I'm stuck on some problem that may en dup silly. I have a DSL defined by the following grammar : struct dimen_grammar : bp::or_< ////////////////////////////////////////////////////////////////////////// // Terminals are dimensional values or integral values ////////////////////////////////////////////////////////////////////////// bp::or_< bp::terminal< dimensions<bp::_,bp::_> > , integer_grammar > ////////////////////////////////////////////////////////////////////////// // dimen can be added together or scaled by an integer // or shifted left or right by an integer ////////////////////////////////////////////////////////////////////////// ,bp::or_< bp::plus<dimen_grammar,dimen_grammar> ,bp::multiplies<dimen_grammar,integer_grammar> ,bp::multiplies<integer_grammar,dimen_grammar> ,bp::shift_left<dimen_grammar,integer_grammar> ,bp::shift_right<dimen_grammar,integer_grammar> > > {}; I've defined a proper callable_context that take care of my special terminals and built a terminal class. At this point, everythign works perfect : i can build expressions and evaluate them. Now I add a grammar with semantic action that helps me computes an aspect of those expression : struct dimen_size : bp::or_< ////////////////////////////////////////////////////////////////////////// // dimensionnal values terminal contains its own size value ////////////////////////////////////////////////////////////////////////// bp::when< bp::terminal< dimensions<bp::_,bp::_> > , dimension_of< bp::_value>() > ////////////////////////////////////////////////////////////////////////// // integral values has a size of 1 ////////////////////////////////////////////////////////////////////////// , bp::when<integer_grammar, bm::size_t<1>()> ////////////////////////////////////////////////////////////////////////// // size(d1+d2) = max(size(d1),size(d2)) ////////////////////////////////////////////////////////////////////////// , bp::when< bp::plus<dimen_grammar,dimen_grammar> , bm::max<dimen_size(bp::_left),dimen_size(bp::_right)>() > ////////////////////////////////////////////////////////////////////////// // size(d1*i) = size(d1) ////////////////////////////////////////////////////////////////////////// , bp::when< bp::multiplies<dimen_grammar,integer_grammar> , dimen_size(bp::_left) > ////////////////////////////////////////////////////////////////////////// // size(i*d2) = size(d2) ////////////////////////////////////////////////////////////////////////// , bp::when< bp::multiplies<integer_grammar,dimen_grammar> , dimen_size(bp::_right) > ////////////////////////////////////////////////////////////////////////// // size(d1<<i) = size(d1) ////////////////////////////////////////////////////////////////////////// , bp::when< bp::shift_left<dimen_grammar,integer_grammar> , dimen_size(bp::_left) > ////////////////////////////////////////////////////////////////////////// // size(d1>>i) = size(d1) ////////////////////////////////////////////////////////////////////////// , bp::when< bp::shift_right<dimen_grammar,integer_grammar> , dimen_size(bp::_left) > ////////////////////////////////////////////////////////////////////////// // Other returns 0 ////////////////////////////////////////////////////////////////////////// , bp::when<bp::_, bm::size_t<0>()> > {}; This works. If I remove the bp::when<bp::_, bm::size_t<0>()>, it fails to compile when I try to use a hand-overloaded operator<< for displaying expression values on standard output. So, is there anything I missed (like deactivating operator<< in the grammar somehow ?) ? Other question : When defining custom tag to build new function, why does the make_expr tempalte doesn't do check on its argument to see if it's match the grammar of the domain it's bound to use ? Seems I'm forced to add additional layer of SFINAE to prevent those function to get called on w/e types. Thanks in advance for help -- ___________________________________________ Joel Falcou - Assistant Professor PARALL Team - LRI - Universite Paris Sud XI Tel : (+33)1 69 15 66 35