Why is using “vector.at(x)” better than “vector[x]” in C++?

Question

If I want to get to a value in vector I can use two options : use the [] operator. Or I might use the function .at example for using :

vector ive         


        

Answer 1

The only difference between at and [] is that at performs a range check, and [] does not. If you have checked the range already or have constructed your index in such a way that it cannot get out of range, and need to access an item repeatedly, you could save a few CPU cycles by opting for [] instead of an at.

Example of a single check and multiple accesses:

size_t index = get_index(vect);
if (index < 0 || index >= vect.size()) return;
if (vect[index] > 0) {
    // ....
} else if (vect[index] > 5) {
    // ....
} else ....

Example of a case when the index is constructed to be inside limits:

for (size_t i = 0 ; i != vect.size() ; i++) {
    if (vect[i] > 42) {
        // ....
    }
}