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STRUCTURES AND UNIONS.
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A STRUCTURE is a collection of one or more variables, frequently of
different types, grouped together under a single name for convenience of
handling. In Turbo Pascal and many other languages structures are called
'records', with each individual variable in the record located in a
'field'. In C, each individual variable is called a member or element
of the structure.
A typical simple example of a structure could relate to the stores of
components or products in a factory. STRUCTUR.C is a program to
initialize a file, named 'product.dat', of structures, or records, of
5 products and to update and read the structures.
typedef struct { /* Opening the declaration of a structure*/
int itemno; /* */
char name[20]; /* The members of the structure */
int instock; /* Variables of different types */
char supplier[24]; /* */
} product ; /* Closing the declaration and defining */
/* the struct type 'product' */
This declaration and definition of a structure type 'product' is placed
at the start of the program and can subsequently be used to declare
specific structure variables of type 'product' as follows:
product stock[5] /* An array of 5 structures of type product */
Each variable within the array of structures could then be initialized as
follows:
product stock[5] = {0,"",0,"", /* itemno,name,instock,supplier */
1,"",0,"",
2,"",0,"",
3,"",0,"",
4,"",0,""
};
The dot notation can be used to refer to a specific member of a structure
and combined with the reference to a particular member of the array of
structures typical program instructions would be:
printf("\nEnter stock name ( <20 char ) ");
scanf("%20s",stock[i].name);
If i = 2, then this would change the name of itemno. 2 in the array of
structures.
Clearly for such a database to be usefully applied it must be stored in a
disk file. This can be achieved by opening a file (see notes on Input
and Output) and then using an instruction such as:
fwrite(&stock[i],sizeof(product),1,fstock);
where &stock[i] is the starting address for the i th item of the structure
sizeof(product) is the size of the structure
1 indicates that there is only one item of data - the structure
fstock is a pointer to type FILE
This use of an array of structures is fully illustrated in the program
STRUCTUR.C which can also be run as an executable file.
It is possible to declare pointers to structure, just as pointers can be
declared for other data types and there is a special symbol -> for
referrring to the member of a structure pointed to by a pointer. This
symbol is formed from the minus sign - and the greater than sign > and
is illustrated in the program LINKLIST.C where a pointer 'this' points
to a structure containing the members 'name' and 'job'. Thus these two
members are referenced by 'this->name' and 'this->job' respectively.
Note that the single inverted comma is used here in the text for clarity
and is not included in the actual program.
When using pointers to structures, dynamic variables are generally used,
involving a call to a memory allocation function such as malloc(). This
is illustrated in the program LINKLIST.C and the note on dynamic memory
management ALLOCMEM.TXT should also be consulted.
A UNION is a variable that can hold, at different times, values of
different types and sizes, with the compiler ensuring that size and
alignment are maintained. Different kinds of data can therefore be
accommodated in a single area of storage, in a way analogous to variant
records in Turbo Pascal.
Unions use the same syntax as structures, but whereas a structure might
hold an int, a double and a char, a union can hold an int or a double or a
char.