Why wstring can accept WCHAR[] while string doesn't accept UCHAR[] - c++

Question

I am trying to print the returned value of NtQueryValueKey which is UCHAR Data[1]; i have tried printf, cout, and string(Data, DataLengh), with the first two printing only 1 character and the last one throws an exception... Basically if i changed the Data Type to WCHAR Data[1] and used wstring(Data) it accepts it normally without any complain... also wprintf prints the value normally.
Edit: I meant NtQueryValueKey using the KEY_VALUE_PARTIAL_INFORMATION, I am using VS 2015 btw...

Answer 1

You must have mixed something up. You did not specify what value from the KEY_NAME_INFORMATION enumeration you are using for the second parameter to specify the data type, but a quick look at MSDN shows that all of the structures contain WCHAR Name[1]; or something similar as the last member (which I guess is the one you are interested in). Can you elaborate and provide the link or other means of documentation that states you actually need to use UCHAR ?

Answer 2

WCHAR is an alias for wchar_t. std::wstring operates with wchar_t elements. A WCHAR[] can decay to a wchar_t*, and thus can be assigned directly to a std::wstring.
UCHAR is an alias for unsigned char. std::string operates with char elements instead. A UCHAR[]/UCHAR* cannot be assigned directly to a std::string without a type-cast to char*, as char and unsigned char are distinct data types.
unsigned char is commonly used to represent 8bit bytes (it is the same data type used for BYTE).
NtQueryKey() returns strings as UTF-16LE encoded bytes using WCHAR[] character arrays, not UCHAR[] byte arrays. So your code is declaring things wrong if you are using UCHAR[] to begin with. But even so, you can use UCHAR if you pay attention to the encoding and byte length, and use appropriate type-casts.
Any associated Length value reported by NtQueryKey() is expressed in bytes, not characters. sizeof(UCHAR) is 1 and sizeof(WCHAR) is 2. So every 2 UCHARs represents 1 WCHAR. And the strings are not null-terminated, so you have to take the Length into account when printing or converting.
In Latin-based languages, most commonly used Unicode characters will be <= U+00FF, and thus every other UCHAR in UTF-16LE will usually be 0. That is interpreted as a null terminator when UTF-16 is printed with printf() or std::cout. You need to use wprintf() or std::wcout instead.
Converting Data to a std::string is a valid operation and should not be raising an exception:
std::string((char*)Data, DataLength)
Provided that:
Data is a valid pointer.
DataLength is an accurate byte count.
The only way this could raise an exception is if either:
Data is not pointing at valid memory.
the value of DataLength is more than the actual number of bytes allocated for Data.
available memory is too low to allocate std::string's internal buffer.
memory is corrupted.
Assigning Data by itself to a std::wstring without taking DataLength into account is not a valid operation because the strings are not null-terminated. You must specify the length:
std::wstring(Data, DataLength / sizeof(WCHAR))
If Data is UCHAR then use a type-cast:
std::wstring((WCHAR*)Data, DataLength / sizeof(WCHAR))
When printing Data directly with wprintf(), you must pass DataLength as an input parameter:
wprintf(L"%.*s", DataLength / sizeof(WCHAR), Data);
When printing Data directly with std::wcout, you should use write() instead of operator<< so you can pass DataLength as an input parameter:
std::wcout.write(Data, DataLength / sizeof(WCHAR));
If Data is UCHAR then use a type-cast:
std::wcout.write((WCHAR*)Data, DataLength / sizeof(WCHAR));