Question

bool hasId(string id, vector<User>& map)
{
    User ans = *(find_if(map.begin(), map.end(), [&](User d)
    {   return (id==(d).uid());}));
    return ans.uid() == id;
}

Answer 1

If no matches are found then find_if returns last(), in your case, map.end(). end() does not return a valid iterator (it’s one past the last element), yet you assume that a match is always found and proceed to dereference the return value unconditionally. This is a problem.

You need to perform a check before the dereference. Without the check your function simply assumes that a match is always found as ans.uid() == id is the predicate to find_if (and thus redundant), so you may as well just replace the entire thing with return true; as it stands 🙂

bool hasId(string id, vector<User>& map)
{
    return map.end() != find_if(map.begin(), map.end(), [&](User d)
    {   
        return id == d.uid();
    });   
}

On a side note, it’s kinda weird to call a vector a map, confusing at least.

---

Documentation for find_if

Relevant bit:

template< class InputIt, class UnaryPredicate >
InputIt find_if( InputIt first, InputIt last, UnaryPredicate p );

Return value

Iterator to the first element satisfying the condition or last if no such element is found.