c++ cannot fill a stringstream anew

Question

I emptied a stringstream, then I tried to fill it again without any success. II don't understand why. Here is my code:

#include <string> #include <iostream> #include <sstream> using namespace std; int main(int argc, char* argv[] ) { stringstream ss1, ss2; ss1 << "0 1 2 3 40 5 6 76 8 9"; // Stream is filled with one string of chars vector <int> V; string st; int number; while(!ss1.eof() ) { ss1 >> number; // Next int found in ss1 is passed to number V.push_back(number); ss2 << number << " "; // ss2 is filled with number + space in each iteration. } // Basically here, the content of ss1 has been passed to ss2, leaving ss1 empty. ss1 << "helloooo"; getline(ss1, st); cout << st << endl; // <--- Here, st appears to be empty... Why ? return 0; }

while (!eof()) is really buggy. Use while (ss1 >> number). — Qaz
– Qaz, Commented Aug 25, 2013 at 17:45
It works your way too, but why would eof() be buggy? Anyway my ss1 stringstream still can't take another string :( How comes? — Alexandre Toqué
– Alexandre Toqué, Commented Aug 25, 2013 at 17:54
The problem with eof() is that it may never be reached, e.g., when you add a non-digit into your string: the stream will go into failure mode (i.e., std::ios_base::failbit gets set) but unless you get over that, it won't ever reach EOF (i.e., std::ios_base::eofbit will never be set). More importantly, you need to check your stream after you tried to read because the stream doesn't know what you are going to attempt. — Dietmar Kühl
– Dietmar Kühl, Commented Aug 25, 2013 at 17:57

Dietmar Kühl · Accepted Answer · 2013-08-25 17:52:22Z

First off, you should check if reading from the stream is successful by converting the stream to a Boolean value after you tried to read from it, e.g.:

while (ss1 >> number) { ... }

Not testing after the input tends to result in processing the last input twice. Now, once this loop terminates, ss1 is in failure state, i.e., it has std::ios_base::failbit set. At this point the stream will refuse to do anything else until the bit is cleared. You can use clear() to reset the stream state:

ss1.clear();

After that the stream should be in good shape again.

Last input was indeed repeated twice. Thank you it perfectly worked :)

youdontneedtothankme · Accepted Answer · 2013-08-25 17:52:54Z

Since you hit eof, the stream is in an error state. You have to reset it before you can use it again. In your case, I would drop the reset, and just use a fresh stringstream object.

Oh, and after ss1 >> number you should check the state of ss1 before using number. eof() doesn't return true before the last read failed

Jerry Coffin · Accepted Answer · 2013-08-25 18:35:39Z

Although you'll still need to clear() the stream to write to it after you finish reading, you might consider using istream_iterators to read the data from the file:

stringstream ss1("0 1 2 3 40 5 6 76 8 9"); // initialize V from ss1 vector <int> V{std::istream_iterator<int>(ss1), std::istream_iterator<int>()}; // write values from V to ss2 std::copy(V.begin(), v.end(), std::ostream_iterator<int>(ss2)); ss1.clear(); ss1 << "helloooooo";

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