VS2019: [C6386] Buffer Overrun while to - c++

Question

void Counting_Sort(vector<int>& A)
{
const int size = A.size();
int max = A[0];
for (int i = 1; i < size; i++)
if (max > A[i])
max = A[i];
int* C = new int[max + 1]{ 0 };
for (int i = 0; i < max + 1; i++)
C[A[i]] = C[A[i]] + 1;
for (int i = 1; i < max + 1; i++)
C[i] = C[i] + C[i - 1];
int* B = new int[size];
for (int i = size-1; i >= 0; i--)
B[C[A[i]] - 1] = A[i]; // <-- Warning here
}
I'm not really sure why I get the warning or what exactly it means. Setting size-1 in for loop to size-2 removes the warning, but I don't uderstand why.

Answer 1

I notice four separate issues with your sample code:
The computation of maximum is incorrect. Your condition should be testing if (A[i] > max)
The Counting Sort algorithm's "accumulation step" should be iterating over the input data. That is, the loop should be the following (up to size, not up to max + 1):
for (int i = 0; i < size; i++)
C[A[i]] = C[A[i]] + 1;
The algorithm's final loop forgot to update the destination of each "Counting Sort bin". That is, the final loop should be the following:
for (int i = size - 1; i >= 0; i--) {
B[C[A[i]] - 1] = A[i];
C[A[i]] = C[A[i]] - 1;
}
Don't forget to use delete[] on B and C when you are done with them.
Here is the fully edited result:
#include <iostream>
#include <vector>
void Counting_Sort(std::vector<int>& A) {
const int size = A.size();
int max = A[0];
for (int i = 1; i < size; i++)
if (A[i] > max)
max = A[i];
int* C = new int[max + 1]{ 0 };
for (int i = 0; i < size; i++)
C[A[i]] = C[A[i]] + 1;
for (int i = 1; i < max + 1; i++)
C[i] = C[i] + C[i - 1];
int* B = new int[size];
for (int i = size-1; i >= 0; i--) {
B[C[A[i]] - 1] = A[i];
C[A[i]] = C[A[i]] - 1;
}
for (int i = 0; i < size; i++)
std::cout << "B[" << i << "] is " << B[i] << "\n";
delete[] B;
delete[] C;
}
int main() {
std::vector<int> A = {6, 1, 3, 3, 6, 9};
Counting_Sort(A);
return 0;
}

Answer 2

The compliler says that the size of B is 4xsize bytes long but in some cases, 8 bytes might be written, which might overpass the 4xsize bytes length.
Take the example of A = {10} for instance.