This program shows the difference between initializing and not initializing variables. It is wise to initialize a variable before they are declared.
#include<iostream>
using namespace std;
int main(){
int a;
int b = 10;
cout<<"a = "<<a<<" b = "<<b<<endl;
return 0;
}

You can get different trash value instead of 16, that I got here.
#include<iostream>
using namespace std;
int main(){
int a = 30;
int b = 10;
cout<<"a = "<<a<<" b = "<<b<<endl;
return 0;
}

I got the output 30, as per the above program as variable was initialized.
It is better to initialize the variable before using.