difference between new int[100] and new int[100]();

As title

#include <iostream>
int main() {
    auto* a = new float[1000000];
    auto* b = new float[10]();
    for(auto i=0; i<1000000; i++){
        std::cout << "a" << a[i] << std::endl;
    }
    for(auto i=0; i<10; i++){
        std::cout << "b" << b[i] << std::endl;
    }
    return 0;
}

what's the difference? I had tried both output is zero.

In addition what's about smart pointer, how to make sure it can zero initialized.

std::unique_ptr<int[]> p = std::make_unique<int[]>(100);