C++ · References

A reference (int& r = x;) is an alias: a second name for an existing variable. Unlike a pointer:

• it must be initialised and can’t be null
• it can’t be re-pointed to another variable
• there’s no * to dereference it; you just use it like the variable

How references change function calls:

signature	the function gets	can change the caller’s value?	copies?
f(std::string s)	a copy	no	yes
f(std::string& s)	the original	yes	no
f(const std::string& s)	the original, read-only	no	no

The C++ Core Guidelines advise passing cheap types (int, double) by value and bigger ones by const&.

#include <iostream>
#include <string>

void byValue(std::string s)        { s += "!"; }
void byRef(std::string& s)         { s += "!"; }
std::size_t length(const std::string& s) { return s.size(); }   // no copy, no change

void swapInts(int& a, int& b) { int t = a; a = b; b = t; }

int main() {
    int x = 1;
    int& r = x;
    r = 99;
    std::cout << "x=" << x << '\n';

    std::string msg = "hello";
    byValue(msg); std::cout << msg << '\n';
    byRef(msg);   std::cout << msg << '\n';
    std::cout << "length " << length(msg) << '\n';

    int a = 3, b = 8;
    swapInts(a, b);
    std::cout << "a=" << a << " b=" << b << '\n';
}

Output:

x=99
hello
hello!
length 6
a=8 b=3