• Minas Mina (45/45) Aug 13 2012 I'm writing an insert() function for a binary tree in D:
• Chris Cain (9/10) Aug 13 2012 It's not really a bug. You're using insert before its return type

"Minas Mina" <minas_mina1990 hotmail.co.uk> writes:

I'm writing an insert() function for a binary tree in D:

Note: Node is a value type (struct)

Node!(T)* insert(T) (Node!(T)* root, T val)
{
	if( root is null )
	{
		root = new Node!T();
		root.value = val;
		root.left = root.right = null;
	}
	else
	{
		if( val < root.value )
			root.left = insert(root.left, val);
		else
			root.right = insert(root.right, val);
	}
	
	return root;
}


This works (compiles).


auto insert(T) (Node!(T)* root, T val)
{
	if( root is null )
	{
		root = new Node!T();
		root.value = val;
		root.left = root.right = null;
	}
	else
	{
		if( val < root.value )
			root.left = insert(root.left, val); // line x
		else
			root.right = insert(root.right, val); // line y
	}
	
	return root;
}

This doesn't compile.

test.d(x): Error: forward reference to inferred return type of 
function call insert((*root).left,val)
test.d(y): Error: forward reference to inferred return type of 
function call insert((*root).right,val)

Is it a bug with auto or something else?

"Chris Cain" <clcain uncg.edu> writes:

On Monday, 13 August 2012 at 19:09:04 UTC, Minas Mina wrote:
 Is it a bug with auto or something else?

It's not really a bug. You're using insert before its return type 
has been inferred. Hence, it says "forward reference to inferred 
return type".

However, in this case, it should be possible to infer the return 
type because they type of root is known. Maybe an enhancement 
request?

In any case, the solution, as you've found, is to explicitly 
define the return type if you're going to be using it recursively.