Re: [Boost-users] [proto] Matching grammars in the presence of adapted types

On 6/10/2010 11:57 AM, Sebastian Redl wrote:

So I've figured out the problem. Basically, I was doing something like this:

---------------------

struct SmartContext {

template struct eval;

template<typename Expr> struct eval { /* implementation for terminals */ };

template<typename Expr> struct eval { /* one implementation for non-terminals */ };

template<typename Expr> struct eval { /* another implementation for non-terminals */ };

};

---------------------

Unfortunately, those specializations are all tested at the same time. Thus, even for terminal expressions, the stuff looking at the children was evaluated.

Using SFINAE in proto context types means you should probably be using transforms instead. Contexts are useful when you can dispatch on node tag type. Anything fancier and transforms are the way to go.

However, the "child" of a terminal is the underlying value implementation, so this stuff tried to access the value implementation as if it was an expression. Thus the compilation errors.

Is there a particular reason why terminals are implemented this way? Does it make Proto implementation considerably more convenient? Because if not, it would be nice to change it so that proto::child() and proto::value() are not synonyms, but instead simply don't compile for terminals and non-terminals, respectively. That would have helped a lot in detecting this error.

Actually, a static_assert would suffice. Something like: <snip>

I'll consider adding it, but I worry about breaking people who are unknowingly and incorrectly relying on this behavior. Hmm... -- Eric Niebler BoostPro Computing http://www.boostpro.com