using namespace std;

int main()
    string a;
    vector<string> b;
    vector< vector<string> > s;
    while(cin>>a)  {


    return 0;

Any problems with this? If line A2 is deleleted,the program can run.So I think s does exist,but when I add line A2 hoping to see the last element of vector of s ,Cannot run! My IDE:codeblocks(svn build rev 4977 (2008-03-29 18:01:31) gcc 4.2.1 Windows/unicode)
Somebody give a hand? Thanks.

That will not work because the elements in 's' are vector<string> vectors themselves,
ostream operator << has no support for that kind of thing by default.
If you really want to pass e.g. a vector<string> to cout, you can write;

ostream & operator <<(ostream & o, const vector<string> & v)
	for(vector<string>::const_iterator it = v.begin(); it != v.end(); ++ it)
		o << *it;

	return o;