C++ wstring how to assign from NULL-terminated wchar_t array

Question

Most texts on the C++ standard library mention wstring as being the equivalent of string, except parameterized on wchar_t instead of char, and then proceed to demonstrate string only.

Well, sometimes, there are some specific quirks, and here is one: I can't seem to assign a wstring from an NULL-terminated array of 16-bit characters. The problem is the assignment happily uses the null character and whatever garbage follows as actual characters. Here is a very small reduction:

typedef unsigned short PA_Unichar;
PA_Unichar arr[256];
fill(arr); // sets to 52 00 4b 00 44 00 61 00 74 00 61 00 00 00 7a 00 7a 00 7a 00
// now arr contains "RKData\0zzz" in its 10 first values
wstring ws;
ws.assign((const wchar_t *)arr);
int l = ws.length();

At this point l is not the expected 6 (numbers of chars in "RKData"), but much larger. In my test run, it is 29. Why 29? No idea. A memory dump doesn't show any specific value for the 29th character.

So the question: is this a bug in my standard C++ library (Mac OS X Snow Leopard), or a bug in my code? How am I supposed to assign a null-terminated array of 16-bit chars to a wstring?

Thanks

@obelix, a null character is the same both big- and little-endian. — Nick Meyer
– Nick Meyer, Commented Aug 27, 2009 at 11:57
@Nick - yep. i saw binary and thought it might be endianness — obelix
– obelix, Commented Aug 27, 2009 at 11:59

sbi · Accepted Answer · 2009-08-28 17:45:52Z

9

Under most Unixes (Mac OS X as well), whar_t represents UTF-32 single code point, and not 16bit utf-16 point like at windows.

So you need to:

Either:
```
ws.assing(arr,arr + length_of_string);
```
That would use arr as iterator and copy each short int to wchar_t. But this would work only if your characters lay in BMP or representing UCS-2 (16bit legacy encoding).
Or, correctly work with utf-16: converting utf-16 to utf-32 -- you need to find surrogate pairs and merge them to single code point.

edited Aug 28, 2009 at 17:45

sbi

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answered Aug 27, 2009 at 11:58

Artyom

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Comments

Logan Capaldo · Accepted Answer · 2009-08-27 12:36:38Z

3

Just do it. You didn't in your code, you assigned an array of unsigned shorts to a wstring and you used a cast to shut the compiler up. wchar_t != unsigned short. You certainly can't assume they have the same size.

answered Aug 27, 2009 at 12:36

Logan Capaldo

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Comments

Managu · Accepted Answer · 2009-08-27 12:04:34Z

0

I'd think your code would work, just by inspection. But you could always work around the trouble:

ws.assign(static_cast<const wchar_t*>(arr), wcslen(arr));

answered Aug 27, 2009 at 12:04

Managu

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1 Comment

Nick Meyer Over a year ago

If ws.assign can't find the proper terminating point of the string by picking out the null character, why would wcslen? I think Artyom hit the nail on the head -- wchar_t != unsigned short.

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