[boost] Re: [spirit] rule<> and copying

From: Eric Niebler (eric_at_[hidden])
Date: 2004-05-27 18:25:21

David Abrahams wrote:

> "Eric Niebler" <eric_at_[hidden]> writes:
>>Why is that confusing? That's how most other types in C++ behave:
>>
>>int a,b; // a and b are undefined
>>a = b;   // a and b are still undefined
> 
>   ^^^^^
> This is undefined behavior, actually.

Oops, you're right. Bad example. How about:

int a=0,b=0;
a=b;
b=1; // b is 1, a is still 0

My point was that rule<> doesn't "do as the ints do", which can be 
surprising to some. I would love for this to work:

rule<> a;

{
   rule <> b;

   // match balanced, nested parens.
   // b refers to itself by reference
   // (Note: ! means optional, no special syntax
   // required to get by-ref semantics here)
   b = '(' >> !b >> ')';

   // a assumes b's behavior
   a = b;
}

// a still has b's behavior.
// this matches balanced, nested parens
parse( "(())", a );

Currently, this crashes because 'a' holds 'b' by reference and tries to 
access it after 'b' has gone out of scope and been destroyed. I haven't 
yet thought of a way to make this work, but I'm not convinced it's 
impossible.

I now agree that rules should have by-ref semantics when embedded in 
another rule. But in simple assignment statements ("a=b;"), it would be 
nice if rules behaved as the ints do. I'm sure this will draw fire from 
the EBNF purists who want rule assignment to have by-ref semantics, but 
IMO C++ types that don't have normal copy and assignment semantics are 
just hard to work with.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com