Question

I saw a piece of code like this:

#include <windows.h>
static VOID (WINAPI * pFun) (DWORD seconds) = NULL;

void foo(PVOID * par){
    return;
}

int main(){
    foo(&(PVOID&) pFun); // type-cast 1
    foo((PVOID *) (&pFun)); // type-cast 2
    return 0;
}

I can understand the type-cast 2, but how does type-cast 1 work?

Answer 1

First of all, you’re using C++.

In windows.h VOID and PVOID are defined as void and void* respectively.

#define VOID void

typedef void *PVOID;

Let’s look at a basic program:

#include <iostream>

using namespace std;

void foo(void** pp)
{
  int i = *((int*) *pp);
  cout << i << endl;
}

int main()
{
  int i = 10;
  int *p = &i;

  foo((void**) &p); // call #1
  foo(&(void*&) p); // call #2
}

In the first case, we’re taking the address of p, which is int**, and passing it to foo() by casting it to void**.

In the second case, we’re first casting p to void*& and then taking its address, which would be void**, and passing that to foo(). The reason it’s void*& and not void* is that you can use the unary & operator (address of) only on an lvalue.