It is useful to give basic information here. C++ is a strongly typed language, which means that you have to declare the type of the variables that you use. The reason for this is that the compiler needs to know how much memory to allocate for the variable, and it can determine this by the type of the variable. In addition, the compiler needs to know how to initialize a variable, if it has not been explicitly initialized, and to perform this initialization the compiler needs to know the type of the variable.

C++ variables can be declared anywhere in your code as long as they are declared before they are used. Where you declare a variable determines how you use it (this is called the scope of the variable). In general, it is best to declare the variable as close as possible to where you will use it, and within the most restrictive scope. This prevents name clashes, where you will have to add additional information to disambiguate two or more variables.

You may, and should, give your variables descriptive names. This makes your code much more readable and easier to understand. C++ names must start with an alphabetic character, or an underscore. They can contain alphanumeric characters except spaces, but can contain underscores. So, the following are valid names:

    numberOfCustomers 
    NumberOfCustomers 
    number_of_customers

C++ names are case-sensitive, and the first 2,048 characters are significant. You can start a variable name with an underscore, but you cannot use two underscores, nor can you use an underscore followed by a capital letter (these are reserved by C++). C++ also reserves keywords (for example, while and if), and clearly you cannot use type names as variable names, neither built in type names (int, long, and so on) nor your own custom types.

You declare a variable in a statement, ending with a semicolon. The basic syntax of declaring a variable is that you specify the type, then the name, and, optionally, any initialization of the variable.

Built-in types must be initialized before you use them:

    int i; 
    i++;           // C4700 uninitialized local variable 'i' used 
    std::cout << i;

There are essentially three ways to initialize variables. You can assign a value, you can call the type constructor (constructors for classes will be defined in Chapter 4, Classes) or you can initialize a variable using function syntax:

    int i = 1; 
    int j = int(2); 
    int k(3);

These three are all legal C++, but stylistically the first is the better because it is more obvious: the variable is an integer, it is called i, and it is assigned a value of 1. The third looks confusing; it looks like the declaration of a function when it is actually declaring a variable. 

Chapter 4, Classes will cover classes, your own custom types. A custom type may be defined to have a default value, which means that you may decide not to initialize a variable of a custom type before using it. However, this will result in poorer performance, because the compiler will initialize the variable with the default value and subsequently your code will assign a value, resulting in an assignment being performed twice.