why i need clear stringstream before reuse?

Question

For example:

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

int main() {
    int num = 10;
    string str;
    stringstream toString;
    toString << num;
    toString >> str;
    cout << str << "\n"; //10

    int num2 = 20;
    string str2;
    toString << num2;
    toString >> str2;
    cout << str2 << "\n"; //str2 is empty
    return 0;
}

I know that I must clear this like:

toString.str("");
toString.clear();

But why doesn't it clear automatically after using operator >>?

Why would it? What if there's more stuff in the stream that you want to get later? — Borgleader, Jan 12 '15 at 16:53
You may wish to use a series of >> operators before checking the flags. — Neil Kirk, Jan 12 '15 at 16:54
Similar reason to why `cout` doesn't automatically print a new line after every `<<` — Alex, Jan 12 '15 at 16:54
[Don't use `using namespace std;`](http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) — Baum mit Augen, Jan 12 '15 at 17:44
`toString` already has a specific meaning to other programmers. Almost any other name would be preferable; even `myStream` or simply `s` would suffice. — Robert Harvey, Jan 12 '15 at 17:55

score 6 · Accepted Answer · answered Jan 12 '15 at 16:54

6

If I do toString >> a >> b >> c and the first one fails, I don't want the flag to be cleared so that the final state appears to have succeeded.

answered Jan 12 '15 at 16:54

Jonathan Wakely

166,810
27
341
521

lrineau · Answer 2 · 2015-01-12T17:57:45.650

After the first read toString >> str;. then the rdstate() of toString is std::ios_base::eofbit, because during the reading of str the end of the string has been reached.

Then the line toString << num2; does not modify the string stored in toString but sets the failbit of toString. That is the standard behavior of all formatted output.

The line toString >> str2; does nothing: the failbit is already set, and no reading is performed: str2 stays empty.

The clear() function resets the rdstate() of toString to std::ios_base::goodbit.

The reasons why the >> must not call clear() are:

in case there is an error, then one must be able to test it using the functions bad(), fail(), eof() (or rdstate() directly),

and one can use the operator >> several times:

#include <iostream>
#include <sstream>

int main() {
  std::stringstream sstr("10 20");
  int i, j;
  sstr >> i >> j;
  if(!sstr.fail()) {
    std::cout << i << " " << j << "\n";
    return 0;
  }
  return 1;
}

why i need clear stringstream before reuse?

2 Answers2