Sending a dummy variable to a function as a compound literal - c

Question

Let's say I have the following I'm using to call a function:
Item dummy;
while (!QueueIsEmpty(pq))
DeQueue(pq, &dummy);
Is there a way to put the dummy parameter into the call itself? Something like:
while (!QueueIsEmpty(pq))
DeQueue(pq, &(Item)NULL);

Answer 1

It looks like the method dequeues an object and copies its value to the dummy variable. Where would you expect it to be copied without providing a place to copy it to?
Do you want to just throw it away and clear the queue this way?
If so I would expect that you have to pass it inside unless there is another method that just clears the queue without even copying it. Maybe some clear method.

Answer 2

This isn't valid C [AFAICT]
From my top comment, you might mean: DeQueue(pq,(Item *) NULL) or simply: DeQueue(pq, NULL)?
You're trying to create a compound literal, but it's being initialized from a [NULL] pointer.
Here's the test program I created:
#include <stdio.h>
typedef struct {
int x;
} Queue;
typedef struct {
int y;
int z;
} Item;
int
QueueIsEmpty(Queue * pq)
{
return 0;
}
void
DeQueue(Queue * pq, Item * item)
{
}
int
main(void)
{
Item dummy;
Queue *pq = NULL;
while (!QueueIsEmpty(pq))
DeQueue(pq, &dummy);
// Is there a way to put the dummy parameter into the call itself?
// Something like:
while (!QueueIsEmpty(pq))
DeQueue(pq, &(Item) NULL);
return 0;
}
Here's the output of gcc:
x.c:38:3: error: conversion to non-scalar type requested
Here's the output of clang:
x.c:38:16: error: used type 'Item' where arithmetic or pointer type is required
What you may have wanted is:
DeQueue(pq, &(Item) { 0 })
But, passing that is a bit of [too much] trickery.
Personally, I've never done something like that. I've always just passed down a NULL pointer.
If you do really want a "dummy" parameter, I'd do the Item dummy; thing [vs the compound literal]. It's actually faster because you don't add the overhead of the initialization.
To me, the compound literal is getting "too cute"

Answer 3

If the function requires the address of a valid object, you can pass in the address of a compound literal:
DeQueue(pq, &((Item){0}) );