Hi all,

is there any way to prevent a struct from being created directly? Basically, I want to prevent this:

{
  non_null!(T) a;
}

I want people to have to use provided functions to create a structure:

{
  auto a = non_null!(T)(new T);
}

  -- Daniel

Hello Daniel,

> Hi all,
> 
> is there any way to prevent a struct from being created directly?
> Basically, I want to prevent this:
> 
> {
> non_null!(T) a;
> }
> I want people to have to use provided functions to create a structure:
> 
> {
> auto a = non_null!(T)(new T);
> }
> -- Daniel
> 

struct S { private static S opCall(); } //???

On Mon, 26 Jan 2009 20:40:01 +0300, BCS <none@anon.com> wrote:

> Hello Daniel,
>
>> Hi all,
>>  is there any way to prevent a struct from being created directly?
>> Basically, I want to prevent this:
>>  {
>> non_null!(T) a;
>> }
>> I want people to have to use provided functions to create a structure:
>>  {
>> auto a = non_null!(T)(new T);
>> }
>> -- Daniel
>>
>
> struct S { private static S opCall(); } //???
>
>

Nope:

S s; //okay

I think it would really suck to introduce special cases to forbid default initialization of structs. And assuming T was the type of the struct, what would T.init do? Or typeid(T).init()?

Use a class instead.

If you really need a struct, you could use a private field, that signals if the struct was properly initialized.

E.g.

struct Foo {
	debug private bool initialized;
	
	static Foo opCall() {
		Foo n;
		debug n.initialized = true;
		return n;
	}
	
	void foo() {
		assert (initialized);
		//do something useful
	}
}

grauzone wrote:
> Use a class instead.

That would defeat the purpose of defining a non_null template in the first place.

  -- Daniel

Daniel Keep wrote:
> Hi all,
> 
> is there any way to prevent a struct from being created directly?
> Basically, I want to prevent this:
> 
> {
>   non_null!(T) a;
> }
> 
> I want people to have to use provided functions to create a structure:
> 
> {
>   auto a = non_null!(T)(new T);
> }
> 
>   -- Daniel

struct non_null(T : class)
{
	// this should be private, but no can do
	// an alternative is to obscure _member somehow,
	// say, as a void*, just to make sure that nobody
	// uses it without it being obvious that they're doing
	// something bad
	T _member;

	T opDot ()
	{
		if (_member is null) _member = new T;
		return T;
	}
	
	non_null!(T) opAssign(T value)
	{
		demand (value !is null);
		_member = value;
	}
}