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

Answer 1

My answer uses an intermediate buffer, but perhaps you will be happy that it uses iterators to initialize the string from the buffer.

std::vector<char> buffer(128); // create a buffer
ifs.read( &buffer[0], buffer.size() ); // read to buffer
std::string first128( buffer.begin(), buffer.end() ); // copy from vector

To me it seems like they got a little too cute with the implementation of iostreams. Trying to use iterators for stream I/O is overly complicated.

By the way, I suspect the implementation that you were attempting will, under the covers, do a variety of intermediate buffering (perhaps some in the kernel, some in the library) as well as re-allocating and copying the string several times as it grows.

One other idea: Do you really need the result in a standard string? You might just work from the vector -- avoiding the final step of copying to a string. Or, if you are feeling adventurous, you could create your own string class that does allow you to expose the internal buffer in the same way that vector does.

Answer 2

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.

Answer 3

char buffer[129];
ifs.read (buffer,128);
buffer[128] = '\0';
first128 = buffer;

How about this then:

template <typename Itr, typename Out>
void copy_n(Itr it, size_t count, Out out)
{
    for(size_t i=0;i<count;++i)
      out = *it++;
} 

...

std::string first128; 
std::istreambuf_iterator<char> it(ifs);
copy_n( it, 128,
  std::back_inserter<std::string>(first128) );