a question about ostringstream

[Aman JIANG]

hello, i am a beginner to stringstream.

int main()
{
ostringstream oss;
for (int i = 0; i < 100; i++) {
oss << "word " << endl;
cout << oss.tellp() << " ";
}
}

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

why the result of ostringstream::tellp() is always zero ?

but ofstream::tellp() works well:

int main()
{
ofstream ofs("test.txt");
for (int i = 0; i < 100; i++) {
ofs << "word " << endl;
cout << ofs.tellp() << " ";
}
}

please 'tell' me why