Re: The fear of new things... 3 ideas for C++.
* Robert Hairgrove:
Alf P. Steinbach wrote:
I wonder why we, or at least I, react with so much fear to new ideas.
In Germany, there is a saying: "The farmer won't eat something
unfamiliar to him" (i.e., "Was der Bauer nicht kennt, det frisst er
net...")
Or even old ideas!
ditto...
[snip]
* Post-constructors and pre-destructors.
Advanced by Andrei over in clc++m. He thought it would help especially
with X-windows programming. I then thought it was ridiculous, fearing
an escalation to N-phase initialization -- instead of looking for
solutions to the bad-usage problems I envisioned.
Do you have a link to that thread?
I see Keith Duggar has provided one, if it is that thread. I would be reluctant
to provide such link because as I recall that thread reflected so extremely
poorly on me, resorting to really bad & inexcusable ad hominem argumentation. On
the other hand, perhaps that behavior proves that I'm human, not a furry ALF
from Tralfamadore or wherever (of course I'm just pretending to not remember...).
I have often encountered situations
where I would like to have special initializations performed after the
constructor has run because the constructor has no access to overrides
in the derived class while it is still running. This can be implemented
using a member object with type of some helper class which can be
customized on a per-class basis. Unfortunately, it is next to impossible
to call member functions of such data members except as the last line in
the constructor body, which means that there are still similar
limitations as to which functions one may safely call because the
constructor hasn't finished yet... Post-constructors might come in handy
here.
There are two main solutions within the scope of C++98.
(1)
The idea Dynamic Binding During Initialization is discussed in the FAQ at <url:
http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.6>. Quite
simply, you pass up, either via constructor argument or via templating, the
functionality needed in the Base class constructor, e.g. a factory for API level
widgets. It is just a kind of separation of concerns.
Like most other regulars here I've contributed a lot of times in small doses to
the FAQ, but the above is my main contribution. It's Marshall Cline's writing,
though. And the DBDI acronym is his invention.
This is the intrusive solution, the class has to be designed to support DBDI.
(2)
The second solution, which is non-intrusive, is to derive from the class and
forward constructor arguments.
General constructor argument forwarding is directly supported in C++0x (by a
combo of rvalue references, variadic template arguments and variadic function
args) but how to do that in C++98, e.g. for a templated bottom level class?
Well, consider the following (actual, working test-) code:
<code origin="my cppx lib, which is Under Construction">
// Copyright (c) Alf P. Steinbach, 2010.
#include <progrock/cppx/argument_forwarding.h>
#include <assert.h>
#include <iostream>
#include <iomanip>
using namespace progrock;
using namespace cppx;
using namespace std;
namespace {
static bool test( char const expr[], bool const value )
{
cout << setw( 75) << left << expr << " " << value << endl;
return value;
}
#define TEST( expected, e ) assert( !!expected == !!test( #e, e ) )
struct Base
{
Base( int x, char const s[] )
{
cout << x << ", " << s << endl;
}
};
struct Wrapped
: ConstructorArgForwarder< Base >
{
template< class ArgPack >
Wrapped( char const s[], ArgPack const& args )
: ConstructorArgForwarder< Base >( args )
{
cout << "Wrapped: " << s << endl;
}
};
} // namespace anon
void testArgumentForwarding()
{
Wrapped( "at bottom", args( 42, "blah blah" ) );
}
</code>
The trick here is to have suitable ArgPack and ConstructorArgForwarder
(templated) classes! I'm guessing that you can probably use Boost's Parameters
Library for the ArgPack. However, you'll have to then roll your own
ConstructorArgForwarder, or use the cppx one if/when I post the code for that
library.
Hm, I'll have to blog about this! :-)
Cheers, & hth.,
- Alf
(who has a BLOG at <url: http://alfps.wordpress.com/> :-) )