[c] How do you pass a function as a parameter in C?

I want to create a function that performs a function passed by parameter on a set of data. How do you pass a function as a parameter in C?

Pass address of a function as parameter to another function as shown below

#include <stdio.h>

void print();
void execute(void());

int main()
{
    execute(print); // sends address of print
    return 0;
}

void print()
{
    printf("Hello!");
}

void execute(void f()) // receive address of print
{
    f();
}

Also we can pass function as parameter using function pointer

#include <stdio.h>

void print();
void execute(void (*f)());

int main()
{
    execute(&print); // sends address of print
    return 0;
}

void print()
{
    printf("Hello!");
}

void execute(void (*f)()) // receive address of print
{
    f();
}

This question already has the answer for defining function pointers, however they can get very messy, especially if you are going to be passing them around your application. To avoid this unpleasantness I would recommend that you typedef the function pointer into something more readable. For example.

typedef void (*functiontype)();

Declares a function that returns void and takes no arguments. To create a function pointer to this type you can now do:

void dosomething() { }

functiontype func = &dosomething;
func();

For a function that returns an int and takes a char you would do

typedef int (*functiontype2)(char);

and to use it

int dosomethingwithchar(char a) { return 1; }

functiontype2 func2 = &dosomethingwithchar
int result = func2('a');

There are libraries that can help with turning function pointers into nice readable types. The boost function library is great and is well worth the effort!

boost::function<int (char a)> functiontype2;

is so much nicer than the above.

You need to pass a function pointer. The syntax is a little cumbersome, but it's really powerful once you get familiar with it.

Functions can be "passed" as function pointers, as per ISO C11 6.7.6.3p8: "A declaration of a parameter as ‘‘function returning type’’ shall be adjusted to ‘‘pointer to function returning type’’, as in 6.3.2.1. ". For example, this:

void foo(int bar(int, int));

is equivalent to this:

void foo(int (*bar)(int, int));

Since C++11 you can use the functional library to do this in a succinct and generic fashion. The syntax is, e.g.,

std::function<bool (int)>

where bool is the return type here of a one-argument function whose first argument is of type int.

I have included an example program below:

// g++ test.cpp --std=c++11
#include <functional>

double Combiner(double a, double b, std::function<double (double,double)> func){
  return func(a,b);
}

double Add(double a, double b){
  return a+b;
}

double Mult(double a, double b){
  return a*b;
}

int main(){
  Combiner(12,13,Add);
  Combiner(12,13,Mult);
}

Sometimes, though, it is more convenient to use a template function:

// g++ test.cpp --std=c++11

template<class T>
double Combiner(double a, double b, T func){
  return func(a,b);
}

double Add(double a, double b){
  return a+b;
}

double Mult(double a, double b){
  return a*b;
}

int main(){
  Combiner(12,13,Add);
  Combiner(12,13,Mult);
}

It's not really a function, but it is an localised piece of code. Of course it doesn't pass the code just the result. It won't work if passed to an event dispatcher to be run at a later time (as the result is calculated now and not when the event occurs). But it does localise your code into one place if that is all you are trying to do.

#include <stdio.h>

int IncMultInt(int a, int b)
{
    a++;
    return a * b;
}

int main(int argc, char *argv[])

{
    int a = 5;
    int b = 7;

    printf("%d * %d = %d\n", a, b, IncMultInt(a, b));

    b = 9;

    // Create some local code with it's own local variable
    printf("%d * %d = %d\n", a, b,  ( { int _a = a+1; _a * b; } ) );

    return 0;
}

I am gonna explain with a simple example code which takes a compare function as parameter to another sorting function. Lets say I have a bubble sort function that takes a custom compare function and uses it instead of a fixed if statement.

Compare Function

bool compare(int a, int b) {
    return a > b;
}

Now , the Bubble sort that takes another function as its parameter to perform comparison

Bubble sort function

void bubble_sort(int arr[], int n, bool (&cmp)(int a, int b)) {

    for (int i = 0;i < n - 1;i++) {
        for (int j = 0;j < (n - 1 - i);j++) {
            
            if (cmp(arr[j], arr[j + 1])) {
                swap(arr[j], arr[j + 1]);
            }
        }
    }
}

Finally , the main which calls the Bubble sort function by passing the boolean compare function as argument.

int main()
{
    int i, n = 10, key = 11;
    int arr[10] = { 20, 22, 18, 8, 12, 3, 6, 12, 11, 15 };

    bubble_sort(arr, n, compare);
    cout<<"Sorted Order"<<endl;
    for (int i = 0;i < n;i++) {
        cout << arr[i] << " ";
    }
}

Output:

Sorted Order
3 6 8 11 12 12 15 18 20 22