Eric Niebler <eric <at> boost-consulting.com> writes:
Markus Werle wrote:
Hi!
In the section "Expression Nodes as Fusion Sequences" you show the follwing code
struct fun_t {}; terminal<fun_t>::type const fun = {{}};
// ... fusion::for_each( fusion::transform( // pop_front() removes the "fun" child fusion::pop_front(fun(1,2,3,4)) // Extract the ints from the terminal nodes , functional::arg<>() ) , display() );
It did not became clear to me why a terminal<fun_t>::type can be initialized with 4 integer arguments.
"fun" is an object. It is already initialized.
I should be careful about how I ask, sorry for the stupid error. I meant operator call not initialization, but I had in mind that fun(1, 2, 3, 4) initializes some sort of expr<>, see questions below.
I thought terminal<fun_t>::type is equivalent to expr< tag::terminal, args0< fun_t >, 0 > and should take no args at all. Could you clarify this?
"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)
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? 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? 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.
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. Markus