Copy a streambuf's contents to a string

问题

Apparently boost::asio::async_read doesn't like strings, as the only overload of boost::asio::buffer allows me to create const_buffers, so I'm stuck with reading everything into a streambuf.
Now I want to copy the contents of the streambuf into a string, but it apparently only supports writing to char* (sgetn()), creating an istream with the streambuf and using getline().

Is there any other way to create a string with the streambufs contents without excessive copying?

回答1:

I don't know whether it counts as "excessive copying", but you can use a stringstream:

std::ostringstream ss;
ss << someStreamBuf;
std::string s = ss.str();

Like, to read everything from stdin into a string, do

std::ostringstream ss;
ss << std::cin.rdbuf();
std::string s = ss.str();

Alternatively, you may also use a istreambuf_iterator. You will have to measure whether this or the above way is faster - i don't know.

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

Note that someStreamBuf above is meant to represent a streambuf*, so take its address as appropriate. Also note the additional parentheses around the first argument in the last example, so that it doesn't interpret it as a function declaration returning a string and taking an iterator and another function pointer ("most vexing parse").

回答2:

It's really buried in the docs...

Given boost::asio::streambuf b, with size_t buf_size ...

boost::asio::streambuf::const_buffers_type bufs = b.data();
std::string str(boost::asio::buffers_begin(bufs),
                boost::asio::buffers_begin(bufs) + buf_size);

回答3:

Another possibility with boost::asio::streambuf is to use boost::asio::buffer_cast<const char*>() in conjunction with boost::asio::streambuf::data() and boost::asio::streambuf::consume() like this:

const char* header=boost::asio::buffer_cast<const char*>(readbuffer.data());
//Do stuff with header, maybe construct a std::string with std::string(header,header+length)
readbuffer.consume(length);

This won't work with normal streambufs and might be considered dirty, but it seems to be the fastest way of doing it.

回答4:

For boost::asio::streambuf you may find a solution like this:

    boost::asio::streambuf buf;
    /*put data into buf*/

    std::istream is(&buf);
    std::string line;
    std::getline(is, line);

Print out the string :

    std::cout << line << std::endl;

You may find here: http://www.boost.org/doc/libs/1_49_0/doc/html/boost_asio/reference/async_read_until/overload3.html

回答5:

One can also obtain the characters from asio::streambuf using std::basic_streambuf::sgetn:

asio::streambuf in;
// ...
char cbuf[in.size()+1]; int rc = in.sgetn (cbuf, sizeof cbuf); cbuf[rc] = 0;
std::string str (cbuf, rc);

回答6:

The reason you can only create const_buffer from std::string is because std::string explicitly doesn't support direct pointer-based writing in its contract. You could do something evil like resize your string to a certain size, then const_cast the constness from c_str() and treat it like a raw char* buffer, but that's very naughty and will get you in trouble someday.

I use std::vector for my buffers because as long as the vector doesn't resize (or you are careful to deal with resizing), you can do direct pointer writing just fine. If I need some of the data as a std::string, I have to copy it out, but the way I deal with my read buffers, anything that needs to last beyond the read callback needs to be copied out regardless.

回答7:

A simpler answer would be to convert it in std::string and manipulate it some what like this

 std::string buffer_to_string(const boost::asio::streambuf &buffer)
 {
  using boost::asio::buffers_begin;
  auto bufs = buffer.data();
  std::string result(buffers_begin(bufs), buffers_begin(bufs) + buffer.size());
 return result;
}

Giving a very concise code for the task.

回答8:

I think it's more like:


streambuf.commit( number_of_bytes_read );

istream istr( &streambuf );
string s;
istr >> s;

I haven't looked into the basic_streambuf code, but I believe that should be just one copy into the string.

回答9:

I tested the first answer and got a compiler error when compiling using "g++ -std=c++11" What worked for me was:

        #include <string>
        #include <boost/asio.hpp>
        #include <sstream>           
        //other code ...
        boost::asio::streambuf response;
        //more code
        std::ostringstream sline;
        sline << &response; //need '&' or you a compiler error
        std::string line = sline.str();

This compiled and ran.

来源：https://stackoverflow.com/questions/877652/copy-a-streambufs-contents-to-a-string

标签

c++

string

boost

boost-asio

streambuf