Vector going out of bounds without giving error

Question

I have a std::vector. I check its size which is 6 but when I try to access vec[6] to check whether it will give error, I get no error but some number instead. Should not it give an error?

edit: something like:

struct Element
{
    std::vector<double> face;
};

int main()
{
    Element elm;

    .... // insert 6 elements into elm.face

    std::cout << elm.face.size() << std::endl; // answer is 6
    std::cout << elm.face[6] << std::endl; // answer is some number
}

Answer 1

STL vectors perform bounds checking when the .at() member function is called, but do not perform any checks on the [] operator.

When out of bounds, the [] operator produces undefined results.