Why accessing out of bounds not producing any error in my code?

Question

This is my function, I have run with -std=c++17 -Wall -Wextra with clang and gcc on godbolt;

both compilers did not produced any errors;

function returns 0, when empty needle; returns -1 when needle not found and returns index of first occurrence when match.

int strStr(string haystack, string needle) {

        if ( needle.empty( ) ) return 0;

        std::size_t n_len = needle.size();

        if( needle.size() > haystack.size() ) return -1;

        int index = 0;

        for ( auto it = haystack.begin(); it != haystack.end(); ++it )
        { 

            std::cout << std::string( it, it + n_len ) << std::endl; 
            if ( std::string( it, it + n_len ) == needle ) return index; // hear also
            //if ( !*(it + n_len ) ) break;
            index++;
        }

        return -1;

    }

I know I can get the results using find or substr;

I have tested with below, it should produce error, when I access out of bound, but no errors;

int main()
{
    
    std::cout << strStr( "aabaaaababaababaa", "bbbb") << std::endl; // prints -1
    std::cout << strStr( "hello", "ll") << std::endl; // prints 2
    std::cout << strStr( "mississippi", "issip") << std::endl; // prints 4
    std::cout << strStr( "ab", "a") << std::endl; // prints 0
}

Thank you for your time and assistance;

Answer 1

it should produce error, when I access out of bound

No.

The behaviour of accessing an array out of bounds is undefined. There is no guarantee that you would get an error.

but I have tested same code leetcode platform, it produced runtime error

There is also no guarantee that you would not get an error. There are no guarantees whatsoever.