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References and pointers are some of the most powerful features in C++; they allow programmers to directly manipulate memory – the most critical and scarce resource in computer – in order to optimize performance.

References and Pointers

So now we learned what a pointer is and how to create one, but is there a way to obtain the value pointed to by the pointer?

The asterisk sign `*` a.k.a. the dereference operator is used to obtain the value pointed to by a variable. This can be done by preceding the name of a pointer variable with `*`.

```cpp
int blah = *ptr;
```

The double meaning of the `*` symbol can be tricky at first, so make sure to note:

- When `*` is used in a declaration, it is creating a pointer.
- When `*` is not used in a declaration, it is a dereference operator.


Dereference

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:

```cpp
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.


Pass-By-Reference

In C++, a _pointer_ variable is mostly the same as other variables, which can store a piece of data. Unlike normal variables, which store a value (such as an `int`, `double`, `char`), a pointer stores a memory address.

While references are a newer mechanism that originated in C++, pointers are an older mechanism that was inherited from C. We recommend avoiding pointers as much as possible; usually, a reference will do the trick. However, you will see pointers a lot in the wild, particularly in older projects, where they are used in a very similar way to references.

Pointers must be declared before they can be used, just like a normal variable. They are syntactically distinguished by the `*`, so that `int*` means "pointer to `int`" and `double*` means "pointer to `double`".

```cpp
int* number;
double* decimal;
char* character;
```

So suppose we have a variable called `gum`:

```cpp
int gum = 8;
```

We can create a pointer to it by:

```cpp
int* ptr = &gum;
```

- `int*` makes it a pointer rather than a normal variable.
- `ptr` is the pointer name.
- `&gum` is the memory address of the other variable `gum`.

So now `ptr` has a value of `gum`'s memory address.

<img src="https://content.codecademy.com/courses/learn-cpp/references-and-pointers/pointers1.png" alt="Pointers" height="300">

[Credit: XKCD 138](https://xkcd.com/138)

**Note:** Syntactically, spaces around `*` do not matter, but the best practice is to have it after the data type.

```cpp
int* number;
int *number;
int * number;
```


Pointers

In C++, a _reference_ variable is an alias for something else, that is, another name for an already existing variable. 

So suppose we make Sonny a reference to someone named Songqiao. You can refer to the person as either Sonny or Songqiao.

Let's take a look at how we can do this with code. Suppose we have an `int` variable already called `songqiao`, we can create an alias to it by using the `&` sign in the declaration:

```cpp
int &sonny = songqiao;
```

So here, we made `sonny` a reference to `songqiao`.

Now when we make changes to `sonny` (add 1, subtract 2, etc), `songqiao` also changes.

Two things to note about references:

1. Anything we do to the reference also happens to the original.
2. Aliases cannot be changed to alias something else.

References

Remember `const`? The `const` keywords tells the compiler that we won't change something.

For example, in the following code, we are telling the compiler that the `double` variable `pi` will stay at `3.14` through out the program:

```cpp
double const pi = 3.14;
```

If we try to change `pi`, the compiler will throw an error.

Sometimes, we use `const` in a function parameter; this is when we know for a fact that we want to write a function where the parameter won't change inside the function. Here's an example:

```cpp
int triple(int const i) {

  return i * 3;

}
```

In this example, we are not modifiying the `i`. If inside the function `triple()`, the value of `i` is changed, there will be a compiler error.

So to save the computational cost for a function that doesn't modify the parameter value(s), we can actually go a step further and use a `const` reference:

```cpp
int triple(int const &i) {

  return i * 3;

}
```

This will ensure the same thing: the parameter won't be changed. However, by making `i` a reference to the argument, this saves the computational cost of making a copy of the argument.



Pass-By-Reference with Const

Congratulations! You have officially done it. 🎉🎉🎉

In this lesson, you have learned:

- References
  - Pass-by-reference
  - Pass-by-reference with `const`
- Memory addresses and how to access them
- Pointers
  - Dereferencing a pointer
  - `nullptr`

```cpp
// Reference
int &reference = original;

// Pointer
int* pointer = &original;
```

Review

A computer's _memory_ is a sequence of [bytes](https://en.wikipedia.org/wiki/Byte). We can number the bytes from 0 to the last one. Each number, known as an _address_, represents a location in the memory.

Everything we put into memory has an address. For example, when we declare and initialize an `int` variable named `power`:

```cpp
int power = 9000;
```

This will set aside an `int`-size piece of memory for the variable `power` somewhere and put the value `9000` into that memory. But where is "somewhere"? How is it useful?

In this lesson, we will answer these questions by learning about:

- References
- Pointers

These are some of the most powerful features in C++; they allow programmers to directly manipulate memory – the most critical and scarce resource in computer – in order to optimize performance. 

However, references and pointers are also sometimes considered two of the most complex and difficult features in C++.

So let's get started.

Introduction

So we have learned about references (aliases), which are created by using the `&` symbol in a variable declaration. But the `&` sign can have another meaning.

The “address of” operator, `&`, is used to get the _memory address_, the location in the memory, of an object.

Suppose we declare a variable called:

```cpp
int porcupine_count = 3;
```

Have you wondered where the variable `porcupine_count` is stored on the computer? We can find out by printing out `&porcupine_count`:

```cpp
std::cout << &porcupine_count << "\n";
```

It will return something like:

```bash
0x7ffd7caa5b54
```

This is a memory address represented in [hexadecimal](https://en.wikipedia.org/wiki/Hexadecimal). A memory address is usually denoted in hexadecimal instead of binary for readability and conciseness.

The double meaning of the `&` symbol can be tricky at first, so make sure to note:

- When `&` is used in a declaration, it is a reference operator.
- When `&` is not used in a declaration, it is an address operator.


Memory Address

When we declare a pointer variable like so, its content is not intialized:

```cpp
int* ptr;
```

In other words, it contains an address of "somewhere", which is of course not a valid location. This is [dangerous](https://en.wikipedia.org/wiki/Uninitialized_variable)! We need to initialize a pointer by assigning it a valid address.

But suppose we don't know where we are pointing to, we can use a null pointer.

`nullptr` is a new keyword introduced in C++11. It provides a typesafe pointer value representing an empty pointer.

We can use `nullptr` like so:

```cpp
int* ptr = nullptr;
```

**Note:** In older C/C++ code, `NULL` was used for this purpose. `nullptr` is meant as a modern replacement to `NULL`.