PrimitiveRef ?

[Baz via Digitalmars-d-learn]

On Monday, 23 March 2015 at 16:58:49 UTC, Andre wrote:
> Hi,
>
>  " (needed for specifying reference behavior in a type tuple).
> I need exactly that behavior. I am currently unsure whether it 
> is possible at all to have such a construct which works at user 
> side exactly like a boolean (booleans can be assigned and 
> directly compared with ==) but internally are pointers...
>
> (The coding does not compile)
>
> Kind regards
> André
>
>
> struct PrimitiveRef(T)
> {
> 	private T* _value;
>
> 	alias  _value this;
> 	
> 	void opAssign(T v)
> 	{
> 		_value = v;
> 	}
> 	
> }
> alias BoolRef = PrimitiveRef!bool;
>
> void test(BoolRef b)
> {
> 	b = true;
> }
>
> void main()
> {
> 	BoolRef b = false;
> 	test(b);
> 	assert(b == true);	
> }

even if the code would compile you'd get an AV because _value is 
null.
Then when you initalize b with false this could not work because 
false is not a a pointer to a bool.

struct PrimitiveRef(T)
{
     private T* _value;
     alias _value this;
     void opAssign(T v)
     {
         *_value = v;
     }
	
}
alias BoolRef = PrimitiveRef!bool;

void test(BoolRef b)
{
     b = true;
}

bool btarget;
void main()
{
     BoolRef b;
     b._value = &btarget;
     test(b);
     assert(*b == true);	
}

I don't know where you've read this, but i 'm interseted too. 
Maybe it's an ABI thing: since the only member is a reference (so 
`size_t.sizeof` bytes) the `struct` can be passed in a register 
during the function call, instead of being copied on the stack...

---
void foo(BoolRef aRef)
{
     // x86: aRef is in EAX
     // X86_64 aRef is in RAX
}
---

What is the source of "I read that if a structure only contains a 
reference, it will behave like a reference type in function 
calls" ?