Boost logo

Boost :

Subject: Re: [boost] boost::mpl::for_each and value_initialized
From: Domagoj Saric (domagoj.saric_at_[hidden])
Date: 2009-09-02 05:11:12

"Peter Foelsche" <peter_foelsche_at_[hidden]> wrote in message
> what is the use of value_initialized in boost::mpl::for_each?
> It should be sufficient if the value is initialized by the constructor.
> value_initialized() calls memset() in before calling the constructor.
> This memset() is not being optimized away with gnu-c++ 4.4.1.
> What class does require zeroed memory to work correctly?

i second that...
...msvc++ 8.0 and 9.0 ("with all available optimizations and maximum
inlining depth") are also "unable" to remove the redundant memset() call (it
is only sometimes replaced with an intrinsic implementation)...

there was a somewhat related discussion ( ) dealing, among
other things, with the problem of default construction in mpl::for_each ...
in the current implementation you must either pay for default construction
or pay for added verbosity/ugliness of the "wrap" approach...
...i must admit i did not quite understand the argument of Aleksey Gurtovoy
that the proposed "aux::unwrap(f, 0)::operator()<arg>()" solution would
"would rule out ordinary function objects" (can you even use them with
mpl::for_each?) and that "it's not portable" (the ::operator()<arg>() syntax
is not standard?)...
(apologies if this should go to a seperate seemed related enough

 "That men do not learn very much from the lessons of history is the most
important of all the lessons of history."
 Aldous Huxley 

Boost list run by bdawes at, gregod at, cpdaniel at, john at