Performance comparison between string::at and string::operator[] in C++

Check following testPalindrome_index.cpp program which utilizes string::operator[]:

#include <string>

bool isPalindrome(
    std::string& s,
    std::string::size_type start,
    std::string::size_type end) {

        auto count = (end - start + 1) / 2;
        for (std::string::size_type i = 0; i < count; i++) {
            if (s[start] != s[end]) {
                return false;
            }
            start++;
            end--;
        }

        return true;
}

int main() {
        std::string s(1'000'000'000, 'a');

        isPalindrome(s, 0, s.size() - 1);
        return 0;
}

My compile is clang++, and measure the execution time without & with optimization:

# c++ -std=c++14 testPalindrome_index.cpp -o index
# time ./index
    0m13.84s real     0m12.77s user     0m01.06s system
# c++ -std=c++14 -O2 testPalindrome_index.cpp -o index
# time ./index
    0m01.44s real     0m00.42s user     0m01.01s system

We can see the time differences are so large (13.84 vs 1.44)!

Then change the code to use string::at:

#include <string>

bool isPalindrome(
    std::string& s,
    std::string::size_type start,
    std::string::size_type end) {

        auto count = (end - start + 1) / 2;
        for (std::string::size_type i = 0; i < count; i++) {
            if (s.at(start) != s.at(end)) {
                return false;
            }
            start++;
            end--;
        }

        return true;
}

int main() {
        std::string s(1'000'000'000, 'a');

        isPalindrome(s, 0, s.size() - 1);
        return 0;
}

Compile and test again:

# c++ -std=c++14 testPalindrome_at.cpp -o at
# time ./at
    0m07.31s real     0m06.36s user     0m00.96s system
# c++ -std=c++14 -O2 testPalindrome_at.cpp -o at
# time ./at
    0m06.42s real     0m05.45s user     0m00.97s system

We can see the time gap is nearly 1 second, and not outstanding as the first case. But the time with “-O2” optimization is 6.42, far bigger than 1.44which uses string::operator[].

The conclusion is if the string is long enough, the performance bias of using string::operator[] and string::at is remarkable. So this factor should be considered when decide which function should be used.

P.S., the full code is here.