cout a stringstream twice ends up in corrupted cout (minimal example)

Question

I have the following code, and I dont quite understand why the results happens to be like the one below:

#include <iostream>
#include <sstream>

using namespace std;
int main () {

   std::stringstream s;

   std::streambuf *backup;
   backup = cout.rdbuf();


   s << "Oh my god" << endl;


   cout << "Before1" << endl;
       cout << s.rdbuf();
   cout << "After1" << endl;


   cout << "Before2" << endl;
   //s.seekg(0, std::ios_base::beg); // If that is in: After 2 is printed!
   cout << s.rdbuf();
   //cout.rdbuf(backup); // If that is in, also After 2 is printed!
   cout << "After2" << endl;

}

Output:

Before1
Oh my god
After1
Before2

Where is the rest??¿ It only gets outputted when We uncomment the above lines... What happens internally? Does anybody know? =) Would be interesting...

Answer 1

Check whether the fail bit is set on cout. You could also just clear the fail bit, with cout.clear().

---

Here's the rule from the Standard (section 27.7.3.6.3) that requires that the fail bit will be set in this case:

basic_ostream<charT,traits>& operator<<(basic_streambuf<charT,traits>* sb);

Effects: Behaves as an unformatted output function. After the sentry object is constructed, if sb is null calls setstate(badbit) (which may throw ios_base::failure).

Gets characters from sb and inserts them in *this. Characters are read from sb and inserted until any of the following occurs:

• end-of-file occurs on the input sequence;
• inserting in the output sequence fails (in which case the character to be inserted is not extracted);
• an exception occurs while getting a character from sb.

If the function inserts no characters, it calls setstate(failbit) (which may throw ios_base::failure). If an exception was thrown while extracting a character, the function sets failbit in error state, and if failbit is on in exceptions() the caught exception is rethrown.

Returns: *this.