When and How Are Variables Automatically Initialized in C ?

Variable Initialization in C : When and Where Does It Occur Automatically?

Understanding variable initialization in C is crucial for developing robust code. Contrary to the assumption that int variables are automatically initialized to 0, the following code snippet demonstrates that this is not the case:

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

This code will output a random value for the uninitialized variable i. To address this, it is essential to understand the rules governing initialization in C .

Automatic Initialization Rules

In C , variables are automatically initialized only under specific conditions:

• Class/Struct Instances: Default constructor initialization will initialize all primitive types within a class or struct instance.
• Array Initializer Syntax: Using curly braces to initialize arrays, e.g., int a[10] = {} or int a[10] = {1,2}, will automatically zero out any uninitialized elements.
• Global/Extern Variables: Variables declared outside of function scope (global or extern) are automatically initialized to 0.
• Static Variables: Variables with the static keyword are automatically initialized to 0, regardless of their scope.

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