why c++ std::string.length() is slower than strlen() on VS2017?

Question

I have tested c++ std::string.length() vs strlen() function on VS2017 as below. The result is:

what surprised me is that string.length() is 7x slower than strnlen(). But I suppose string.length() is an O(1) operation, and strlen() is an O(n) operation.

I have also tested it on GNU GCC v7.1.1 on coding ground

And it shows string.length() is slight faster than strlen() (not as much as I expected.)

Why is that? something wrong in my test code?

class stopwatch
{
public:
    stopwatch()
    {
        start = std::chrono::system_clock::now();
    }
    ~stopwatch()
    {
        auto end = std::chrono::system_clock::now();
        auto elasped = std::chrono::duration_cast<std::chrono::microseconds>(end - start);
        cout << "elasped: " << elasped.count() << endl;
    }

private:
    chrono::system_clock::time_point start;
};

void test_std_str(const string& ss)
{
    stopwatch sw;
    const int max = 100000;
    int sum = 0;
    for (int i = 0; i < max; i++)
    {
        sum += ss.length();
    }
    cout<< "test_string: sum = " << sum << endl;
}

void test_c_str(const char* str)
{
    stopwatch sw;
    const int max = 100000;
    int sum = 0;
    for (int i = 0; i < max; i++)
    {
        sum += strlen(str);
    }
    cout << "test_c_str: sum = " << sum << endl;
}

int main()
{
    std::string ss = "abcdef";
    const char* str = "abcdef";
    int x;
    cin >> x;
    if (x > 0)
    {
        ss = "helloworld";
        str = "helloworld";
    }
    test_std_str(ss);
    test_c_str(str);
    return 0;
}

Answer 1

To answer my own question:

As suggested by @Ken Y-N and @PaulMcKenzie, I localize the scope of stopwatch to exclude the time of cout, and build it with Release build. The result is make sense to me now!