Re: [boost] assignable()

From: David Abrahams (dave_at_[hidden])
Date: 2002-11-29 17:13:26

"David B. Held" <dheld_at_[hidden]> writes:

> On c.l.c++.m, I argue that operator=() should not use the swap idiom
> (largely based on comments I've seen Dave Abrahams make), but that a
> named assignment function should instead provide assignment with the
> strong guarantee.  

When neccessary. It's not as though a "strong guarantee assignment" is
a fundamental operation which people often need.

> Since this is such a trivial function, I thought
> it would be nice to encapsulate it in a simple class, provided
> below:
>
> template <class T>
> struct assignable
> {
>     T& assign(T t)
>     {
>         BOOST_STATIC_ASSERT(is_base_and_derived<assignable, T>::value);
>         T* const me = static_cast<T*>(this);
>         me->swap(t);
>         return *me;
>     }
> };
>
> I'm not sure about the type trait name.  

Which type trait name?

> I just added that in for safety.

Added what?

> Of course, this class assumes that T has a non-throwing member
> function named swap().  Obviously, it can be used like so:
>
> class my_class : public assignable<my_class>
> {
> // ...
> };

Don't you want the static assertion in the constructor or in the class
body itself? Do you want to have to instantiate assign() to find out
you've misused this template?

> void foo(void)
> {
>     my_class a, b;
>     // ...
>     a.assign(b);
> }
>
> The nice thing about this approach, I think, is that if all the data
> members have a basic guarantee assignment, then the default
> operator= works just fine, and now you can add strong guarantee
> assignment with one line (assuming you already had swap, of course).
>
> If people like the idea, perhaps it could go into utility.hpp?

I'm not sure it's useful enough to warrant putting it in
utility.hpp. Also I have concerns about the name "assign()", since
that is used in the standard library to mean something slightly
different.

-- 
                       David Abrahams
   dave_at_[hidden] * http://www.boost-consulting.com
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