Functions are reusable blocks of code that perform a specific task. They help in reducing redundancy and making the code more readable and maintainable.

Always! Break down complex problems into smaller, manageable functions.

• Call overhead: $O(1)$
• Execution: Depends on function body.

cpp Standard
#include <iostream>

// Declaration
int add(int a, int b);

int main() {
    int sum = add(5, 10); // Call
    std::cout << "Sum: " << sum << std::endl;
    return 0;
}

// Definition
int add(int a, int b) {
    return a + b;
}

• Not providing a function prototype if the definition is after main().
• Returning the wrong type from a function.
• Defining a function inside another function (not allowed in standard C++).