Kim Kuen Tang wrote:
Hi all,
consider the following code:
std::vector<double> a(10,1.0), b(10,2.0); BOOST_AUTO( temp , proto::as_expr<MixedDomain>(a+b+0.1) ); // is the function proto::as_expr<MixedDomain>() needed? <snip>
But the code crashes during runtime. The reason is that the number 0.1 is held by reference. So the number 0.1 goes out of scope before i try to evaluate the expression temp[0];
A way to solve this problem is to deep copy the expression with deep_copy. But on the other hand i do not want to copy the two vectors a and b.
Deep-copying only some terminals is fundamentally unsafe. But if you really don't want to deep-copy the vector terminals, you'll need to implement your own deep-copy algorithm. It's easiest to write a grammar with transforms that does it for you. #include <vector> #include <boost/proto/proto.hpp> namespace boost { namespace proto { // make std::vector a proto terminal extension template<typename T, typename Al> struct is_extension<std::vector<T, Al> > : mpl::true_ {}; }} namespace proto = boost::proto; using proto::_; // Deep-copy all terminals except vectors struct DeepCopyNotVectors : proto::or_< proto::when< proto::terminal<std::vector<_,_> > // The vector itself will be held by reference, but the // temporary proto terminal expression wrapping the // reference should be held by value. , boost::remove_const<boost::remove_reference<_> >(_) > , proto::when<proto::terminal<_>, proto::_deep_copy(_)> , proto::otherwise< proto::nary_expr<_, proto::vararg<DeepCopyNotVectors> > > > {}; int main() { using namespace proto::exops; std::vector<int> a, b; BOOST_AUTO(tmp0, a + b + 0.1); BOOST_AUTO(tmp1, DeepCopyNotVectors()(a + b + 0.1)); std::cout << typeid(tmp0).name() << "\n\n"; std::cout << typeid(tmp1).name() << "\n"; } int main() { using namespace proto::exops; std::vector<int> a, b; BOOST_AUTO(tmp0, a + b + 0.1); BOOST_AUTO(tmp1, DeepCopyNotVectors()(a + b + 0.1)); std::cout << typeid(tmp0).name() << "\n\n"; std::cout << typeid(tmp1).name() << "\n"; } -- Eric Niebler BoostPro Computing http://www.boostpro.com