Teach Yourself Borland C++ in 14 Days - portal

56 Day 2
The extern keyword tells the compiler, “I’m going to be using a variable in this source file
that you will find declared in another source file.” The compiler sorts it all out at compile time
and makes sure you get access to the correct variable.
While global variables are convenient, they aren’t particularly OOP friendly. Usually there
are better solutions (which you will learn about when I discuss classes). In addition, global
variables consume memory for the life of the program. Local variables use up memory only
while they are in scope. Use local variables whenever possible, and keep the use of global
variables to a minimum.
Structures
A structure is a collection of related data rolled up into a single storage unit. For instance, let’s
say you wanted to keep a mailing list. It would be convenient to be able to have a single data
variable that could be used to hold all the fields needed in a typical mailing list. A structure
will allow you to do that. You first declare a structure and then later create an instance of that
structure when you want to put the structure to use. A structure is declared with the struct
keyword:
struct mailingListRecord {
char firstName[20];
char lastName[20];
char address[50];
char city[20];
char state[4];
int zip;
bool aFriend;
bool aFoe;
};
Each of the elements of a structure is called a data member. You will notice that each of the
data members must be declared just as it would be if it were a variable in a code block. In this
example I have five char arrays, one int, and two bool data members. (My apologies to my
friends around the world if this looks like a U.S.-slanted mailing-list record.) Finally, make
note of the semicolon following the closing brace of the structure declaration. This is a
requirement for structure and class declarations.
A structure is a collection of related data identified as a single storage unit. After a
structure has been declared, an instance of that structure can be created for use. Each
of the elements of a structure is called a data member.
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