Re: Const method returning a functor to non-const method
On 22 Aug., 08:47, ta0k...@yahoo.com wrote:
On Aug 21, 5:02 pm, peter koch larsen <peter.koch.lar...@gmail.com>
wrote:
In this case - as is usually the case - there is everything wrong with
a const_cast. We must presume that the caller wants to use the
function, and if he does so, our constant object will be changed -
possibly resulting in undefined behaviour.
The function doesn't modify the instance with the 'const_cast', and
what happens after the function's return isn't the concern of that
function.
But it is of concern for those who of us who want the compiler to
assist us in writing maintainable code. The const was invented for
precisely this purpose.
Also note that a const prevents us to return pointers or references to
modifiable member-variables. In this situation, however, one could
imagine just reading the variable. When you return a function object
there is not much else to do with the object than to call the
function.
Sure, if the object is unconditionally 'const' then that
would be undefined behavior, but otherwise the 'const'-ness of the
function isn't compromised. In this case it's the class' author's
responsibility to document that calling the 'getAction' function on a
'const'-defined instance causes undefined behavior.
I can only say that i can not at all agree with you on that point.
Circumventing the type system is a very bad thing to do.
In fact, I can't
think of a *more* appropriate circumstance to use a 'const_cast',
though such a move requires minor documentation for proper usage, as
does nearly every other non-trivial interface.
A const_cast is primarily a means to cope with legacy libraries that
fails to respect const-correctness - perhaps because of the age of
that library, perhaps because it is written in a language without the
notion of constness. I can't really find any other legitimate usage
for const_casts.
I don't believe I've ever created a library that consciously broke
constness, and I doubt anyone could provide a motivating case for
that.
/Peter
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