[boost] Re: boost::any: explicit constructor?

From: Bjorn.Karlsson_at_[hidden]
Date: 2004-02-26 06:05:24

> From: Kevlin Henney [mailto:kevlin_at_[hidden]]

> I have yet to see a convincing reason to make it explicit. Autoboxing
> has been a part of boost::any's design from day 1, which 
> existed before
> Boost -- and, indeed, the "autoboxing" terminology. Supporting such
> conversions was intentional. There is no more reason to make the
> converting constructor of boost::any explicit than there is 
> to make the
> converting constructor of std::string explicit, or the conversion from
> int to long explicit, or the conversion from T * to void *, or indeed
> any other widening conversion.

Perhaps you could add this design decision as a FAQ in the documentation for
any? Something like this (assuming a faq.xml among the any docs):

<qandaentry>
  <question><para>Why isn't the parameterized constructor for
<classname>boost::any</classname> explicit?</para></question>
   <answer>
    <para>The most important reason is that
<classname>boost::any</classname> favors 
      the convenience of implicit widening conversions over the additional
safety of an explicit constructor. 
      This design decision is fundamental for <code><classname
alt="boost::any">any</classname></code>, so making the constructor 
      explicit would require very convincing reasons.  
    </para>
    <para>One useful idiom when the implicit construction of
<code><classname alt="boost::any">any</classname></code> is considered a
    problem is to always pass
    <classname>boost::any</classname> by reference rather than by reference
to <code>const</code>, or by value. 
    This avoids implicit conversions of arguments to functions accepting
<classname>boost::any</classname>. For example, the following example shows 
    how to disable autoboxing for calls to the function <code>sample</code>:
    </para>
    <programlisting name="any.faq.pass_by_reference">void
sample(boost::any&amp; a) {}

int main() {
   boost::any a(5);
   sample(a); // This works
   sample(5); // This won't compile
}</programlisting>
</answer>
</qandaentry>

If you like it, I'll update the docs.

Bjorn