Re: [boost] Pervasive dependency upon mpl::bool_

Subject: Re: [boost] Pervasive dependency upon mpl::bool_
From: Peter Dimov (lists_at_[hidden])
Date: 2014-06-03 06:17:36

Andrey Semashev wrote:

> Yes, I had similar idea. I'll try to extract minimal bits of MPL in a 
> separate branch in Core, maybe dropping some workarounds in the process to 
> reduce the amount of code.

Extracting the commonly used parts of MPL such as mpl::bool_, mpl::if_, 
mpl::and_ will help, although I'd argue that they need to go into module 
mpl_core, not into module core.

But this will not solve the problem that using boost::is_pod adds MPL to the 
dependency graph. It will only let us specialize it without depending on 
MPL.

On Tue, Jun 3, 2014 at 11:49 AM, John Maddock <boost.regex_at_[hidden]> 
wrote:

> Right, what's wrong with: [...]

This is mpl::bool_:

// bool_fwd.hpp:

#include <boost/mpl/aux_/adl_barrier.hpp>

BOOST_MPL_AUX_ADL_BARRIER_NAMESPACE_OPEN

template< bool C_ > struct bool_;

// shorcuts
typedef bool_<true> true_;
typedef bool_<false> false_;

BOOST_MPL_AUX_ADL_BARRIER_NAMESPACE_CLOSE

BOOST_MPL_AUX_ADL_BARRIER_DECL(bool_)
BOOST_MPL_AUX_ADL_BARRIER_DECL(true_)
BOOST_MPL_AUX_ADL_BARRIER_DECL(false_)

// bool.hpp:

BOOST_MPL_AUX_ADL_BARRIER_NAMESPACE_OPEN

template< bool C_ > struct bool_
{
    BOOST_STATIC_CONSTANT(bool, value = C_);
    typedef integral_c_tag tag;
    typedef bool_ type;
    typedef bool value_type;
    operator bool() const { return this->value; }
};

#if !defined(BOOST_NO_INCLASS_MEMBER_INITIALIZATION)
template< bool C_ >
bool const bool_<C_>::value;
#endif

BOOST_MPL_AUX_ADL_BARRIER_NAMESPACE_CLOSE

So, at minimum, we need integral_c_tag. I'm not sure about the ADL barriers.

Here's the tag:

#include <boost/mpl/aux_/adl_barrier.hpp>
#include <boost/mpl/aux_/config/static_constant.hpp>

BOOST_MPL_AUX_ADL_BARRIER_NAMESPACE_OPEN
struct integral_c_tag { BOOST_STATIC_CONSTANT(int, value = 0); };
BOOST_MPL_AUX_ADL_BARRIER_NAMESPACE_CLOSE
BOOST_MPL_AUX_ADL_BARRIER_DECL(integral_c_tag)

So it looks tractable in principle, but putting those things into core is, 
to use your characterization, abhorrent.