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References and Pointers
Pass-By-Reference

So what’s a good use case for references? Let’s take a look.

Previously, when we passed parameters to a function, we used normal variables and that’s known as pass-by-value. But because the variables passed into the function are out of scope, we can’t actually modify the value of the arguments.

Pass-by-reference refers to passing parameters to a function by using references. When called, the function can modify the value of the arguments by using the reference passed in.

This allows us to:

  • Modify the value of the function arguments.
  • Avoid making copies of a variable/object for performance reasons.

The following code shows an example of pass-by-reference. The reference parameters are initialized with the actual arguments when the function is called:

void swap_num(int &i, int &j) { int temp = i; i = j; j = temp; } int main() { int a = 100; int b = 200; swap_num(a, b); std::cout << "A is " << a << "\n"; std::cout << "B is " << b << "\n"; }

Notice that the int &i and int &j are the parameters of the function swap_num().

When swap_num() is called, the values of the variables a and b will be modified because they are passed by reference. The output will be:

A is 200 B is 100

Suppose we didn’t pass-by-reference here and have the parameters as simply int i and int j in the swap_num() function, then i and j would swap, but a and b wouldn’t be modified.

And the output will then be:

A is 100 B is 200

To reiterate, using references as parameters allows us to modify the arguments’ values. This can be very useful in a lot cases.

Instructions

1.

Take a look at the program in the code editor. There is a function called triple() and there is a main() where we call triple() twice.

What do you think this code will output?

Run the program to see if you are correct.

2.

Change the parameter from int i to int &i.

We are doing pass-by-reference instead of pass-by-value. What do you think this will output now?

Run the code again.

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