iterator to a vector of vector of int

Question

I have an error in the following code where I want to print the first element in each sub-vector:

vector<vector<int>> logs{{0, 0}, {1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
for (auto beg = logs.begin(); beg != logs.end(); beg++) {
    cout << *beg[0] << endl;
}

the error is from cout << *beg[0]...:

Indirection requires pointer operand ('std::vector<int, std::allocator<int>>' invalid)

So my question is: the dreference of the iterator should a sub-vector in the vector "logs", so I use subscript to access the first element of it. Why there's a std::vector<int, std::allocator<int>> object? Where does the std::allocator<int> come from? How can I access the elements in the sub-vectors?

Answer 1

The problem(cause of the mentioned error) is that due to operator precedence, the expression *beg[0] is grouped as(or equivalent to):

*(beg[0])

which can't work because beg is an iterator and has no [] operator. This is because the operator [] has higher precedence than operator *.

To solve this replace cout << *beg[0] << endl; with:

cout << (*beg)[0] << endl;  //or use beg->at(0)

Demo