Creating Variables in C++

In the previous lesson, we constructed a list like this, to show some of the data we would need to describe one of our monsters:

In this lesson, we will see how we can create the variables to store this data.

Lets start with the bool that represents whether or not the monster is currently dead:

1bool isDead;
2

This line is an instruction that we want to create a variable of type bool, and we will be using the name isDead to identify it. An instruction like this is sometimes called a statement and, in C++, we end each statement with a semicolon ;

This statement will cause our program to make a request to the operating system, asking that we be allocated some space in memory. Specifically, it is requesting enough memory to store a boolean value.

From then on, we can use the isDead identifier any time we want to access the data stored in that memory location.

Comments in C++

By using // in our code, we can add a comment.

1// Is the monster currently dead?
2bool isDead;
3

Comments are mainly to help developers describe in plain language what their code is doing, and why. They are ignored by the processes that convert our code into an executable.

Anything after the // on a line of code is treated as a comment. In the above example, the entire line is a comment. But comments can also be on the same line as functional code:

1bool isDead; // This is a comment
2

As an alternative to a line comment using //, we can also place comments between /* and */ tokens:

1bool isDead; /* This is a comment */
2

This style of comment can go in the middle of a line:

1bool /* This is a comment */ isDead;
2

And it can also span multiple lines:

1/*
2This is a comment
3on multiple lines
4*/
5bool isDead;
6

We'll use comments quite a lot in the code samples of this lesson to explain what is happening.

It is often worth adding comments to your own code too, especially as a beginner. They can help you understand what is happening if you come back to your code later, and need a reminder of what certain lines are doing.

Variable Values

Once we have created a variable, we can store a value in it like this:

1// Is the monster currently dead?
2bool isDead;
3isDead = false;
4

The isDead = false; statement visits the memory location created for the isDead variable. It then updates that location to represent the boolean value false.

We can provide an initial value for a variable at the same time we first declare it. This is called initialising the variable. This code will request some space to store a boolean, and initialise that boolean to contain the value of false:

1// Is the monster currently dead?
2bool isDead { false };
3

The { and } characters are commonly called braces and are used a lot in programming. On US/UK keyboard layouts, they are typically to the right of the P key.

When initialising a variable with a value, we can also do it with the = symbol

1// Is the monster currently dead?
2bool isDead = false;
3

1bool isDead = false;
2

1bool isDead { false };
2

Lets also create an int and a string variable:

1int Level { 5 };
2
3// Remember, strings need wrapped in double quotes
4string Name { "Goblin Warrior" };
5

Outputting Variables

In the first lesson, we were working with this basic code structure to stream information to the console.

1#include <iostream>
2using namespace std;
3
4int main()
5{
6  cout << "Hello World!";
7}
8

Lets see how our variables integrate with this. Until we learn more, I'd suggest creating all our variables after the using namespace std; line, and doing all cout statements between the { and the } of the main code block.

1int LookAtMe { 5 };
2

1int ThisWontWork { "Hello World!" };
2

1int DontDoThis { true };
2

The following is an example of a program using our new knowledge of variables:

1#include <iostream>
2using namespace std;
3
4string Name { "Goblin Warrior" };
5int Level { 10 };
6
7int main()
8{
9  cout << "The monster's name is "<< Name;
10  cout << " and its level is " << Level << endl;
11}
12

When we compile and run this, we should see a message like this logged out to the terminal:

1The monster's name is Goblin Warrior and its level is 10
2

Note, if we try to cout boolean values, you might not get what you expect. true will log out as 1 and false will log out as 0. That's not very useful right now, but we will explore booleans in more detail soon.

C++ Error: Redefinition of Variable

The most common error beginners will make with variables is repeating the type when trying to update a variable that was already created.

This error will generally be reported as "redefinition of (your variable name)"

We only need to set the type of the variable when we first create it.

The following would be invalid code, as it is attempting to create two different variables, using the same name:

1bool isDead;
2
3// Error!  We already have a variable called "isDead"
4bool isDead = false;
5

When we want to access or update a variable we already created, we just use the name, not the type:

1bool isDead;
2
3isDead = false;
4

C++ Error: Expected ; after expression

Another common error is misuse of the brace syntax. We only use { and } to initialise a variable, not to update it.

Therefore, the following example would also generate an error, which will generally be reported as "Expected ; after expression":

1bool isDead { false };
2
3// Error! This brace syntax is only for initialisation
4isDead { true };
5

We should use = to update a variable

1bool isDead { false };
2
3isDead = true;
4

Copying Variables

We can initialize a variable using the value of another variable.

In the below code, we are copying the value of the MaxHealth variable into a new variable called CurrentHealth.

1int MaxHealth { 150 };
2int CurrentHealth { MaxHealth };
3

After this code runs, we will have two variables that initially have the same value, but can then be updated individually.

More Examples

The code sample below shows more examples of how we can create and work with variables.

1// Initialise a new integer variable
2int Health { 150 };
3
4// We can also initialize a variable using =
5int ExperienceLevel = 1;
6
7/*
8We can update the value of an existing
9variable later in our code using the = sign
10*/
11Health = 200;
12
13// We can copy booleans too
14bool isDead { true };
15bool canBeLooted { isDead };
16
17/*
18Remember, we don't redeclare the variable's
19type when we update its value.  We only
20declare its type when we first create it.
21Line 24 is an error because Health
22was already initialised
23*/
24int Health = 250; // error!
25
26/*
27However, when updating a variable, we cannot
28use the { braces } syntax.  Braces are only
29used for for initialisation.  Therefore,
30line 32 will cause an error
31*/
32ExperienceLevel { 3 }; // error!
33

Let's continue our initial exploration of variables by looking at numbers in a bit more depth!