Why does the code throw illegal memory access error - c++

Question

I want to know why the error appears when the code just gets into the function '
sort'
I made some check points using standard output. So I know where the error occurs.
I use repl.it to build this code
...
/*return pivot function*/
int partition(...){
...
}
void sort(vector<int> array, int left, int right){\
/*********"sort start" string dose not appear in console***********/
cout << "sort start";
// one element in array
if(left == right);
// two elements in array
else if( left +1 == right){
if(array.at(0) > array.at(1)){
int temp;
swap(array.at(0),array.at(1),temp);
}
}
// more then 3 elements in array
else{
int p = partition(array,left,right);
sort(array,left,p-1);
sort(array,p+1,right);
}
}
int main() {
vector<int> array;
array.push_back(1);
array.push_back(2);
array.push_back(3);
array.push_back(4);
cout << "array is ";
for(int i = 0 ; i < array.size(); i++){
cout << array.at(i) << " ";
}
cout << endl;
sort(array,0,array.size()-1);/***************sort is here*************/
cout << "sorting..." << endl;
cout << "array is ";
for(int i = 0 ; i < array.size(); i++){
cout << array.at(i) << " ";
}
return 0;
}
When I run this code console output is
array is 4 3 2 2
terminate called after throwing an instance of 'std::out_of_range'
what(): vector::_M_range_check: __n (which is 18446744073709551615) >=
this->size() (which is 4)
exited, aborted
But what I expected is
array is 4 3 2 2
sorting...
sort start
array is 2 2 3 4

Answer 1

You are trying to access an element at index -1, which, when converted into 64-bit unsigned value, is 18446744073709551615. Live demo: https://wandbox.org/permlink/jAhJZS3ANjkDDOUr.
There are multiple problems with your code and, first of all, you don't show us the definition of your partition and swap. Moreover, your code does not match the provided output (1 2 3 4 vs 4 3 2 2 in the first line).
Anyway, one of the problems is that you don't check for cases where left is higher than right. That can easily happen. Consider that in the very first call of sort, partition returns 0 (pivot position). Then, you call:
sort(array, left, p - 1);
which turns into
sort(array, 0, -1);
That's where negative indexes can be generated.