Boost C++ Libraries: Ticket #5637: use of mpl::print imposes default constructible requirement on argument

Robert Ramey — Fri, 24 Jun 2011 01:15:46 GMT

Managed to make it work for my GCC system as well !

template <class T>
struct print
    : mpl::identity<T *>
#if defined(__MWERKS__)
    , aux::print_base
#endif
{
#if defined(BOOST_MSVC)
    enum { n = sizeof(T) + -1 };
#elif defined(__MWERKS__)
    void f(int);
#else
    enum {
        n =
# if defined(__EDG_VERSION__)
           aux::dependent_unsigned<T>::value > -1
# else
           sizeof(T) / 0//sizeof(T) > -1
# endif
        };
#endif
};

Robert Ramey

Robert Ramey — Sat, 25 Jun 2011 16:59:31 GMT

Damn - my patch mpl::identity<T> <- mpl::identity<T *> - broke type printing of reference types. So I withdraw that suggestion. The other change:sizeof(T) > -1 <- sizeof(T) / 0 still stands however - at least from my version of GCC.

Robert Ramey

status changed; resolution set

Dave Abrahams — Wed, 13 Jul 2011 18:55:48 GMT

status new → closed
resolution → invalid

Replying to 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 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.

Dave Abrahams — Wed, 13 Jul 2011 18:57:48 GMT

Replying to ramey:

The other change:sizeof(T) > -1 <- sizeof(T) / 0 still stands however - at least from my version of GCC.

As far as I know, division by zero is still a hard error. mpl::print is supposed to generate warnings. If you're satisfied with an error, I'm sure you can figure out how to do that without using print.

pmachata@… — Thu, 18 Jul 2013 18:03:23 GMT

It seems that using NULL to initialize the enum value tickles GCC the right way to produce a warning if the template is actually used (unlike division by zero, which produces one warning always, and optionally more warnings if print is actually "invoked"). It's also enabled by default, unlike signed/unsigned compare that's used in the current solution.

One downside is that we get a warning only for the first failure in a translation unit. What's considered more important: that we have one warning always, or that we get all citations, but only with -Wall?

pmachata@… — Thu, 18 Jul 2013 19:39:25 GMT

... alternatively, I suppose, there could be both.

pmachata@… — Thu, 18 Jul 2013 21:45:45 GMT

Apologies, I was interpreting the output wrong. Initialization by NULL appears to do exactly what we need.

From what I can figure out from GCC sources, this warning is controlled by -Wconversion-null, which has been on by default since it was introduced in 2010 (4.5.0). Before that, the warning was emitted as well, keyed to -Wconversion, which was off even in -Wall. From the time it was written in 2000 until 2006 (4.1.0), the warning was always emitted and couldn't even be turned off. So after 4.1.0 up until 4.5.0, it's more advantageous to use the signed/unsigned comparison (which is on at least in -Wall), otherwise the NULL conversion is better.

pmachata@… — Thu, 18 Jul 2013 23:29:21 GMT

Comparison of values belonging to different enums is even better. That appears to be available since 1996, and is bound to always-on -Wenum-compare since 2008.

attachment set

pmachata@… — Fri, 19 Jul 2013 00:09:35 GMT

attachment → boost-1.54.0-mpl-print.patch

Fix mpl::print in GCC

status changed; resolution deleted

Dave Abrahams — Sat, 20 Jul 2013 17:08:36 GMT

status closed → reopened
resolution invalid

owner, status changed

Dave Abrahams — Sat, 20 Jul 2013 17:09:02 GMT

owner changed from Dave Abrahams to Aleksey Gurtovoy
status reopened → new

Tue, 15 Sep 2015 14:36:29 GMT

Hmm, the GCC patch no longer seems to work, or I just can't figure out what mpl::print is meant to do.

Tue, 15 Sep 2015 14:44:48 GMT

It stopped working with GCC 4.8