Why does modifying an std::string out-of-range cause no runtime errors?

Question

I was playing around with C++'s std::string class, and for fun, I declared a string and tried to modify out-of-bounds characters:

#include <iostream>
#include <fstream>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>

using namespace std;

int main()
{
    string str = "Hello World!";
    str[5] = 20;
    str[20] = 10;
    str[200] = 4;

    cout << str << endl;

    return 0;
}

To my surprise, nothing happened during the program's execution at all (except for str[5] = 20). I expected the program to crash with a Segmentation Fault, and I'm not quite sure why this doesn't happen. Is this a side effect of "undefined behavior", or is something else at work here?

EDIT:

I even tried adding

cout << str[-1] << endl;
cout << str[200] << endl;

And no runtime errors occurred; it just printed a blank string for both of those lines.

Checks will be only done with [`std::string::at()`.](https://en.cppreference.com/w/cpp/string/basic_string/at) — πάντα ῥεῖ, Sep 28 '20 at 16:34
Not a side effect but the very meaning of undefined behavior. It's undefined which mean anything could happen. Anything includes appearing to work. — TheUndeadFish, Sep 28 '20 at 16:34
Yes, this is Undefined Behavior in action. It could "work", it could crash, and do many other things in-between. — NathanOliver, Sep 28 '20 at 16:35

Why does modifying an std::string out-of-range cause no runtime errors?

0 Answers0