C++ Passing by Reference

Declaring a function parameter with & makes it a reference to the caller's argument, letting the function modify the original variable directly instead of working on a copy.

In this page:

Pass by Value vs Pass by Reference
Modifying the Caller's Variable
Returning Multiple Values via Reference Parameters
Passing Large Objects by Reference for Performance
const Reference Parameters

Pass by Value vs Pass by Reference

By default, C++ passes arguments by value, giving the function its own copy, but adding & to a parameter's type makes it a reference, letting the function operate on the caller's original variable instead.

Example: Pass by Value vs Pass by Reference

cpp

#include <iostream>

void byValue(int x) { x = 99; }
void byReference(int &x) { x = 99; }

int main() {
	int a = 1, b = 1;
	byValue(a);
	byReference(b);
	std::cout << "a=" << a << " b=" << b << std::endl;
	return 0;
}

Modifying the Caller's Variable

A reference parameter shares the same memory as the caller's argument, so any assignment made to the parameter inside the function immediately changes the value the caller sees.

Example: Modifying the Caller's Variable

cpp

#include <iostream>

void reset(int &score) {
	score = 0;
}

int main() {
	int score = 50;
	reset(score);
	std::cout << score << std::endl;
	return 0;
}

Returning Multiple Values via Reference Parameters

Since a function can only return one value directly, reference parameters are a common technique for a function to effectively hand back several results to its caller.

Example: Returning Multiple Values via Reference Parameters

cpp

#include <iostream>

void minMax(int a, int b, int &minVal, int &maxVal) {
	if (a < b) { minVal = a; maxVal = b; }
	else { minVal = b; maxVal = a; }
}

int main() {
	int lo, hi;
	minMax(8, 3, lo, hi);
	std::cout << "min=" << lo << " max=" << hi << std::endl;
	return 0;
}

Passing Large Objects by Reference for Performance

Passing a large object like a big string or a struct by reference avoids the cost of copying it entirely, since the function receives a reference to the existing data instead of a duplicate.

Example: Passing Large Objects by Reference for Performance

cpp

#include <iostream>
#include <string>

void printLength(const std::string &text) {
	std::cout << "Length: " << text.length() << std::endl;
}

int main() {
	std::string bigText = "A long string that would be costly to copy";
	printLength(bigText);
	return 0;
}

const Reference Parameters

Combining const with a reference parameter, written as const Type&, passes the argument efficiently without copying it while also preventing the function from accidentally modifying the caller's original data.

Example: const Reference Parameters

cpp

#include <iostream>
#include <string>

void show(const std::string &name) {
	std::cout << "Hello, " << name << std::endl;
	// name = "Changed"; // compile error: cannot modify a const reference
}

int main() {
	show("Alex");
	return 0;
}

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