Pete Vilder mentioned this a few days back. This is the "Compiler-Firewall
Idiom", or pimple. It's similar to an approach taken in some Java designs.
AAMOF, when I saw it in Java, my first reaction was "Oh by Þor! It's
freakin' headers all over again!"
My greatest misgiving about the approach is that it seems overly
complicated. I /feel/ as if there is a better more 'conventional' way of
dealing with the separation of interface and implementation. OTOH, it's
clear to me that the people who have advocated it are not stupid.*
Here's the discussion: http://www.gotw.ca/publications/mill05.htm
What do you make of it?
*(Sutter does serve the Dark Lord, but that's a different story. Lippman,
too has passed into darkness.)
--
p->m == (*p).m == p[0].m http://www.kdevelop.org http://www.suse.com http://www.mozilla.org
Jul 22 '05
34  4580 
Steven T. Hatton wrote: Pete Vilder mentioned this a few days back. This is the "Compiler-Firewall
Idiom", or pimple. It's similar to an approach taken in some Java
designs. AAMOF, when I saw it in Java, my first reaction was "Oh by Þor! It's
freakin' headers all over again!"
My greatest misgiving about the approach is that it seems overly
complicated. I /feel/ as if there is a better more 'conventional' way of
dealing with the separation of interface and implementation. OTOH, it's
clear to me that the people who have advocated it are not stupid.*
"Pimpl" is an idiom that one refactors into existing code to compile faster.
Refactors change design while leaving behavior the same. The Pimpl refactor
leaves logical design the same while tweaking physical design, so
translation units needn't see the details of other classes. That prevents
excessive recompiles when you change a highly reused class.
But Pimpl is for emergencies, after a design is committed. C++ was designed
to permit compilation firewalls based on abstract types; Pimpl abstracts a
concrete type.
Robert C. Martin's "Dependency Inversion Principle" describes this design
goal. http://www.objectmentor.com/resources/articles/dip.pdf
It implies that only classes with a few pure-virtual methods should be
widely re-used. This is both a valid design technique and a system that, in
C++, prevents runaway re-compiles.
--
Phlip http://www.xpsd.org/cgi-bin/wiki?Tes...UserInterfaces
"Steven T. Hatton" <su******@setid ava.kushan.aa> wrote: Pete Vilder mentioned this a few days back. This is the "Compiler-Firewall
Idiom", or pimple. It's similar to an approach taken in some Java designs.
AAMOF, when I saw it in Java, my first reaction was "Oh by Þor! It's
freakin' headers all over again!"
My greatest misgiving about the approach is that it seems overly
complicated. I /feel/ as if there is a better more 'conventional' way of
dealing with the separation of interface and implementation. OTOH, it's
clear to me that the people who have advocated it are not stupid.*
Here's the discussion: http://www.gotw.ca/publications/mill05.htm
What do you make of it?
The only time I have used the pimpl idiom is when I need compile time
polymorphism. For example when I have a class that is implemented
completely differently for two different architectures but have the same
interface. I will have one header file, and two (or three) source files
for the class. (Foo.h, FooCommon.cpp, FooMac.cpp, FooWindows.cpp for
example) This allows all programs to use the same header even though the
two architectures use completely different internal variables and
implementations .
"Steven T. Hatton" <su******@setid ava.kushan.aa> wrote: Pete Vilder mentioned this a few days back. This is the "Compiler-Firewall
Idiom", or pimple. It's similar to an approach taken in some Java designs.
AAMOF, when I saw it in Java, my first reaction was "Oh by Þor! It's
freakin' headers all over again!"
My greatest misgiving about the approach is that it seems overly
complicated. I /feel/ as if there is a better more 'conventional' way of
dealing with the separation of interface and implementation. OTOH, it's
clear to me that the people who have advocated it are not stupid.*
Here's the discussion: http://www.gotw.ca/publications/mill05.htm
What do you make of it?
The only time I have used the pimpl idiom is when I need compile time
polymorphism. For example when I have a class that is implemented
completely differently for two different architectures but have the same
interface. I will have one header file, and two (or three) source files
for the class. (Foo.h, FooCommon.cpp, FooMac.cpp, FooWindows.cpp for
example) This allows all programs to use the same header even though the
two architectures use completely different internal variables and
implementations .
"Steven T. Hatton" <su******@setid ava.kushan.aa> wrote in message
news:RY******** ************@sp eakeasy.net... Nick Hounsome wrote:
"Steven T. Hatton" <su******@setid ava.kushan.aa> wrote in message
news:ea******** ************@sp eakeasy.net... Pete Vilder mentioned this a few days back. This is the
"Compiler-Firewall
Idiom", or pimple. It's similar to an approach taken in some Java designs. AAMOF, when I saw it in Java, my first reaction was "Oh by Þor! It's
freakin' headers all over again!"
My greatest misgiving about the approach is that it seems overly
complicated. I /feel/ as if there is a better more 'conventional' way
of dealing with the separation of interface and implementation.
Well come on then - bless us all with your insight.
I don't see how there can logically be a "better more 'conventional'
way" since this IS the conventional way.
You may notice I have '/feel/' emphasized, and ''conventional' ' in single
quotes. The intent of the former was to stress that I do not have
a /reasoned/ alternative. The intent of the latter was that I didn't
really mean the CFI (Compiler Firewall Idiom) is unconventional.
What do you see as the motive for it? Do you use it ubiquitously? I'll
have to start examining the headers in some of the programs I have the source
for to see if and how it is used there.
2 motives: reduce compile time. reduce dependencies between programmers
/groups in large project
or between library and user.
I don't use it much lately because I'm only doing smallish stuff on my own.
On small projects the compiler does most of its work parsing all the
standard headers that you tend to need so the saving would be minimal
anyway.
I did recently have cause to use it when doing embedded work as it allowed a
different pimpl to be used when testing on the host without having to #ifdef
all the headers that only existed in the target.
I suppose that people would say that factory is the pattern for this but it
would have been overkill and
you still need a handle to factory generated object which is not unlike a
class and its pimpl.
I really don't think that there is a better way in C++.
"Steven T. Hatton" <su******@setid ava.kushan.aa> wrote in message
news:RY******** ************@sp eakeasy.net... Nick Hounsome wrote:
"Steven T. Hatton" <su******@setid ava.kushan.aa> wrote in message
news:ea******** ************@sp eakeasy.net... Pete Vilder mentioned this a few days back. This is the
"Compiler-Firewall
Idiom", or pimple. It's similar to an approach taken in some Java designs. AAMOF, when I saw it in Java, my first reaction was "Oh by Þor! It's
freakin' headers all over again!"
My greatest misgiving about the approach is that it seems overly
complicated. I /feel/ as if there is a better more 'conventional' way
of dealing with the separation of interface and implementation.
Well come on then - bless us all with your insight.
I don't see how there can logically be a "better more 'conventional'
way" since this IS the conventional way.
You may notice I have '/feel/' emphasized, and ''conventional' ' in single
quotes. The intent of the former was to stress that I do not have
a /reasoned/ alternative. The intent of the latter was that I didn't
really mean the CFI (Compiler Firewall Idiom) is unconventional.
What do you see as the motive for it? Do you use it ubiquitously? I'll
have to start examining the headers in some of the programs I have the source
for to see if and how it is used there.
2 motives: reduce compile time. reduce dependencies between programmers
/groups in large project
or between library and user.
I don't use it much lately because I'm only doing smallish stuff on my own.
On small projects the compiler does most of its work parsing all the
standard headers that you tend to need so the saving would be minimal
anyway.
I did recently have cause to use it when doing embedded work as it allowed a
different pimpl to be used when testing on the host without having to #ifdef
all the headers that only existed in the target.
I suppose that people would say that factory is the pattern for this but it
would have been overkill and
you still need a handle to factory generated object which is not unlike a
class and its pimpl.
I really don't think that there is a better way in C++.
* "Steven T. Hatton" <su******@setid ava.kushan.aa> schriebt: Phlip wrote:
But Pimpl is for emergencies, after a design is committed. C++ was
designed to permit compilation firewalls based on abstract types; Pimpl
abstracts a concrete type.
That was the kind of answer I was looking for.
I'm hereby giving up on you. You chose the only response that was all dress
and no actual content. That fancy dressed "girl" is a boy, Steven.
--
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
* "Steven T. Hatton" <su******@setid ava.kushan.aa> schriebt: Phlip wrote:
But Pimpl is for emergencies, after a design is committed. C++ was
designed to permit compilation firewalls based on abstract types; Pimpl
abstracts a concrete type.
That was the kind of answer I was looking for.
I'm hereby giving up on you. You chose the only response that was all dress
and no actual content. That fancy dressed "girl" is a boy, Steven.
--
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
Alf P. Steinbach wrote: * "Steven T. Hatton" <su******@setid ava.kushan.aa> schriebt: Phlip wrote:
> But Pimpl is for emergencies, after a design is committed. C++ was
> designed to permit compilation firewalls based on abstract types; Pimpl
> abstracts a concrete type.
That was the kind of answer I was looking for.
I'm hereby giving up on you. You chose the only response that was all
dress
and no actual content. That fancy dressed "girl" is a boy, Steven.
I didn't say it was the correct answer. All I intended was that it seems to
suggest there are better ways to address some or most circumstances in
which the CFI would be used. I have to say the concern about compile time
makes me a bit uneasy. If anything, I would say the long compile times are
a symptom not the disease.
So, I was looking at Martin's book, but I bought these instead: http://www.josuttis.com/libbook/index.html http://www.oreilly.com/catalog/cpluspluspr/
Have I gained any redemption?
--
STH
Hatton's Law: "There is only One inviolable Law"
KDevelop: http://www.kdevelop.org SuSE: http://www.suse.com
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