C++ initializes value for variable even though I left it empty, why does this happen?

Question

For some reason, the value of final_sum is 16 but I didn't initialize a value for that variable, why's that? Isn't supposed to start with 0 ?

#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

int main()
{
    int n, sum, final_sum;

    cout<<"ff: "<<final_sum<<endl;
    cout<<"Input the value for n: ";
    cin>>n;

    for(int i=1; i<=n; i++){
        sum += i;
        final_sum += sum;
        cout<<"sum: "<<sum<<endl;
        cout<<"final sum: "<<final_sum<<endl<<endl;
    }
    return 0;
}

Answer 1

Isn't supposed to start with 0

No, its initial value is undefined. See https://en.cppreference.com/w/cpp/language/default_initialization

the objects with automatic storage duration (and their subobjects) are initialized to indeterminate values.

If you do

 int final_sum{}; 

Or

 int final_sum = 0; 

You will have a 0 in your variable.

---

I suggest you to use -Wuninitialized (or better -Wall ). The compiler will raise a warning if you forgot and initialization

Answer 2

If you don't initialize a variable before reading/using it, its undefined behavior . Its value can then be anything, it's unpredictable and always a bad idea. Use

int final_sum = 0;

to initialize it.

From documentation :

The value in an uninitialized variable can be anything – it is unpredictable, and may be different every time the program is run. Reading the value of an uninitialized variable is undefined behaviour – which is always a bad idea. It has to be initialized with a value before you can use it.

and

The only place where they are useful is when you are about to read the variable in from some input stream.

Edit: Some may ask "Why my variable still print 0 even though I haven't initialize it?"

From this post:

That is because variables with automatic storage duration are not automatically initialized to zero in C++. In C++, you don't pay for what you don't need, and automatically initializing a variable takes time (setting to zero a memory location ultimately reduces to machine intruction(s) which are then translated to electrical signals that control the physical bits).

So when you do:

int final_sum;

The final_sum is just being reserved a memory location, and anything currently inside that location will be printed out by cout . It just happened that there's a big 0 in that spot.

More info: (Why) is using an uninitialized variable undefined behavior?

Answer 3

Declaring the variable globally does the job

int final_sum;
int main() {
     your code ...
}

is the same as

int final_sum = 0;
int main() {
    your code here ...
}

However, you probably don't want to use global variables and it is always recommended to initialize variables with some value.