按一定长度截取utf-8编码的字符串

int getChLength(unsigned char c)
{

if (c < 0x7F)
{
return 1;
}else if ((c & 0xE0) == 0xC0)
{
return 2;
}else if ((c & 0xF0) == 0xE0)
{
return 3;
}else if ((c & 0xF8) == 0xF0)
{
return 4;
}else if ((c & 0xFC) == 0xF8)
{
return 5;
}else if ((c & 0xFE) == 0xFC)
{
return 6;
}else
return -1;
}

std::string sUtf8Substr(std::string sFormerStr, int iRsltLen)
{
if (sFormerStr.size() <= iRsltLen)
{
return sFormerStr;
}

char* cbuf  = new char[iRsltLen];
memset(cbuf, 0, iRsltLen);
int i = 0;
int j = 0;
while (i < iRsltLen)
{
j = getChLength(sFormerStr[i]);

if (j == -1)
break;

strcat(cbuf, sFormerStr.substr(i, j).c_str());
i += j;
}

std::string sTmp(cbuf);
delete [] cbuf;

if (sTmp.size() > iRsltLen)
{
sTmp.erase(sTmp.size() - 1 - j, j);
}

return sTmp;
}