Re: [Boost-bugs] [Boost C++ Libraries] #5637: use of mpl::print imposes default constructible requirement on argument

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #5637: use of mpl::print imposes default constructible requirement on argument
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2011-07-13 18:55:48

#5637: use of mpl::print imposes default constructible requirement on argument
-------------------------------+--------------------------------------------
  Reporter:  ramey             |       Owner:  dave     
      Type:  Tasks             |      Status:  closed   
 Milestone:  To Be Determined  |   Component:  mpl      
   Version:  Boost 1.46.1      |    Severity:  Problem  
Resolution:  invalid           |    Keywords:  mpl print
-------------------------------+--------------------------------------------
Changes (by dave):

  * status:  new => closed
  * resolution:  => invalid

Comment:

 Replying to [ticket:5637 ramey]:
 > I'm using the following to debug some TMP code that looks like the
 following.
 > {{{
 > #include <boost/mpl/print.hpp>
 > struct X {
 >     X(int);
 > };
 >
 > boost::mpl::print<
 >     X
 > >::type x;
 >
 > boost::mpl::print<
 >     X
 > >::type y;
 > }}}
 >
 > I get an error message that the type X is not default constructible.

 I don't think mpl::print is supposed to guarantee you can do that.  Why
 don't you just  do

 {{{
 boost::mpl::print<X> y;
 }}}

 instead?


 > A little investigation yields the definition of mpl::print (simplified
 for exposition)
 >
 > {{{
 > template <class T>
 > struct print : identity<T>
 > {
 >     enum { n = sizeof(T) + -1 };
 > };
 > }}}
 >
 > So invoking the print creates an instance with the default constructor

 No.  Even if you treat `mpl::print` as a metafunction, invoking it doesn't
 create an instance.  Constructing an instance of its result… creates an an
 instance (which is of course tautological, sorry).

 > which provokes the error in this case.  For my purposes, I made the
 following change which seems to address the problem:
 >
 > {{{
 > template <class T>
 > struct print : identity<T *>
 > {
 >     enum { n = sizeof(T) + -1 };
 > };
 > }}}
 >
 > I'm using MSVC 9.0.

 Unfortunately, `mpl::print` [#1900 is undocumented], but the intention is
 that as a metafunction, it returns `T`, not `T*`, so that change wouldn't
 work.

 > A couple of misceleanous issues besides this:
 >
 > On my current version of GCC - 4.3.4 I get no warning at all.

 If you can figure out how to generate an appropriate warning for that
 compiler, we'd love to have a patch.

 > If the same type is printed more than once, I only get the warning on
 the first instance.

 Nothing we can do about the latter; it's just an artifact of your
 compiler's behavior.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/5637#comment:3>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.