Re: [boost] Re: Formal Review: FOREACH macro

From: Dave Harris (brangdon_at_[hidden])
Date: 2005-05-02 12:57:34

In-Reply-To: <4276462D.1020501_at_[hidden]>
eric_at_[hidden] (Eric Niebler) wrote (abridged):
> It injects 5 local variables. I haven't checked, but I suspect that 
> some can be optimized away sometimes.

I doubt they will be in debug mode, which is when I need to look at them.

> I'm not sure why users will need to understand how FOREACH uses
> its hidden variables. Can you explain?

When I am debugging, I need to understand what the code is doing else how 
I can I understand what it is doing wrong?

For example, from looking at the implementation it seems that the macro 
caches the end() value in a variable called _foreach_end. This means that 
if some random memory corruption changes that variable the loop may never 
terminate. I need to know.

Incidently, if it is caching then code like:

    BOOST_FOREACH( int i, vec )
        if (condition(i))
            vec.push_back( filter(i) );

will not work the same as:

    for (iterator i = vec.begin(); i != vec.end(); ++i)
        if (condition(*i))
            vec.push_back( filter(*i) );

Which at least needs to be documented. Again the implementation is complex 
enough that I can't tell at a glance whether these are the same, so I may 
have this wrong, but I strongly suspect they are different.

-- Dave Harris, Nottingham, UK.