Reading directly from an std::istream into an std::string

Question

Is there anyway to read a known number of bytes, directly into an std::string, without creating a temporary buffer to do so?

eg currently I can do it by

boost::uint16_t len;
is.read((char*)&len, 2);
char *tmpStr = new char[len];
is.read(tmpStr, len);
std::string str(tmpStr, len);
delete[] tmpStr;

Answer 1

std::string has a resize function you could use, or a constructor that'll do the same:

boost::uint16_t len;
is.read((char*)&len, 2);

std::string str(len, '\0');
is.read(&str[0], len);

This is untested, and I don't know if strings are mandated to have contiguous storage.

Answer 2

You could use a combination of copy_n and an insert_iterator

void test_1816319()
{
    static char const* fname = "test_1816319.bin";
    std::ofstream ofs(fname, std::ios::binary);
    ofs.write("\x2\x0", 2);
    ofs.write("ab", 2);
    ofs.close();

    std::ifstream ifs(fname, std::ios::binary);
    std::string s;
    size_t n = 0;
    ifs.read((char*)&n, 2);
    std::istream_iterator<char> isi(ifs), isiend;
    std::copy_n(isi, n, std::insert_iterator<std::string>(s, s.begin()));
    ifs.close();
    _unlink(fname);

    std::cout << s << std::endl;
}

no copying, no hacks, no possibility of overrun, no undefined behaviour.

Answer 3

You could use something like getline:

#include <iostream>
#include <string>
using namespace std;

int main () {
  string str;
  getline (cin,str,' ');
}