Tutorial :How to read entire stream into a std::string?



Question:

I'm trying to read an entire stream (multiple lines) into a string.

I'm using this code, and it works, but it's offending my sense of style... Surely there's an easier way? Maybe using stringstreams?

void Obj::loadFromStream(std::istream & stream)  {     std::string s;      std::streampos p = stream.tellg();  // remember where we are      stream.seekg(0, std::ios_base::end); // go to the end    std::streamoff sz = stream.tellg() - p;  // work out the size    stream.seekg(p);        // restore the position      s.resize(sz);          // resize the string    stream.read(&s[0], sz);  // and finally, read in the data.  


Actually, a const reference to a string would do as well, and that may make things easier...
const std::string &s(... a miracle occurs here...)  


Solution:1

How about

std::istreambuf_iterator<char> eos;  std::string s(std::istreambuf_iterator<char>(stream), eos);  

(could be a one-liner if not for MVP)

post-2011 edit, this approach is now spelled

std::string s(std::istreambuf_iterator<char>(stream), {});  


Solution:2

I'm late to the party, but here is a fairly efficient solution:

std::string gulp(std::istream &in)  {      std::string ret;      char buffer[4096];      while (in.read(buffer, sizeof(buffer)))          ret.append(buffer, sizeof(buffer));      ret.append(buffer, in.gcount());      return ret;  }  

I did some benchmarking, and it turns out that the std::istreambuf_iterator technique (used by the accepted answer) is actually much slower. On gcc 4.4.5 with -O3, it's about a 4.5x difference on my machine, and the gap becomes wider with lower optimization settings.


Solution:3

You could do

std::string s;  std::ostringstream os;  os<<stream.rdbuf();  s=os.str();  

but I don't know if it's more efficient.

Alternative version:

std::string s;  std::ostringstream os;  stream>>os.rdbuf();  s=os.str();  


Solution:4

You can try using something from algorithms. I have to get ready for work but here's a very quick stab at things (there's got to be a better way):

copy( istreambuf_iterator<char>(stream), istreambuf_iterator<char>(), back_inserter(s) );  


Solution:5

Well, if you are looking for a simple and 'readable' way to do it. I would recomend add/use some high level framework on your project. For that I's always use Poco and Boost on all my projects. In this case, with Poco:

    string text;      FileStream fstream(TEXT_FILE_PATH);      StreamCopier::copyToString(fstream, text);  


Solution:6

Perhaps this 1 line C++11 solution:

std::vector<char> s{std::istreambuf_iterator<char>{in},{}};  

Note:If u also have question or solution just comment us below or mail us on toontricks1994@gmail.com
Previous
Next Post »