Inheritance of purity

Steven Schveighoffer schveiguy at
Fri Feb 17 12:38:51 PST 2012

On Thu, 16 Feb 2012 21:49:40 -0500, Walter Bright  
<newshound2 at> wrote:

> Given:
>      class A { void foo() { } }
>      class B : A { override pure void foo() { } }
> This works great, because is covariant with, meaning it can  
> "tighten", or place more restrictions, on foo. But:
>      class A { pure void foo() { } }
>      class B : A { override void foo() { } }
> fails, because tries to loosen the requirements, and so is not  
> covariant.
> Where this gets annoying is when the qualifiers on the base class  
> function have to be repeated on all its overrides. I ran headlong into  
> this when experimenting with making the member functions of class Object  
> pure.
> So it occurred to me that an overriding function could *inherit* the  
> qualifiers from the overridden function. The qualifiers of the  
> overriding function would be the "tightest" of its explicit qualifiers  
> and its overridden function qualifiers. It turns out that most functions  
> are naturally pure, so this greatly eases things and eliminates annoying  
> typing.
> I want do to this for @safe, pure, nothrow, and even const.
> I think it is semantically sound, as well. The overriding function body  
> will be semantically checked against this tightest set of qualifiers.
> What do you think?

I think it feels like a (welcome) about face from previous stances.  But I  
share some concern with others in that we are separating the attributes of  
a function from its declaration to the point where we need a tool to  
determine what the exact attributes of a function are.

I shudder to think about how I would understand some of the CSS I have to  
deal in my daily work if I didn't have firebug.  I hope D doesn't become  

Of course, if we get to the point where DDoc is full-featured, one will  
almost never have to look at function declarations when using them.

Also, I like how you *can* repeat the attributes if it's necessary to.

Could there be a compiler option for requiring overriding functions to  
repeat base attributes?  Or at least print out where they are while  
compiling?  At least then you can see where your attributes aren't  


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