Re: [boost] Bind documentation - request for clarification

From: Peter Dimov (pdimov_at_[hidden])
Date: 2001-12-26 09:27:52

From: "David Abrahams" <david.abrahams_at_[hidden]>
> The documentation for bind says:
>
> "Note that the first argument - the bound function object - is an
exception
> to the nesting rules. A nested bind expression passed to bind as a first
> argument is not treated differently from any other function object: "
>
> But it never says explicitly what "the nesting rules" are. With some work
I
> can infer that bind objects passed as arguments to bind (other than the
> first), are evaluated when the outer bind is called, and the results are
> used in place of the bind objects themseves.

Here's what the documentation says:

"Some of the arguments passed to bind may be nested bind expressions
themselves:

bind(f, bind(g, _1))(x);               // f(g(x))

The nested subexpressions are evaluated when the function object is called."

This looks pretty similar to your explanation.

> One reason I find the passage confusing is that in the example, it says
g(5,
> 8) is called, but that appears to violate arity constraints since x is
> passed as an argument but apparently never used.

The example is a bit contrived, but so is the situation. The point is that
the first subexpression (the bound function object) is not evaluated when
it's a nested bind.

It is a feature of bind that unreferenced arguments are silently ignored;
the bound function object's arity is enforced, but the function object
returned by bind is flexible (it enforces only the minimal arity.)

--
Peter Dimov
Multi Media Ltd.