在rdbuf之后重用stringstream

Question

Is it possible to reuse a stringstream s after pushing the buffer to another stream stream with the .rdbuf() function?

I reconstructed the circumstances:

http://ideone.com/JoPJ1E

#include <iostream>
using namespace std;
#include <fstream>
#include <sstream>
#include <assert.h>

ofstream f("t.txt");

void dump(stringstream & s){
    f << s.rdbuf();
    assert(f.good()); // THIS ASSERT FAILS IN my code (see main)
}

void doit1(){

  static std::stringstream s;

  s.str("");
  s.clear();
  s.seekp(0);
  s.seekg(0);

  s << "1";
  dump(s);

}

void doit2(){
  // your code goes here
  std::stringstream s;
  s << "2";
  dump(s);

}

int main() {
    // your code goes here
    doit2();
    doit1(); // ASSERT FAILS HERE
}

My program does not crash, and there is no output in the text file! The assert fails exactly by calling doit1(), why does doit2 set the stream f in a bad state??

Any idea what could be wrong here?

Answer 1

It seems a long-standing MSVC design issue by Dinkumware (the STL-provider for Microsoft) when setting a 0-position with seekp on a stream associated with an empty content. Apparently they did this to have the compiler conform to the Perennial C++ test suite and that the standard dictated that.

I find this not very clear with N3797 since §27.7.3.5 basic_ostream seek members/p3 says

basic_ostream& seekp(pos_type pos);

3 Effects: If fail() != true, executes

rdbuf()->pubseekpos(pos, ios_base::out) . In case of failure, the function calls setstate(failbit) (which may throw ios_base::failure).

4 Returns: *this.

And calling pubseekpos (equivalent) directly doesn't trigger any bug.

Tested with MSVC2013Update4:

int main() {
    std::stringstream s;
    s.str("");
    if (s.fail())
        cout << "bad"; // not printed
    s.seekp(0);
    // s.rdbuf()->pubseekpos(0, ios_base::out); // Equivalent as per 27.7.3.5/3
    if (s.fail())
        cout << "bad"; // printed
}

Clang and gcc work just fine.