Constructor within a template

[joatmon]

This is my first experiment with templates, and I'm a little confused. I'm writing a constructor and have this:

D_Node<Item>::D_Node( const Item &init_data = Item( ), 
			   D_Node *init_fore = NULL, D_Node *init_back = NULL )
    {
        data_field = init_data;
        link_fore = init_fore;
        link_back = init_back;
    }

However, this won't compile. I get the following error (actually, three of them, one for each variable:

default argument given for parameter 1 of 'my_namespace::D_Node<Item>::D_Node(const Item&, my_namespace::D_Node<Item>*, my_namespace::D_Node<Item>*)'

'after previous specification in 'my_namespace::D_Node<Item>::D_Node(const Item&, my_namespace::D_Node<Item>*, my_namespace::D_Node<Item>*)'


I think that I understand what the error message it telling me, but this is exactly how I have been taught to write this constructor (or at least how I THINK I've been taught...). What am I doing wrong?

Thanks.

[joatmon]

Follow up: I found something on this error that says:

"This occurs when the implementation file (.cxx) specifies a default argument. Only header files (.h) should ever give a default value. "

I can make this go away by inlining the variable assignments within the header file. But I still don't understand this. Why can't I set these variables within the implementation file? That doesn't make any sense to me.

Thanks again.

[DSLq5Di1]

It is to prevent you from redefining the default value, else you might have a declaration of foo(bar = 0) while in the implementation foo(bar = -1).

[joatmon]

Thanks. What I don't understand is why this is different than any old default constructor. Say that I have a normal old class (not within a template) with private variable x. In my default constructor in the implementation file, I know that I can have a line that says x = 0;. So why is this any different? Is it something about templates?

Thanks again!

[DSLq5Di1]

I see what you mean, but I'm not really sure why that is the case either. I've created a test case and opened a question over @stackoverflow, hopefully they can provide you with an answer. ^^

http://stackoverflow.com/questions/6805507/default-constructor-defined-with-default-arguments-outside-the-class-definition

[joatmon]

Thanks!