Reference

A reference is an alternative name for an existing variable. A reference is declared by specifying the variable type followed by &:

int var = 1;
int& ref = var;

Reading and writing the reference reads and writes the original variable:

ref = 2;
std::cout << var << "\n";  // 2

References must be initialised when declared:

int& ref;  // ERROR.

References cannot rebound to another variable:

int a = 1;
int& ref = a;

int b = 2;
ref = b;  // a is now 2, ref still refers to a.

A const reference can read but not modify the variable it refers to:

int var = 1;
const int& ref = var;
ref = 2;  // ERROR.

References are most often used as function parameters so the function can modify the caller's variable, or to avoid copying large objects:

void increment(int& n) { n++; }
void print(const std::string& s) { std::cout << s; }

int count = 0;
increment(count);  // count is now 1.