Function pointer definition
- C course
- Intermediate
- Lesson 24/116
- 5 min
Learn function pointer syntax and types.
Function pointer types
A function pointer is a variable holding a function's address. The declaration is a return type, a parenthesized star and name, then the parameter list.
Example: int (*fp)(int, int) = add;. Here fp is a pointer to a function taking two ints and returning int. Parentheses matter: writing int *fp(int, int) declares a function returning int *. Parentheses are required to mark it as a pointer. The return and parameter types must match the function signature exactly.
Type matching
When assigning to a function pointer, the return type and parameter list must match exactly. Otherwise the compiler warns and the behavior is undefined.
Example: for int add(int, int); write int (*fp)(int, int) = add;. But int (*fp)(int) = add; mismatches parameter count. Likewise double (*fp)(int, int) = add; mismatches return type. The pointer type carries the function signature as a contract; that's why it's long but safe.
Practice Declare a function pointer
Write int add(int a, int b) that returns a + b. Declare a pointer with int (*fp)(int, int) = add; and assign it to add. Print the result of fp(3, 4) with printf("%d\n", fp(3, 4));.
Starter code
#include <stdio.h>
// Write the add function
// Declare the fp pointer
// Call fp and print
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
int add(int a, int b) {
return a + b;
}
int main(void) {
int (*fp)(int, int) = add;
printf("%d\n", fp(3, 4));
return 0;
}Expected output
7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which is a pointer to a function returning int and taking two ints?
- int *fp(int, int);
- int (*fp)(int, int); Correct answer
- int fp*(int, int);
- (int *)fp(int, int);
Explanation: Parentheses make fp a pointer; int *fp(int, int) is a function declaration.
2 Two-parameter function pointer
int (____)(int, int) = add;int (*fp)(int, int) = add;Explanation: *fp inside the parentheses forms the pointer variable.
3 Match the function pointer parts
| Term | Meaning |
|---|---|
int |
Return type |
(*fp) |
Pointer variable |
(int, int) |
Parameter list |
= add |
Assignment |