What happens to uninitialized variables? C++

int main()
{    
    int a;
    cout << a;
    return 0;
}

I am wondering why the value 0 is being output. I thought if a variable is uninitialized, it would output a garbage value.

However, I also remember hearing that the default value of an integer is 0 so I am a bit confused.

Thanks

Solution 1:

The default behavior of an uninitialized function scope (i.e., local) integer in C++ is for it to be indeterminate, which is fine; however if that value is used before it is defined it introduces undefined behavior, and anything could happen - demons could fly out of your nose.

This page on cppreference provides examples of default integer behavior.

On the other hand, all non-local, thread-local variables, not just integers, are zero initialized. But this case wasn't included in your original example.

(Side note: It is generally considered good practice to simply initialize variables anyway and avoid potential hazards altogether... Especially in the form of global variables. )

There are exceptions to best practice using global variables in rare special cases, such as some embedded systems; which initialize values based off of sensor readings on startup, or during their initial loop iteration... And need to retain a value after the scope of their loop ends.

Solution 2:

I think you are not convinced with the answers/comments given, may be you can try the below code:

#include <iostream>
using namespace std;

int main(){

 int a,b,c,d,e,f,g,h,i,j;

 cout<<a<<endl;
 cout<<b<<endl;
 cout<<c<<endl;
 cout<<d<<endl;
 cout<<e<<endl;
 cout<<f<<endl;
 cout<<g<<endl;
 cout<<h<<endl;
 cout<<i<<endl;
 cout<<j<<endl;

 return 0;
}