Variable initialization in C++

Question

My understanding is that an int variable will be initialized to 0 automatically; however, it is not. The code below prints a random value.

int main () 
{   
    int a[10];
    int i;
    cout << i << endl;
    for(int i = 0; i < 10; i++)
        cout << a[i] << " ";
    return 0;
}

• What rules, if any, apply to initialization?
• Specifically, under what conditions are variables initialized automatically?

Answer 1

It will be automatically initialized if

• it's a class/struct instance in which the default constructor initializes all primitive types; like MyClass instance;
• you use array initializer syntax, e.g. int a[10] = {} (all zeroed) or int a[10] = {1,2}; (all zeroed except the first two items: a[0] == 1 and a[1] == 2)
• same applies to non-aggregate classes/structs, e.g. MyClass instance = {}; (more information on this can be found here)
• it's a global/extern variable
• the variable is defined static (no matter if inside a function or in global/namespace scope) - thanks Jerry

Never trust on a variable of a plain type (int, long, ...) being automatically initialized! It might happen in languages like C#, but not in C & C++.