C++ program crashes when trying to sort a vector of strings

Question

I'm trying to sort an array of strings in C++, but I am getting the following error message:

terminate called after throwing an instance of 'std::logic_error'  
  what():  basic_string::_M_construct null not valid

The following program causes the previous error. I got the error when v has 17 elements, but everything works fine when v has less elements.

Could someone point me out what is the problem? I'm using gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)

#include <vector>
#include <string>
#include <algorithm>

using namespace std;

bool comp (string s1, string s2) {
    if (s1.size() < s2.size())
        return false;
    else
        return true;
}

int main () {   
    vector<string> v = { "a", "a", "a", "a",
                         "a", "a", "a", "a",
                         "a", "a", "a", "a",
                         "a", "a", "a", "a",
                         "a" };
    
    sort(v.begin(), v.end(), comp);
    return 0;
}

Answer 1

The comparator you pass to sort must satisfy the named requirement Compare:

Establishes strict weak ordering relation with the following properties

For all a, comp(a,a)==false
If comp(a,b)==true then comp(b,a)==false
if comp(a,b)==true and comp(b,c)==true then comp(a,c)==true

With your comparator: comp(a,a) == true. As you do not fullfill the preconditions of std::sort your code has undefined behavior.