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In C++, when you pass an argument to a function, there are two main ways to do it: by value or by reference.

Understanding this distinction is key to both program correctness and performance optimization.

Pass by Value (Copy)

By default, C++ passes arguments by value. This means a copy of the data is created in memory. Any modification made inside the function will not affect the original variable outside:

#include <iostream>

void increment(int n) {
    n++; // Modifies only the local copy
}

int main() {
    int x = 5;
    increment(x);
    std::cout << x << std::endl; // Prints 5, x remains unchanged!
    return 0;
}

Pass by Reference (&)

If you want the function to be able to modify the original variable, or if you want to avoid copying large objects (like long strings or vectors), you must use pass by reference.

A reference is declared by adding the & symbol after the parameter's type:

#include <iostream>

void incrementRef(int& n) {
    n++; // Modifies the original variable directly
}

int main() {
    int x = 5;
    incrementRef(x);
    std::cout << x << std::endl; // Prints 6!
    return 0;
}

The parameter int& n is not a copy, but an alias (a direct reference) to the variable x passed when calling the function.

Try it yourself