why UChar* is not working with this ICU conversion?

When converting from UTF-8 to ISO-8859-6 this code didn't work:

UnicodeString ustr = UnicodeString::fromUTF8(StringPiece(input));

const UChar* source = ustr.getBuffer();

char target[1000];
UErrorCode status = U_ZERO_ERR开发者_JS百科OR;
UConverter *conv;
int32_t     len;

// set up the converter
conv = ucnv_open("iso-8859-6", &status);
assert(U_SUCCESS(status));

// convert 
len = ucnv_fromUChars(conv, target, 100, source, -1, &status);
assert(U_SUCCESS(status));

// close the converter
ucnv_close(conv);

string s(target);
return s;

images: (1,2)

However when replacing UChar* with a hard-coded UChar[] it works well!! image : (3)

It looks like you're taking the difficult approach. How about this:

static char const* const cp = "iso-8859-6";
UnicodeString ustr = UnicodeString::fromUTF8(StringPiece(input));
std::vector<char> buf(ustr.length() + 1);
std::vector<char>::size_type len = ustr.extract(0, ustr.length(), &buf[0], buf.size(), cp);
if (len >= buf.size())
{
    buf.resize(len + 1);
    len = ustr.extract(0, ustr.length(), &buf[0], buf.size(), cp);
}
std::string ret;
if (len)
    ret.assign(buf.begin(), buf.begin() + len));
return ret;