Search notes:
C: function pointers
Three functions with the same signature
Here are three functions. All of the have the same signature: they take a char const* and a char and they return an unsigned int:
unsigned int count(char const *text, char c);
unsigned int first(char const *text, char c);
unsigned int last (char const *text, char c);
Here is their implementation:
unsigned int count(char const *text, char c) {
unsigned int returnValue = 0;
for (const char* t=text; *t; t++) {
if (*t == c) returnValue ++;
}
return returnValue;
}
unsigned int first(char const *text, char c) {
unsigned int returnValue = 0;
for (const char* t=text; *t; t++) {
if (*t == c) return returnValue;
returnValue ++;
}
return -1;
}
unsigned int last(char const *text, char c) {
unsigned int returnValue = -1;
for (const char* t=text; *t; t++) {
if (*t == c) returnValue = t-text;
}
return returnValue;
}
Using a function pointer
The following short code declares a function pointer, assigns a function to it and executes the function via the function pointer
#include <stdio.h>
#include "funcs.h"
int main() {
//
// Declare a function pointer:
//
unsigned int (*funcPtr) (char const*, char);
//
// Assign a function to the function pointer
//
funcPtr = &first;
//
// Execute function pointer
//
printf("%d\n", (*funcPtr)("Hello world", 'o'));
}
Passing a function pointer to a function
A function whose first argument is a function pointer. The function executes the function pointer with the other two arguments it receives:
#include <stdio.h>
#include "funcs.h"
int execute_funcPtr(unsigned int (*funcPtr)(char const*, char), char const* arg_1, char arg_2) {
printf("funcPtr returned %d\n", (*funcPtr)(arg_1, arg_2));
}
int main() {
execute_funcPtr(&count, "Hello world", 'o');
execute_funcPtr(&first, "Hello world", 'o');
execute_funcPtr(&last , "Hello world", 'o');
}
Returning a function pointer
Similarly, it's possible to declare a function that returns a function pointer:
#include <stdio.h>
#include "funcs.h"
#define FUNC_COUNT 1
#define FUNC_FIRST 2
#define FUNC_LAST 3
unsigned int (*getFuncPointer(int funcNo))(char const*, char) {
if (funcNo == FUNC_COUNT) return &count;
if (funcNo == FUNC_FIRST) return &first;
if (funcNo == FUNC_LAST ) return &last ;
return NULL;
}
int main() {
unsigned int (*funcPtr) (char const*, char);
funcPtr = getFuncPointer(FUNC_LAST);
printf("%d\n", (*funcPtr)("Hello world", 'o'));
}