Re: [boost] about function.hpp

From: Doug Gregor (gregod_at_[hidden])
Date: 2001-04-30 23:02:08

On Monday 30 April 2001 05:34, you wrote:
> i have a question about the just uploaded function
> Callback/function-04-29.zip by Doug Gregor (great stuff btw):
>     is it possible to make the operator () methods const? or is/was there a
> good reason for making them non-const?
> (saves me lots of mutable's  if they're const;-)

I see two options for making it const, each with its semantic problems:
        1) Just make function::operator() const, but constness won't be passed 
through to underlying function objects, so we're dealing with a semantic grey 
area.
        2) Have both a const and a non-const version, and pass an "is_const" flag so 
that function objects will be invoked as const or non-const and the constness 
will be passed through. However, this requires a const version of operator() 
to be defined for any function object regardless of whether it is used or not.

I don't really like #1 because it's inheritently unsafe: if one calls a const 
function, one expects no modifications. However, mutables, global variables, 
and free functions all break this constness guarantee already, so would it be 
so bad if constness was lost?

I don't really like #2 because it requires const operator() on functors, 
which may not be reasonable for some function objects. Requiring the user to 
define extra members that won't/can't be used seems to go against the 
intention of C++: don't make the user pay for what s/he doesn't use.

So here's your answer: I dislike both solutions, and could not come up with 
another one very easily. I'm leaning toward #1 because guaranteeing constness 
is impossible given the ability to link to free functions. Any input would be 
very, very helpful, and either solution is reasonably easy to implement.

        Doug Gregor