A local variable is one that occurs within a specific scope. They exist only in the function where they are created.
They are sometimes called automatic variables because they are automatically created when the function starts execution, and automatically go away when the function is finished executing.
The keyword auto can be used to explicitly create these variables, but isn't necessary since auto is the default.
Global Variables and extern
A global variable is a variable that is defined outside all functions and available to all functions.
These variables are unaffected by scopes and are always available, which means that a global variable exists until the program ends.
It is possible to create a global variable in one file and access it from another file. In order to do this, the variable must be declared in both files, but the keyword extern must precede the "second" declaration.
A static variable can be either a global or local variable. Both are created by preceding the variable declaration with the keyword static.
A local static variable is a variable that can maintain its value from one function call to another and it will exist until the program ends.
When a local static variable is created, it should be assigned an initial value. If it's not, the value will default to 0.
A global static variable is one that can only be accessed in the file where it is created. This variable is said to have file scope.
In C, the preprocessor directive #define was used to create a variable with a constant value. This still works in C++, but problems could arise.
When #define is used, the preprocessor will go through the code and replace every instance of the #defined variable with the appropriate value. Well, since the #defined variable exists only in the file where it is created, it is possible to have the same definition in another file with a completely different value. This could lead to disastrous consequences.
To overcome this problem, the concept of a named constant that is just like a variable was introduced to C++.
To create a constant variable in C++, precede the variable declaration with the keyword const. This tells the compiler that "a variable has been created that has a value that cannot be changed"
When creating a constant variable, it MUST be assigned a value.