#include <cstdio>
using namespace std;
int troll(int x){ return x + 1; }
 
int main(){
    int (*f)(int) = troll;
    printf("%d\n", (*f)(2));
    printf("%d\n", f(3));
}
 

So basically, to define a function pointer variable or whatever the correct term is, you pretend you're writing a function declaration without the argument names and body, and you replace the function name with (*something). The function pointer and the function it's being assigned have to have the exact same signature and return value.

To invoke the function, you can explicitly dereference it as in line 7 or, on most recent compilers, just pretend it's a function as in line 8.

Functions with a return type of void

void a() {}

return nothing. The compiler will get mad at you if you do something silly like

int x = a();

fp.cpp: In function ‘int main()’:
fp.cpp:11: error: void value not ignored as it ought to be

"void" is not a type that a variable can hold either.

void x;

fp.cpp: In function ‘int main()’:
fp.cpp:11: error: variable or field ‘x’ declared void

But you knew that already, right?

On the other hand, "void*" is a pointer to anything... so you can do extremely questionable things like

int x = 1;
char y[20] = "windmill";
FILE* hi = stdout;
 
void* p = &x;
printf("%d\n", *((int*)p));
p = y;
printf("%s\n", ((char*)p));
p = stdout;
fprintf((FILE*) p, "\"\"\"\"\"\n");

The question is, of course,
Why would anybody ever want to do that???

Mainly, this function in <cstdlib>.

qsort(void* base, size_t num, size_t size,
    int ( * comparator ) ( const void *, const void * ) );

Of course you'd probably use STL containers and algorithms if you're actually using C++, and while they still do need function pointers, their templates give you some type safety, so the second part of this post is being rather silly if you're not using C.

Also:

#include <vector>
vector<vector<int> > v;
// MANDATORY SPACE^