++/WinRT and C++/CX even have special methods that tell you directly.
| Slower way | Quicker way | |
|---|---|---|
| ABI | if (WindowsGetStringLen(s) != 0) |
if (s != nullptr) |
| C++/CX | if (s->Length() != 0) |
if (!s->IsEmpty()) |
| C++/WinRT | if (s.size() != 0) |
if (!s.empty()) |
Related: The C++/CX String^ is not an object, even though it wears a hat.
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