Union function

Question

Why dose he create the function before main then define it after main ?
Why not just once ?

And why in main he puts set_id(&item) not (*item) ?
https://code.sololearn.com/crEnRhZjVnFH/?ref=app

Answer 1

"Why does he create the function before main then define it after main?
Why not just once?"

https://en.m.wikipedia.org/wiki/Forward_declaration
https://norasandler.com/2018/06/27/Write-a-Compiler-9.html

"And why in main he puts set_id(&item) not (*item)?"

The function accepts pointer to union instance ...

void set_id (union id *item);

Hence an address to union instance <item> is required, as argument, during function call ...

void set_id(union id *item)
{
item->id_num = 42;
}

Answer 2

Dark
Yes, the indirection operator (with `*` symbol) is used to dereference the data whose address is carried by a pointer.

The address-of operator (with `&` symbol) is used to get the address of some data in memory. The address-of operator is used here (&item) to get address of <item> in memory.

If <item> itself was a pointer, then we can directly use it like so (item) because a pointer carries memory address. In such case, there will be no need to use address-of operator `&`.

"If I defined a function with a pointer as a parameter, when I call it I have to use the adress not the pointer itself as an argument?"

It depends, if the argument you want to pass WAS a pointer, then just go ahead pass it on. But if the argument WAS NOT a pointer, like what we have here - remember <item> is a `union` instance, then we need to pass address of <item>, and that's where and why we use the address-of operator `&`

* About operators:
https://en.cppreference.com/w/c/language/operator_precedence

Answer 3

Ipang u mean *item not same as &item ?
and if i defined a function with a pointer as a parameter, when i call it i have to use the adress not the pointer itself as an argument ?