If you import a single class with a using directive QtCreator is unable to provide autocompletion.

For example, this works:

#include <vector>
using namespace std;
void main()
{
vector blubb;
cout << blubb.size(); // using autocompletion for .size()
}

While this doesn't work:

#include <vector>
using std::vector;
void main()
{
vector blubb;
cout << blubb.size(); // no autocompletion available
}