well anyways here is a way you could do it.. though not sure why you would unless it somehow saves computing time compared to the for loop method. In this example two arrays are created so it requires twice as much memory as a for loop, but maybe less CPU cycles to complete? I don't know, you would have to ask someone else as I've never attempted using memcmp() before. There might be a way to memory compare the entire array x with a 0, but I am not aware of how to do it.
Code:
#include <iostream>
int main()
{
int x[8][8] = {0}; // Your array.
int y[8][8] = {0}; // Array of all zeroes.
std::cout << memcmp(x, y, sizeof(x)); // Should output 0 since x array is all zeroes. MEMCMP() stands for memory compare.
x[5][5] = 5;
std::cout << memcmp(x, y, sizeof(x)); // Should output 1 since x array is not all zeroes anymore.
return 0;
}