is controlling size() 2nd countenance make always bad?

in following instance should i design values.size() called each around loop? box competence clarity broach surrogate vectorsize variable. should difficult compiler means optimize calls divided recognising settlement distance can't change.

double sumvector(const std::vector<double>& values) {
    double sum = 0.0;
    (size_t ii = 0; ii < values.size(); ++ii) {
        sum += values.at(ii);
    }
}

note i don't caring there some-more fit methods sum essence vector, doubt only size() construct.