how can I read exactly 128 bytes from an fstream into a string object? [duplicate]

Answer 1

Here i have some of researching over streambuffer, read directly to string from istream by constructor:

class mystringbuf : public std::stringbuf
{
public:
    explicit mystringbuf(std::istream& istr, size_t n,
                        std::ios_base::openmode __mode = std::ios_base::in )
    {
        _M_string.resize(n);
        std::stringbuf::_M_stringbuf_init(__mode);
        istr.read(gptr(), n);
    }
public:
    std::stringbuf::char_type* gptr() const
    {
        return std::stringbuf::gptr();
    }
    std::string& str_ref(){
        return _M_string;
    }
};
std::ostream& operator << (std::ostream& ostr, mystringbuf& buf){
    ostr << buf.str_ref();
    return ostr;
}

Example to use:

using std::cout;
using std::endl;

int main()
{
    std::stringbuf buffer;          // empty buffer
    buffer.str("abc def ABC DEF "); // not empty now
    std::istream is (&buffer);      // associate stream buffer to stream
    mystringbuf data(is, 10);       // read 10 bytes
    cout << "data=" <<  data << endl;
    return 0;
}

Output:

data=abc def AB

Please forward me if i`am somewhere was wrong.