Pointing to Functions
Before you finish the course for this hour, there is one more interesting thing you need to learn about: pointers to functions.
As with pointers to arrays, you can declare a pointer that is initialized with the left value of a function. (The left value is the memory address at which the function is located.) Then you can call the function via the pointer.
The program in Listing 16.8 is an example that declares a pointer to a function.
TYPE
Listing 16.8. Pointing to a function.1: /* 16L08.c: Pointing to a function */
2: #include <stdio.h>
3: /* function declaration */
4: int StrPrint(char *str);
5: /* main() function */
6: main()
7: {
8: char str[24] = "Pointing to a function.";
9: int (*ptr)(char *str);
10:
11: ptr = StrPrint;
12: if (!(*ptr)(str))
13: printf("Done!\n");
14:
15: return 0;
16: }
17: /* function definition */
18: int StrPrint(char *str)
19: {
20: printf("%s\n", str);
21: return 0;
22: }
After the executable, 16L08.exe, of the program in Listing 16.8 is created and executed, the following output is shown on the screen:
OUTPUT
C:\app>16L08
Pointing to a function.
Done!
C:\app>
ANALYSIS
As usual, a function declaration comes first in Listing 16.8. The StrPrint() function is declared with the int data type specifier and an argument of a char pointer in line 4.
The statement in line 9 gives the declaration of a pointer (ptr) to the StrPrint() function (that is, int (*ptr)(char *str);).
Note that the pointer, ptr, is specified with the int data type and passed with a char pointer. In other words, the format of the pointer declaration in line 9 is quite similar to the declaration of StrPrint() in line 4. Please remember that you have to put the *ptr expression between a pair of parentheses (( and )) so that the compiler won't confuse it with a function name.
In line 11, the left value (that is, the address) of the StrPrint() function is assigned to the ptr pointer. Then, the (*ptr)(str) expression in line 12 calls the StrPrint() function via the dereferenced pointer ptr, and passes the address of the string declared in line 8 to the function.
From the definition of the StrPrint() function in lines 18_22, you can tell that the function prints out the content of a string whose address is passed to the function as the argument. Then, 0 is returned at the end of the function.
In fact, the if statement in lines 12 and 13 checks the value returned by the StrPrint() function. When the value is 0, the printf() function in line 13 displays the string of Done! on the screen.
The output of the program in Listing 16.8 shows that the StrPrint() function has been invoked successfully by using a pointer that holds the address of the function.
- 4 views
