On 9/9/2011 12:47 PM, John Maddock wrote:
Following the guide here: http://www.boost.org/doc/libs/1_47_0/doc/html/proto/users_guide.html#boost_p...
I fell into a nasty trap: as_child stores it's argument as a reference, and as far as I can tell there is no way to force it to store by value, unless you apply deep_copy to the whole expression tree (not just the one child created with as_child).
I found this when trying to protoize a function that looked like:
template <class Exp> some-return-type foo(const Exp&, int i);
In which the value "i" was returned as a reference to a value on the stack which then went out of scope :-(
Even if the function has a signature such as:
template <class Exp> some-return-type foo(const Exp&, const int& i);
There is still a potential trap, because you don't know where "i" came from and when it will go out of scope - OK so the result of the expression will *usually* be calculated before that happens - but you never know.
IMO I think this trap needs better documenting, because the section in question (and text preceding it) read to me "as if" arguments to as_child were stored by value.
I'm sure I'm reading that wrong.... but you know ;-)
You're not reading it wrong. proto::as_expr exists for when you want things stored by value. -- Eric Niebler BoostPro Computing http://www.boostpro.com