Accessing elements of a vector in C++?

Question

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.

Answer 1

std::vector::at() guards you against accessing array elements out of bounds by throwing a std::out_of_range exception unlike the [] operator which does not warn or throw exceptions when accessing beyond the vector bounds.

std::vector is/was considered as a 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 a [] operator for accessing its elements. At the same time, the 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.

Naturally, the std::vector::at() method checks for the size of the vector before dereferencing it and that will be a little overhead (perhaps negligible in most use cases) over accessing elements by [], So std::vector provides you both the options to be safe or to be faster at expense of managing the safety yourself.

Answer 2

As others have mentioned, at() performs bounds checking and [] does not. Two reasons I can think of to prefer [] are:

1. Cleaner syntax
2. Performance. When looping through the elements of a vector, it is often overkill and very costly to perform bounds checking on every iteration.

Answer 3

at()

Pros:

• safe because exception is thrown if array out of bounds

Cons:

• slow access
• more characters to type

operator[]

Pros:

• fast access because of missing bounds checks
• less characters to type
• 'intuitive' array element access

Cons:

• unsafe because of missing bounds checks