C++ References

What is a Reference?

A reference is an alias — another name for an existing variable. Created with &. Both names refer to the same memory location.

#include <iostream>
using namespace std;

int main() {
    int x = 10;
    int &ref = x;   // ref is an alias for x

    cout << x   << endl;   // 10
    cout << ref << endl;   // 10

    ref = 99;
    cout << x   << endl;   // 99 (x changed through ref)
    return 0;
}

Output:

10
10
99

Pass by Reference

By default, C++ passes arguments by value (a copy). Using & in a parameter passes the original variable — changes persist after the function returns.

void addTen(int &n) {   // reference parameter
    n += 10;
}

int main() {
    int score = 50;
    addTen(score);
    cout << score;   // 60 — original was modified
}

// Swap function using references
void swap(int &a, int &b) {
    int temp = a;
    a = b;
    b = temp;
}

int x = 5, y = 10;
swap(x, y);
cout << x << " " << y;   // 10 5

const References

Use const & to pass large objects efficiently without allowing modification:

void print(const string &s) {
    cout << s << endl;
    // s = "change"; // ERROR — const prevents modification
}

print("Hello World");   // no copy made, no modification allowed

#include <iostream>
using namespace std;

void square(int &n) {
    n = n * n;
}

int main() {
    int x = 7;
    square(x);
    cout << "Squared: " << x << endl;   // 49
    return 0;
}

Summary

• A reference is an alias for another variable, using &.
• Pass by reference allows functions to modify the caller's variables.
• const & passes efficiently without allowing modification.