bool try_take_( callable & ca)
{
if ( empty_() )
return false;
callable tmp( queue_.top().ca);
queue_.pop();
ca.swap( tmp);
return ! ca.empty();
}
value_type(
callable const& ca_,
attribute_type const& attr_) :
ca( ca_), attr( attr_)
{ BOOST_ASSERT( ! ca.empty() ); }