In C++, a reference is a type of variable that acts as an alias, or an alternative name, for an existing variable. It allows you to create a new name for a variable and use that name interchangeably with the original variable. References are often used as function parameters to enable pass-by-reference semantics, where changes made to the reference inside a function affect the original variable outside the function.

Here's an example that demonstrates the use of references in C++:

cppCopy code

#include <iostream> void increment(int& num) {    num++; } int main() {    int x = 5;    int& ref = x;  // Creating a reference to variable x    std::cout << "x = " << x << std::endl;     // Output: x = 5    std::cout << "ref = " << ref << std::endl; // Output: ref = 5    ref = 10;  // Assigning a new value to the reference    std::cout << "x = " << x << std::endl;     // Output: x = 10    std::cout << "ref = " << ref << std::endl; // Output: ref = 10    increment(x);  // Passing x by reference to a function    std::cout << "x = " << x << std::endl;     // Output: x = 11    std::cout << "ref = " << ref << std::endl; // Output: ref = 11    return 0; }

In this example, we define a variable x and create a reference ref to x using the & operator. Any changes made to ref will also affect x, and vice versa. Initially, both x and ref are set to 5. When we assign a new value of 10 to ref, x is also updated. Similarly, when we pass x by reference to the increment function and modify num inside the function, it increments the value of x.

Using references can simplify code and avoid unnecessary copying of data. They are particularly useful when working with large objects or when modifying variables within functions.

Note that references must be initialized when declared, and they cannot be re-assigned to refer to a different variable. Additionally, references cannot refer to constants or expressions that do not have an address (e.g., literals).