Assign a function pointer passed in as a parameter

Question

Is it possible to pass in a function pointer to a function and have that function assign the function to use?

Below is an example of what I mean:

void sub(int *a, int *b, int *c) { *c = *a + *b; } void add(int *a, int *b, int *c) { *c = *a - *b; } void switch_function(void (*pointer)(int*, int*, int*), int i) { switch(i){ case 1: pointer = &add; break; case 2: pointer = &sub; break; } } int main() { int a; int b; int c; void (*point)(int*, int*, int*); switch_function(point, 1); // This should assign a value to `point` a = 1; b = 1; c = 0; (*point)(&a, &b, &c); return 0; }

Any help is appreciated.

In this patricular case, you should return a function pointer. — n. m. could be an AI
– n. m. could be an AI, Commented Mar 26, 2018 at 17:50
It should really have a default case where pointer is set to NULL on bad input. Better to crash when that pointer is called than to risk calling some actual function. — ikegami
– ikegami, Commented Mar 26, 2018 at 18:40

Ronan Boiteau · Accepted Answer · 2018-03-26 17:51:31Z

You can create a pointer to a function pointer.

Here is the syntax:

void (**pointer)(int*, int*, int*);

After you initialized it, you can then dereference this pointer to set your function pointer:

*pointer = &add; *pointer = &sub;

So your function would look like this:

void switch_function(void (**pointer)(int*, int*, int*), int i) { switch(i) { case 1: *pointer = &add; break; case 2: *pointer = &sub; break; } }

...and you'd call it like this:

switch_function(&point, 1);

I think you can also tighten this up a bit more and replace (e.g.) &add with add as the latter is already the address of add
Pointer to function pointer makes perfect sense, thanks for the help.

dinoex · Accepted Answer · 2018-03-26 18:04:06Z

The syntax is much easier if you use a typedef for your function pointer. Especially if you use a double indirection in your sample.

typedef void (*fptr)(int*, int*, int*); void sub(int *a, int *b, int *c) { *c = *a - *b; } void add(int *a, int *b, int *c) { *c = *a + *b; } void switch_function(fptr *pointer, int i) { switch(i){ case 1: *pointer = &add; break; case 2: *pointer = &sub; break; } } int main() { int a; int b; int c; fptr point; switch_function(&point, 1); a = 1; b = 1; c = 0; (**point)(&a, &b, &c); return 0; }

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