I often found people use the array brackets  and a normal vector function .at (). Why are there two separate methods? What are the benefits and disadvantages of both? I know that .at () is safer, but are there any situations where .at () cannot be used? And if .at () is always safer, why ever use array brackets .
I searched around but couldn't find a similar question. If a questions like this already exists please forward me to it and I will delete this question.
std::vector::at() guards you against accessing array elements out of bounds by throwing an
out_of_bounds exception unlike
 operator which does not warn or throw exceptions when accessing beyond the vector bounds.
std::vector is/was considered as an c++ replacement/construct for Variable Length Arrays(VLA) in c99. In order for c-style arrays to be easily replacable by
std::vector it was needed that vectors provide a similar interface as that of an array, hence vector provides
 operator for accessing its elements. At the same time, C++ standards committee perhaps also felt the need for providing additional safety for
std::vector over c-style arrays and hence they also provided
std::Vector::at() method which provides it.
at() method checks for the size of the vector before dereferncing it and that will be a little overhead(perhaps negligible in most use cases) over accessing elements by
std::vector provides you both the options to be safe or to be faster at expense of managing the safety yourself.