在C++的char*以及string中，使用的是字节流编码，即sizeof(char) == 1。

也就是说，C++是不去分字符的编码的。

而一个合法UTF8的字符长度可能为1～4位。

现在假设一串输入为UTF8编码，如何能准确的定位到每个UTF8字符的“CharPoint”，而不会错误的分割字符呢？

参考这个页面：http://www.nubaria.com/en/blog/?p=289

可以改造出下面的函数：

const unsigned char kFirstBitMask = 128; // 1000000
const unsigned char kSecondBitMask = 64; // 0100000
const unsigned char kThirdBitMask = 32; // 0010000
const unsigned char kFourthBitMask = 16; // 0001000
const unsigned char kFifthBitMask = 8; // 0000100
  
int utf8_char_len(char firstByte)
{
    std::string::difference_type offset = 1;

    if(firstByte & kFirstBitMask) // This means the first byte has a value greater than 127, and so is beyond the ASCII range.
    {   
        if(firstByte & kThirdBitMask) // This means that the first byte has a value greater than 224, and so it must be at least a three-octet code point.
        {   
            if(firstByte & kFourthBitMask) // This means that the first byte has a value greater than 240, and so it must be a four-octet code point.
                offset = 4;
            else
                offset = 3;
        }   
        else
        {   
            offset = 2;
        }   
    }   
    return offset;
}

每次传入当前UTF8流的第一个char，然后会根据UTF-8的编码结构，返回这个utf8编码的字符占用了多少字节。也即charpoint的位置。