boost::mpl::Assertsの実装の概要

#include <[]boost[]/mpl/bool.hpp>

#include <[]boost[]/mpl/eval_if.hpp>
namespace question {

  struct X {

    int a;

  };

  

  int X::* what_is_this();

  int X::** what_is_that();

  

  inline void test_X() {

    X x;

    // pointer to member a

    int X::*p2a = &X::a;

    x.*p2a = 10;

    p2a = what_is_this();

    // pointer to pointer to member a

    int X::** pp2a = &p2a;

    x.**pp2a = 10;

    pp2a = what_is_that();

  }

} // question

namespace pst {

  namespace mpl = []boost[]::mpl;
  struct failed { };
  // functors

  // "汚い"型名を残すためにtypedefを使わずexecの戻り値の型を使う

  template< typename Pred >

  struct ftor_ret_true_ {

    mpl::true_ exec();

    typedef ftor_ret_true_ type;

  };
  template< typename Pred >

  struct ftor_ret_failed {

    failed************ Pred::************ exec();

    typedef ftor_ret_failed type;

  };
  // dispatcher

  template< typename Pred >

  struct ftor_ret_true_or_failed

    : mpl::eval_if< Pred, ftor_ret_true_<Pred>, ftor_ret_failed<Pred> >::type { };
  // テンプレート引数にはftor_ret_true_or_failed<(pred)>のように

  // ()を付けられない、しかしマクロ引数には()を付けないといけない。そこで、

  template< typename Pred >

  ftor_ret_true_or_failed<Pred>

  make_ftor_ret_true_or_failed( void (*) (Pred) );
  int assertion_failed(mpl::true_);

} // pst
#define PST_MPL_ASSERT(pred) \

enum { \

  pst_mpl_assertion_in_line_12345 = sizeof( \

    pst::assertion_failed( \

      pst::make_ftor_ret_true_or_failed( (void (*) pred)0 ).exec() \

    ) \

  ) \

}\
#include <[]boost[]/type_traits.hpp>
PST_MPL_ASSERT( ( []boost[]::is_same<int, int> ) );

PST_MPL_ASSERT( ( []boost[]::is_same<char, int> ) );