When passing arguments to functions, you can either pass a copy of the data (Pass by Value) or pass the memory address of the original data (Pass by Reference).

• Pass by Value: For small types like int, char, or when you don't want the function to modify the original value.
• Pass by Reference: For large objects (like std::vector or std::string) to avoid expensive copying, and when you NEED the function to modify the original value.

• Pass by Value: $O(N)$ for copying an object of size $N$.
• Pass by Reference: $O(1)$ (only the address is passed).

cpp Standard
void modifyValue(int x) { x = 100; } // Copy
void modifyRef(int &x) { x = 100; }  // Reference

int main() {
    int a = 5, b = 5;
    modifyValue(a); // a is still 5
    modifyRef(b);   // b is now 100
}

• Passing large objects by value in performance-critical loops.
• Modifying a reference when you intended to only read from it (use const & to prevent this).
• Returning a reference to a local variable (Dangling reference).