Re: Learning C++ Memory Management
"Tony" <tony@my.net> wrote:
"Curt Welch" <curt@kcwc.com> wrote in message
For example, when you simply declare:
SomeObject x;
It was not clear to me if x was the memory for the object, of if, like
with
Java, the variable was really a pointer to the object with the object
allocated automatically on the heap. That's the type of detail the C++
books don't start out talking about, but which I, as a long time C
programmer, want to know first off. You can read a few chapters of a
typical C++ book and still have no clue whether "SomeObject x" was
allocating a pointer on the stack or allocating the object itself on
the stack. I eventually figured out it was allocating the object on
the stack,
but that you can declare pointers and allocate memory yourself using
new etc.
The key to being able to understand the above immediately (when coming
from the world of C) is to realize that C++ was NOT developed as a
new/from-the-ground-up language like Java (pretty much) was. The behavior
of the above should be "kinda" obvious: that it is a stack object "like
any other built-in type" that C offers. The problem you had, I think, was
that you had TOO MUCH knowledge of the other languages and you
"over-thought". Had you had ONLY knowledge of C, it would have been
easier to make the correct assumptions.
Yes, that's exactly the case.
But the same sort of confusion was happening for me with the automatic
calls to constructors and destructors. The answer is that it does most
of what it needs to do automagically so C++ books don't start out
talking about it.
I think the answer is there in the books, but it gets glossed-over. The
casually throw out concepts like "concrete data type", "first-class
objects" and don't dwell on or give insight to the behind-the-scenes
compiler activities. You're right, there should be a few sections in a
chapter about that stuff. For me, that kind of information was first
presented to me in Coplien's "Advanced C++" where he establishes that
"concrete data type" means the "orthodox canonical form" that consists of
a default constructor, a copy constructor, a destructor and an assignment
operator. Over the years, I've just gotten what knowledge that builds on
that from a myriad of places (and I do take most of it for granted unless
something weird starts happening and I find myself in the debugger).
Guess what the following does:
class MyObject
{
int val;
public:
MyObject(){}
~MyObject(){}
MyObject& MyObject(const MyObject& obj){ val = obj.val; }
MyObject& operator=(const MyObject& obj) { if(&obj != this) val =
obj.val; return *this; }
MyObject(int v){ val = v; };
};
MyObject obj() = 39;
I didn't even know you could have () for initialization without anything
inside it. So I would have guessed compiler error. Being told you can
specify the default constructor that way, my second guess would be compiler
error for trying to assign an int to the object without the correct =
operator defined. :)
Ah, but then I read your follow up to this message and see you meant to add
the int assignment operator!
But then I still didn't believe it would work so I tested it and it doesn't
on my version of gcc on FreeBSD at least (gcc version 2.95.4 20020320
[FreeBSD]).
MyObject obj() = 39;
Is a function named obj returning a MyObject and you can not initialize
that with 39. At least that's how I read it.
But changing it to what I guess you meant:
MyObject obj = 39;
or
MyObject obj(39);
And that complies.
You might think that the default constructor and then the assignment
operator is called. Tracing that with a debugger shows that the
overloaded constructor with the int argument is called .
Well, with my two rewrites, you would expect the int constructor to be
called since both are valid constructor syntax, so I'm not sure what you
are suggesting.
Certainly C++ constructor syntax is confusing because with a single
argument constructor you can use the = operator syntax which makes one
think that these two would be the same:
MyObject obj = 1;
vs.
MyObject obj;
obj = 1;
When they are not, since the first is really the same as:
MyObject obj(1);
But that's something I did already understand and wasn't confused by.
That by the way is perhaps the best recommendation: experiment with the
compiler/debugger and watch what happens!
Yeah, I do that endlessly when I have issues like this I want to understand
(and to be sure my understanding is correct when I have any doubt).
[snipped relevant, but long, similar example issues]
It's just going to take some time for me to pick all this up. Mostly,
it's
just climbing a learning curve that requires writing a lot of code no
matter what books you are using. The TC++PL book I'm sure will help
however and that's the type of advice I was looking for when I posted.
There's just so much reference material available on C++ these days
it's hard to know where to turn without some advice.
A debugger will probably be your best friend for awhile: it's quicker to
just watch what happens than reading about it, but surely the literature
or other info sources will answer any questions about why it works as it
does. Answers to thornier issues can be gotten from the many gurus who
post in this NG. Oh yea, don't forget about the c.l.c++ FAQ
Yeah, I was just reading over that last night. Lots of good stuff in
there. Lots to keep me busy and thinking for some time as well...
and googling
It's surprising just how well Googling for answers to C++ questions works.
There seem to be so many specific web pages dedicated to C++ Q&A I can
often find specific answers extremely quickly that way.
past posts, for issues you may be encountering, in the C++ newsgroups, as
they are another majorly good source of info and chances are someone has
asked the question a number of times before.
Good Luck!
Tony
Yeah, and checking questions other people ask in the group is a both a good
way to test your knowledge, and expand it. When I know the answer to 99%
of all the questions asked, I'll feel I know the langauge. :) I've got a
long way to go still to get to that point...
--
Curt Welch http://CurtWelch.Com/
curt@kcwc.com http://NewsReader.Com/