The C++ language allows you to create and use data types other than the fundamental data types. These types are called user-defined data types. There are two ways by which a user-defined data type can be created. The first is by using the keyword struct, and the second is by using the keyword class. Data types defined using the keyword struct are called structures.

Structures

In C++, a structure is a collection of variables that are referenced by a single name. The variable can be of different data types. They provide a convenient means of keeping related information together. Structure definition forms a template which is then used to create structure variables. The variables that make up the structure are called structure elements.

All the elements found in the structure are related to each other. For example, the invoice number and the invoice amount found in an invoice would normally be represented in a structure. The following code segment shows how a structue template that defines an invoice number and invoice amount can be created. The keyword struct tells the compiler that a structure template is being declared.

struct invoice
{
int iInvoice_no;
float fInvoice_amt;
};

Notice that the structure declaration is terminated by a semi-colon. This is because a structure declaration is a statement. Also, the structure tag invoice identifies this particular data structure. At this point in the code, no variable has been declared; only the form of the data has been defined.

To declare a variable, you need to use the following statement:

invoice Inv1;

This defines a structure variable of type 'invoice', called Inv1. When you declare a structure, you are, in essence, defining a data type. Memory is not allocated for a structure until a variable of that type is created.

The compiler automatically allocates sufficient memory to accommodate all the elements that make up the structure.

You may also declare two or more variables at the time of declaring a structure. For example:

struct invoice
{
int iInvoice_no;
float fInvoice_amt;
}Inv1,Inv2;

will declare a structure type called invoice and declare the variable Inv1 and Inv2.

If only one structure has to be declared in a program, then following definition would suffice:

struct
{
int iInvoice_no;
float fInvoice_amt;
}Inv1;

The general form of a structure declaration is:

struct <tag>
{
<type> <variable1>;
<type> <variable2>;
...
...
<type> <variableN>;
}<struct vars>;

where <tag> is the name of the structure declaration, effectively the name of the new data type. The structure tag is then used to declare structure variables. <struct vars> are the names of structure variables. Either the <tag> or the <struct vars> can be omitted, but noth both.