Advanced topics... optimising / C++ / OOP / RefCounting / type-safety

[st_ev_fe]

Hi people,

I've been doing C for about 7 years now. But I'm new to C++.

I've decided that C++'s operator overloading could be very handy. I'm
writing something much like auto_ptr, except for my own set of classes.
Basically, it's a class that is supposed to be allocated automatically
on the stack, and possesess one member which is a pointer to an object.

Unlike auto_ptr, my object will actually call a lock or unlock function
upon the object. Each object has a "long RefCount" member, and a
"MyClass*" which is a pointer to some type information. The "class"
pointed to is just a statically allocated struct that contains some
nice info about each instance of itself, such as the size, the
constructor and destructor functions.

Basically I've written a ref-counting memory management system in C++.

I think anyone familiar with the STL string class shouldn't be too
worried about this approach, because most "string" implementations
actually use refcounts internally, anyhow.

And this is my first venture into properly using C++. So by default,
I'll be getting things wrong )

Well the code is correct. It compiles, and runs as I expect it to (took
some debugging but it's there now).

So what's my question?

Basically, is there a faster / smaller / neater way to do what I'm
doing? I've tested my code when setting the compiler to generate small
size code (for Mac PPC), and... well the refcounting system seems to
use A LOT of code just to do something very simple like:

Ref1 = Ref2;

Just that takes up about 52 bytes of in my compiled PPC executable!
Normally assigning a pointer takes 4 bytes, (that's 1 PPC instruction).

Well, obviously that's quite bad right?

The code executing is htis:

class OPObject {

OPStruct* Value;

OPObject& operator=(OPObject& obj) {
OPUnlock(Value);
Value = ref.Value;
OPLock(Value);
return *this;
}


This is actually an inline function. This is my first question, is it
even worth inlining this function?? Especially when I'm using two
inlined functions, OPLock and OPUnlock?

OPLock goes like this:

inline void OPLock( OPStruct* obj ) {
if (obj) {
obj->RefCount++;
}
}


inline void OPUnlock( OPStruct* obj ) {
if (obj) {
long rc = obj->RefCount - 1;
obj->RefCount = rc;
if (!rc) {
OPDeAlloc( obj );
}
}
}

Doesn't seem like much code, but it adds up...

It's a lot just to do "a = b;", really.

Of course, I don't necessarily need to use my OPObjects everywhere...
OPObject is just for memory management really. I can just extract the
OPStruct* and pass that around, knowing that it's a bare pointer that
could be invalid if stored outside of the lifetime of the OPObject.

Would I get faster code if I uninlined some stuff??

What about altering my code? Does anyone know a faster way to do
refcounting? Any OOP experts out there that know a faster way to do
this?? Any OOP experts out there that know how other refcounting
languages do their refcounting? Maybe if someone knows how Java or perl
or VisualBasic or other languages do it....??

The "if (obj)" thing irks me because I just wonder if there is a way to
do without it... but it seems like I must have it, because it's quite
possible to have an OPStruct* to 0, so I need to be able to handle
that. Unless someone can think of a clever way to avoid having to deal
with that? (One that doesn't involve me recoding my entire library
which currently relies on comparing object pointers to 0 to see if they
exist or not.)

Also, this brings me back to the question of neatness and simplicity.

I just don't know really how to make this into a compile-time type
checking safe system.

Right now I have a base class, OPStruct. All my other classes subclass
from this, so I have:

struct MySpecialClass : public OPStruct { /*...*/ };
struct MyOtherClass : public OPStruct { /*...*/ };

etc etc

Now, OPObject (with it's "OPStruct* Value"), doesn't actually specify
any type other than OPStruct.

OPObject has a handy converter to and from OPStruct*, so there is no
problem passing and returning the two types between each other.

So that means I can stuff a "MySpecialClass" into OPObject...

And then call this function MySpecialFunction(OPStruct* obj); like so

MySpecialFunction( MyOPObject ); // OPObject's operator convert to
OPStruct* is called.

because I've not specified any types in MySpecialFunction's params. if
I DID specify a type in MySpecialFunction's param, like so:

MySpecialFunction( MySpecialClass* obj );

then I can't call this:

MySpecialFunction( MyOPObject );

then I'll get a type error, because OPStruct* isn't necessarily a
MySpecialClass*!!

So what do I do?

I want compile-time type checking with a small simple way to manage
this. Can I do it with some special define trickery that lets me define
OPStruct* subclasses with their corresponding OPObject subclassing
owners?

Or is there a better way using templates? I don't want any overhead,
this must strictly be a compile-time type check remember.

Sorry for taking so long to explain myself but I think if I used less
works I wouldn't have stated the situation fully )

Thanks in advance to anyone smart enough and helpful enough to reply!

 

[niklasb]

Hi people,

I've been doing C for about 7 years now. But I'm new to C++.

I've decided that C++'s operator overloading could be very handy. I'm
writing something much like auto_ptr, except for my own set of classes.
Basically, it's a class that is supposed to be allocated automatically
on the stack, and possesess one member which is a pointer to an object.

Implementing a smart pointer type is a fairly advanced project
for someone new to C++. It's true that std::auto_ptr is pretty
limited, but you might be better of using some other existing
implementation such as boost::shared_ptr rather than rolling
your own.

That said, I imagine you're really doing this as an exercise
and are having too much fun to stop. If so, it's still worth
looking at some existing implemetnations such as boost::shared_ptr
and boost::intrusive_ptr. The latter is pretty close to what
you're trying to do -- i.e., a reference-counting smart pointer
in which the ref count is stored as a member of the pointee
(hence "intrusive").

Unlike auto_ptr, my object will actually call a lock or unlock function
upon the object. Each object has a "long RefCount" member, and a
"MyClass*" which is a pointer to some type information. The "class"
pointed to is just a statically allocated struct that contains some
nice info about each instance of itself, such as the size, the
constructor and destructor functions.

It seems like you're manually reinvent stuff that C++ can do
for you. For example, if the exact type of the pointee is known
then just delete it using delete. If the actual type of the pointee
could be a derived type then you can still use delete as long as
you declare the base type with a virtual destructor. No need to
maintain your own function pointer to a destructor function --
C++ does that for you.

class OPObject {

OPStruct* Value;

OPObject& operator=(OPObject& obj) {
OPUnlock(Value);
Value = ref.Value;
OPLock(Value);
return *this;
}

The operand to operator= should generally be a const reference. It's
not in the case of auto_ptr but auto_ptr has really funky assignment
semantics.

Also, you should increment (lock) before you decrement (unlock) so you
don't mistakenly free the pointee in the case of self-assignment.

OPObject& operator=(OPObject const& obj)
{
OPLock(obj.Value);
OPUnlock(Value);
Value = obj.Value;
return *this;
}

I would suggest more descriptive names -- e.g., OPObject is a
pointer-like object that manages the lifetime of its pointee
using reference counting, so why not "CountedPtr" or something.
But I'll use your names here.

This is actually an inline function. This is my first question, is it
even worth inlining this function?? Especially when I'm using two
inlined functions, OPLock and OPUnlock?

You could try it both ways and see what the actual impact is on code
size. Be aware that inline is only a hint to the compiler anyway; it
may choose not to inline the function.

inline void OPLock( OPStruct* obj ) {
if (obj) {
obj->RefCount++;
}
}

inline void OPUnlock( OPStruct* obj ) {
if (obj) {
long rc = obj->RefCount - 1;
obj->RefCount = rc;
if (!rc) {
OPDeAlloc( obj );
}
}
}

These seem like part of the internal implementation of your smart
pointer class so make them private (and optionally static) member
functions if they aren't already. If they're member functions then
you don't need to add prefixes to the names -- just call them Lock
and Unlock.

Doesn't seem like much code, but it adds up...

It's a lot just to do "a = b;", really.

Of course, I don't necessarily need to use my OPObjects everywhere...
OPObject is just for memory management really. I can just extract the
OPStruct* and pass that around, knowing that it's a bare pointer that
could be invalid if stored outside of the lifetime of the OPObject.

Or invoke your smart pointer's operator* to obtain a reference and pass
that around. So whenever a function takes an OPStruct& parameter (or
OPStruct const&) it's clear that there's now change in "ownership"
implied, and it's also very cheap.

If you want to do something that actually imlples ownership, such as
adding the pointee to a container, then you'd typically pass the smart
pointer by const-reference.

The "if (obj)" thing irks me because I just wonder if there is a way to
do without it... but it seems like I must have it, because it's quite
possible to have an OPStruct* to 0...

Well, you own the smart pointer class so you can control its
invariants. That's what constructors and "private" are for. You
could design things such that your internal pointer is never NULL,
and represent the concept "pointer to nothing" by pointing to a
special instance, thus.

class OPObject
{
public:
// default-initialize to point to &NullValue
OPObject(OPStruct* value = &NullValue);

private:
Value* Value;
static OPStruct NullValue;
};

Your Lock/Unlock would then no longer need null checks, but on the
other hand operator-> would probably need to do a check so as to
return null if Value==&NullValue, so whether it's a net win is up
to you to decide.

I just don't know really how to make this into a compile-time type
checking safe system.

As you guessed, the answer is templates. Again, you might want to look
at how boost::intrusive_ptr does this.

 

[twoeyedhuman1111]

Fricken old people and their c stuff. Goodness.... You need to
upgrade EVERYTHING!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!111

 

[st_ev_fe]

Hi Nik,

Thanks for the good answers. I've since corrected the lock order bug
you spotted in my code.

There are some more considerations that I should have said, so I'll say
them not.

My code is for a library that I hope to be usable from C and C++, hence
the tendancy to use structs instead of classes and using ordinary C
functions.

The C++ OPObject that did the refcounting automatically, is just a
syntactic sugar, not a functional requirement. I'd like it to work in
pure C. Not because I like C, but because it is so useful for library
development. A C API can be accessed from many languages or
environments that a C++ API might not.

By making the C++ automatic refcounting class be a superficial (but
very well crafted and incredibly handy) addition ontop of the C layer,
I'll allow for my compiled library to be executable from languages that
can access C APIs but not C++ APIs. Hence all the prefixes, like OPLock
instead of Lock.

Boost looks very useful actually. If I were using a pure C++
environment and not caring about general purpose library development,
I'd look into it. The shared_ptr and intrusive_ptr look pretty much
exactly what I'm doing...

I'd still be concerned about Boost's size. As it is I've managed to
implement a manual refcounting system that has constructors and
destructors, in just one small .cpp file (with an "Extern "C" API), and
an optional auto refcounting C++ class. Boost looks huge and scary )
My "ObjectPlatform" as I call it (hence the OP) is small and familiar.

I see that boost's shared_ptr is recommended for inclusion into the new
C++ standard, which makes sense as refcounting really is much harder to
cause errors such as leaks or dangling pointers, for the developer.

As for the templates for compile time type checking... well I've never
done templates before but I have no doubt I'll learn to like them if I
give it a shot. My only concern is size.

If I template a function that does exactly the same thing on classes of
different types, will I get different functions compiled? All I'm
looking for is type safety, remember. Same code, same run-time
requirements, different compile-time requirements.

One the one hand I expect this code to be compiled most often with
modern compilers like gcc. On the other hand even I still use a decade
old C++ compiler like Apple's old MrCpp. Why MrCpp? Because it creates
smaller faster code than CodeWarrior does. And CodeWarrior compiles
smaller faster code than gcc does. So MrCpp is the best in my
experience.

Basically I worry if enough compilers will be smart enough to use the
same code if templated.

But then it doesn't matter if I am inlining all my C++ functions
anyhow, right? As long as the OPLock and OPUnlock functions are
actually not inline, then I won't be getting too much code duplicated.

I can't really use a C++ class and use C++'s constructor/destructor,
because I need this library to be compatible across different OOP
execution environments. It will run in pure C++ environment, some
basics, and maybe even Java. So I need to have my own class system that
my ObjectPlatform can massage into a shape acceptable by the hosting
OOP environment.

The idea of using a private value which is never null... that seems
like a good idea. I'll try it out, see if the speed/size difference is
even noticable. Coders tend to worry about the wrong things. Small
details get blown out of proportion and big troubles pass under the
radar ) I might have been worrying about nothing with the "if (obj)"
thing.

Thanks for all the good answers )

 

[Neil Cerutti]

As for the templates for compile time type checking... well
I've never done templates before but I have no doubt I'll learn
to like them if I give it a shot. My only concern is size.

If I template a function that does exactly the same thing on
classes of different types, will I get different functions
compiled? All I'm looking for is type safety, remember. Same
code, same run-time requirements, different compile-time
requirements.

One the one hand I expect this code to be compiled most often
with modern compilers like gcc. On the other hand even I still
use a decade old C++ compiler like Apple's old MrCpp. Why
MrCpp? Because it creates smaller faster code than CodeWarrior
does. And CodeWarrior compiles smaller faster code than gcc
does. So MrCpp is the best in my experience.

Basically I worry if enough compilers will be smart enough to
use the same code if templated.

Check out Item 42 in _Effective C++_ (Meyers). There's a design
pattern there, whose name I don't know, that answers those
questions. You implement your interface generically once, to
operate on void*. Users use typesafe templates that inherit that
single generic implementation.