Markus Werle wrote:
Eric Niebler <eric <at> boost-consulting.com> writes:
"fun(1,2,3,4)" is an invocation of an overloaded function call operator.
OK, for me it was not clear from the docs why fun is a function object (or similar thing) that takes 4 arguments - or more. Up to this point in the docs no operator() was mentioned (or I overlooked it)
"fun" is a node in an expression tree. Proto overloads *all* operators for Proto expressions. So you can say "fun + fun" and build a new node representing an addition. Or you can say "fun(fun)" to build a new node representing a function call. You could even say "(fun + fun)(fun, fun fun)". But that would be too much fun. :-)
Proto is an expression template library ... all operators are overloaded to build expression trees.
Now since my reputation is lost anyway, let's go on asking further stupid questions:
1. fun is of type expr< tag::terminal, args0< fun_t >, 0 > - right?
Yes.
2. a terminal expression like "fun" has a set of overloaded operators for an arbitrary number of operators (until some PROTO_MAX_SIZE_WHATEVER) that returns a fusion compatible sequence?
Yes, up to BOOST_PROTO_MAX_ARITY operands.
3.For me a terminal is a "do-nothing", a holder. I get confused now. Here the terminal has some magic behind the scenes which I would not expect at this place. A terminal with arity 0 takes 4 arguments. I do not get that into my thick skull.
It's just an overloaded function call operator. All Proto expression types, terminals and non-terminals, have them.
This operator builds a tree node representing a function invocation.
What is the resulting type of the function call fun(1, 2, 3, 4)? I would have expected to see "whatever(1, 2, 3, 4)" be represented as expr<tag::noop, args4<int, int, int, int> >. Now I read this like the function call itself is stored as a NOOP-leaf with 4 arguments ... getting lost.
The type of "fun(1,2,3,4)" would look something like: expr< tag::function ,args5< ref_<expr<tag::terminal, args0<fun_tag> > const> ,expr<tag::terminal, args0<int const &> > ,expr<tag::terminal, args0<int const &> > ,expr<tag::terminal, args0<int const &> > ,expr<tag::terminal, args0<int const &> >
HTH, -- Eric Niebler Boost Consulting www.boost-consulting.com