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Structures, Unions, and User-Defined Types
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Structure Pointers
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C/C++ allows pointers to structures in the same way it does to other types of variables However, there are some special aspects to structure pointers that you must keep in mind
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THE FOUNDATION OF C++
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Declaring a Structure Pointer
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Structure pointers are declared by placing the * in front of a structure variable s name For example, assuming the previously defined structure addr, the following declares addr_pointer to be a pointer to data of that type:
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struct addr *addr_pointer;
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Using Structure Pointers
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To find the address of a structure variable, the & operator is placed before the structure s name For example, given the following fragment,
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struct bal { float balance; char name[80]; } person; struct bal *p; /* declare a structure pointer */
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then
p = &person;
places the address of the structure person into the pointer p To access the members of a structure using a pointer to that structure, you must use the arrow operator The arrow operator, >, is formed using a minus sign and a greater-than symbol For example, to reference the balance member using p, you would write
p->balance
Borland C++ Builder: The Complete Reference
To see how structure pointers can be used, examine this simple program that prints the hours, minutes, and seconds on the screen using a software timer (The timing of the program is adjusted by changing the definition of DELAY to fit the speed of your computer)
/* Display a software timer */ #include <stdioh> #include <conioh> #define DELAY 128000 struct my_time { int hours; int minutes; int seconds; }; void update(struct my_time *t), display(struct my_time *t); void mydelay(void); int main(void) { struct my_time systime; systimehours = 0; systimeminutes = 0; systimeseconds = 0; for(;;) { update(&systime); display(&systime); if(kbhit()) return 0; } } void update(struct my_time *t) { t->seconds++; if(t->seconds==60) { t->seconds = 0; t->minutes++; }
7:
Structures, Unions, and User-Defined Types
if(t->minutes==60) { t->minutes = 0; t->hours++; } if(t->hours==24) t->hours = 0; mydelay(); } void display(struct my_time *t) { printf("%02d:", t->hours); printf("%02d:", t->minutes); printf("%02d\n", t->seconds); } void mydelay(void) { long int t; for(t=1; t<DELAY; ++t) ; }
THE FOUNDATION OF C++
A global structure called my_time is declared Inside main( ), the structure variable called systime, of type my_time, is declared and initialized to 00:00:00 This means that systime is known directly only to the main( ) function The functions update( ), which changes the time, and display( ), which prints the time, are passed the address of systime In both functions the argument is declared to be a pointer to a structure of type my_time Inside the functions, each structure element is actually referenced through a pointer For example, to set the hours back to 0 when 24:00:00 is reached, this statement is used
if(t->hours==24) t->hours = 0;
This line of code tells the compiler to take the address of t (which points to systime in main( )) and assign 0 to its hours member Use the dot operator to access structure members when operating on the structure itself Use the arrow operator when referencing a structure through a pointer As a final example of using structure pointers, the following program illustrates how a general-purpose integer input function can be designed The function input_xy( ) allows
Borland C++ Builder: The Complete Reference
you to specify the x and y coordinates at which a prompting message will be displayed and then inputs an integer value To accomplish these things it uses the structure xyinput
/* A generalized input example using structure pointers */ #include <stdioh> #include <conioh> #include <stringh> struct xyinput { int x, y; /* screen location for prompt */ char message[80]; /* prompting message */ int i; /* input value */ } ; void input_xy(struct xyinput *info);
return 0; }
/* Display a prompting message at the specified location and input an integer value */ void input_xy(struct xyinput *info) { gotoxy(info->x, info->y); printf(info->message); scanf("%d", &info->i); }
The program uses the functions clrscr( ) and gotoxy( ) to clear the screen and position the cursor, respectively Both functions use the <conioh> header file A function like input_xy( ) is useful when your program must input many pieces of information (In fact, you might want to create several functions like input_xy( ) that input other types of data)
messx = 10; messy = 10; strcpy(messmessage, "Enter an integer: "); clrscr(); input_xy(&mess); printf("Your number squared is: %d", messi*messi);
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