初始化向量的向量大小

Question

How do I declare a vector of size 30 with 30 empty vectors. I've tried

vector< vector<T> > vec(30);

But it returns with errors:

./HashSet.h:62:28: error: expected parameter declarator
         vector< vector<T> > table(30);
                              ^
./HashSet.h:62:28: error: expected ')'
./HashSet.h:62:27: note: to match this '('
    vector< vector<T> > table(30);

Answer 1

This creates a vector of 30 vectors of 30 elements each:

vector< vector<T> > vec(30, vector<T>(30));

But it requires that T is default-constructible.