Reference Parameter

The C Language does not have references; it requires using Pointer parameters to make changes “propagate back” to the calling environment.

A reference is just an alias for another variable name.

In C++, we added reference parameters, which can be used for function arguments by adding &.

Use references whenever possible in C++

References are a C++ feature that are not in C. They are like pointers but less error-prone. We recommend you use references rather than pointers wherever possible.

Using references rather than values is also more space efficient, since you need copy the whole object onto the runtime stack, just a reference to it.

Be careful with references in Queues!

If you make references to things in a queue and those get popped, then it will become a dangling reference, which can lead to undefined behavior.

• Many C++ systems started out life as a C system (and/or being developed by C programmers), so you will often see this style (ptr params) in industrial C++ code

C++ Reference Syntax

Similar to Pointer syntax, the preferred syntax in C++ for references int& a. Do that at interviews and in practice.

Resources

• https://www.tutorialspoint.com/cplusplus/cpp_references.htm

Difference between references and Pointers?

1. You cannot have null references (but you can have nullptr)
2. Once a reference is initialized to an object, it cannot be changed to refer to another object (pointers can point to another object at any time)
3. References must be initialized when it is created (pointers can be initialized at any time).

Passing by Value vs Passing by Reference

Swapping with pointers using a call-by-value, see Pointer. Swapping using Reference Parameters:

void swap(int &x, int &y) {
	const int temp = x;
	x = y;
	y = temp;
}
 
int main() {
	x = 5;
	y = 6;
	swap(x,y); // x=6, y=5
}

Use Cases

In C++, we can use normal variables as references, often to give a more convenient name to something long.

Employee& e = emplLIst[Wloo].find(empNum);
cout << e.getName() << " " << e.getAddr() << endl;

const reference parameter

Sometimes, we make the reference parameter a const so that the reference parameter inside the function doesn’t change (compiler will prevent you from doing so).

// This is more space efficient
string peek(const Stack &s) {
	assert (!isEmpty(s));
	return s.back();
}

Reference Binding

Had asked this question to myself in CS247. When you do something like this:

AbstractBook& br1 = t1;

Does it call operator=, or does it call a copy constructor for example? Obviously not, because you are declaring br1 as a reference. This simply does reference binding.

Types of References

oh god, this is where it gets complicated.

https://www.scaler.com/topics/cpp/pointers-vs-references-in-cplusplus/

References in C++ can be categorized based on the type of variables they can refer to such as lvalue references and rvalue references. Additionally, we can also have constant references in C++ which provides immutability.