判断字符串是否为utf8格式，cpp实现

基于网上其它编程语言实现的版本，此处改写为c、cpp版本。

bool isUtf8Str(const char *str)
{
    int str_len = strlen(str);
    int encodingBytesCount = 0;
    bool allTextsAreASCIIChars = true;

    for (int i = 0; i < str_len; i++)
    {
        char current = str[i];

        if ((current & 0x80) == 0x80)
        {
            allTextsAreASCIIChars = false;
        }
        // First byte
        if (encodingBytesCount == 0)
        {
            if ((current & 0x80) == 0)
            {
                // ASCII chars, from 0x00-0x7F
                continue;
            }

            if ((current & 0xC0) == 0xC0)
            {
                encodingBytesCount = 1;
                current <<= 2;

                // More than two bytes used to encoding a unicode char.
                // Calculate the real length.
                while ((current & 0x80) == 0x80)
                {
                    current <<= 1;
                    encodingBytesCount++;
                }
            }
            else
            {
                // Invalid bits structure for UTF8 encoding rule.
                return false;
            }
        }
        else
        {
            // Following bytes, must start with 10.
            if ((current & 0xC0) == 0x80)
            {
                encodingBytesCount--;
            }
            else
            {
                // Invalid bits structure for UTF8 encoding rule.
                return false;
            }
        }
    }

    if (encodingBytesCount != 0)
    {
        // Invalid bits structure for UTF8 encoding rule.
        // Wrong following bytes count.
        return false;
    }

    // Although UTF8 supports encoding for ASCII chars, we regard as a input stream, whose contents are all ASCII as default encoding.
    return !allTextsAreASCIIChars;
}

//判断string是否utf8格式的字符串。
inline bool isUtf8Str(const std::string &str)
{
    return isUtf8Str(str.c_str());
}