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No thanks.No thanks.Posted Nov 10, 2012 21:48 UTC (Sat) by Cyberax (✭ supporter ✭, #52523)In reply to: No thanks. by nix Parent article: Haley: We're doing an ARM64 OpenJDK port! >Hint, Cyberax: consider a simple RAII object which allocates some memory in its constructor, in two tranches, with a single assignment between.
#include <stdio.h>
#include <memory>
#include <stdexcept>
class tracer
{
public:
tracer() { printf("Constructing\n"); }
~tracer() { printf("Dying\n"); }
};
class tester
{
std::auto_ptr<tracer> t1_;
std::auto_ptr<tracer> t2_;
public:
tester()
{
t1_=std::auto_ptr<tracer>(new tracer());
throw std::runtime_error("Test");
t2_=std::auto_ptr<tracer>(new tracer());
}
};
int main()
{
try
{
tester t;
} catch(const std::runtime_error &ex)
{
//Print exception info
}
return 0;
}
cyberax@cybmac:~/work$ ./a.out
Constructing
Dying
And your point is?
Consider that you have to deal with exactly the same crap if you DO NOT use the exceptions. EVERY function you invoke can cause error conditions (such as out-of-memory). Now please rewrite my example without exceptions, providing the same level of guarantees and functionality.
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No thanks. Posted Nov 10, 2012 23:06 UTC (Sat) by nix (subscriber, #2304) [Link]
You really don't understand, do you?
Consider just these three lines of your example:
t1_=std::auto_ptr<tracer>(new tracer());
Let's replace the stupid throw with something more likely to be actually seen in practice, say:
t1_=std::auto_ptr<tracer>(new tracer());
Now... if any of those constructors but the first throws, what do you do about it? If, for whatever reason, you cannot use auto_ptr (which is, to be honest, simply pushing off the RAII memory-allocation overhead to something else that has already done all the work) how do you prevent the earlier-allocated ones from leaking? That's right, you have to free at appropriate times in a catch handler. Now figure out which bits can throw. It's hard. It's very, very hard. I recommend Scott Meyers's Effective C++ books for more than you can possibly stomach on this (I wish I could reread mine, but I loaned them out and they never came back, I'm sure you know how it is).
No thanks. Posted Nov 10, 2012 23:22 UTC (Sat) by Cyberax (✭ supporter ✭, #52523) [Link]
Try it. I really mean it - try it. It would work just fine. Everything will be correctly destroyed.
>If, for whatever reason, you cannot use auto_ptr (which is, to be honest, simply pushing off the RAII memory-allocation overhead to something else that has already done all the work) how do you prevent the earlier-allocated ones from leaking?
Of course, if you really don't want to do it then you'll have to do the try..catch dance. Exactly like in C where ALMOST EVERY function might result in error and the most common pattern of error handling is "goto err_exit". That's actually why glibc handles OOM by calling abort() - it's simply too complicated to make everything in C error-safe.
PS: I had actually written a simple model verifier to verify my own collection library for upholding exception guarantees. That means that I've obviously read Sutter, Meyers, Stroustroup, Alexandrescu and quite a lot of other C++ writers.
No thanks. Posted Nov 10, 2012 23:25 UTC (Sat) by foom (subscriber, #14868) [Link]
> Now... if any of those constructors but the first throws, what do you do about it?
You don't have to do anything, because auto_ptr's destructor takes care of it for you.
> If, for whatever reason, you cannot use auto_ptr ( which is, to be honest, simply pushing off the RAII memory-allocation overhead to something else that has already done all the work
That's the whole point! You *always* just use auto_ptr (or another smart pointer, e.g. unique_ptr) to take care of it for you! That's exactly why RAII is so nice, and has been best practice in C++ for years!
No thanks. Posted Nov 11, 2012 9:32 UTC (Sun) by paulj (subscriber, #341) [Link]
That smart pointers are required exemplifies the "non-trivial" observation made of exception using code. And remind me again, how long have these been available as part of the standard C++ environment?
No thanks. Posted Nov 11, 2012 10:37 UTC (Sun) by Cyberax (✭ supporter ✭, #52523) [Link]
Since at least 80-s. The problem was mostly in old C programmers who don't know better.
No thanks. Posted Nov 11, 2012 12:53 UTC (Sun) by paulj (subscriber, #341) [Link]
Rubbish. I have books from the 90s on C++ that fail to mention smart pointers anywhere, written by C++ committee members.
Could good C++ programmers have created their own smart pointers in the 80s? I guess so. My strong impression though is that C++ programmers would have been *way* ahead of the state of understood wisdom in the C++ world at that time. My impression
To claim smart pointers are trivial, widely understood techniques since the 80s stretches credibility somewhat. If they're so trivial, so widely understood for so long, why did it take 20+ years to standardise some? Why is an only relatively recently standardised form of smart pointer already being deprecated in the next C++ standard? (auto_ptr for unique_ptr). My sense is that smart pointers are so trivial that even C++ standards committee members hadn't really fully grokked the implications of copying, moving and sharing them until relatively recently - but I guess they're just "old C programmers who don't know better".
Unfortunately, in the real world, many programmers just aren't quite as brilliant as you.
No thanks. Posted Nov 11, 2012 13:04 UTC (Sun) by Cyberax (✭ supporter ✭, #52523) [Link]
> Rubbish. I have books from the 90s on C++ that fail to mention smart pointers anywhere, written by C++ committee members.
[citation needed]
> Could good C++ programmers have created their own smart pointers in the 80s? I guess so. My strong impression though is that C++ programmers would have been *way* ahead of the state of understood wisdom in the C++ world at that time. My impression
http://www.stroustrup.com/bs_faq2.html#finally
>If they're so trivial, so widely understood for so long, why did it take 20+ years to standardise some? Why is an only relatively recently standardised form of smart pointer already being deprecated in the next C++ standard? (auto_ptr for unique_ptr).
However, Boost and other libraries had no shortage of various smart pointers.
No thanks. Posted Nov 11, 2012 14:56 UTC (Sun) by paulj (subscriber, #341) [Link]
Herbert Schildt, "C++ Nuts & Bolts for Experienced Programmers", Osborne McGraw-Hill, 1995. (Yes, Schildt has gotten flack for technical errors / flaws on C++ in his books, and yes, this book wasn't intended to be a reference).
When was Boost created? late 90s. Before Boost there was the HP and SGI STL, which I gather inspired the standardised STL (and Boost?), and iostream. That work dates from the early 90s AFAICT. The SGI STL page doesn't seem to have any kind of smart pointer, dated 1994. C++98 has the auto_ptr, but it apparently is flawed. The early Boost pages reference this: http://web.archive.org/web/19991222182604/http://www.boos... which references books discussing smart pointer programming *patterns* from '92, '94.
I know this stuff is all trivial to you, but it does seem to me that it took about 10 to 15 years (80s to mid/late 90s) for leading C++ programmers to agree on patterns for dealing with exception safety, such as RAII, then getting some kind of smart pointer support in the language (standard or generally available libraries) in order to cope with remaining pitfalls in C++ RAII+exceptions. That smart pointer support then needed further refinement in the mid/late 90s and early 2000s to get to where we are today.
That doesn't quite suggest to me that this stuff is quite as trivial as you suggest to is. Finally, even with smart pointers, I still don't find exception handling code that uses them to be non-trivial! However, you do seem be a superior breed of programmer to the rest of us, so perhaps that's the reason for the discrepancy in views ;). (Intended as a gentle jibe, I hope you'll accept it in that spirit).
No thanks. Posted Nov 12, 2012 2:05 UTC (Mon) by Cyberax (✭ supporter ✭, #52523) [Link]
>Herbert Schildt, "C++ Nuts & Bolts for Experienced Programmers", Osborne McGraw-Hill, 1995. (Yes, Schildt has gotten flack for technical errors / flaws on C++ in his books, and yes, this book wasn't intended to be a reference).
He's not a member of C++ committee.
>When was Boost created? late 90s. Before Boost there was the HP and SGI STL, which I gather inspired the standardised STL (and Boost?), and iostream.
Exceptions appeared in GCC 2.7 in 1997, I think. Before that there was little reason to use them and to create best practices for them. Though the usefulness of RAII was understood earlier.
In Java exceptions with try..finally also appeared at about the same time. But without RAII.
No thanks. Posted Nov 12, 2012 14:35 UTC (Mon) by paulj (subscriber, #341) [Link]
Well, Schildt's wikipedia page says he was involved in C++ standardisation in the 90s. I don't know myself if that's correct or not.
As for exceptions, they were developed in the late 80s, standardised in the early 90s, and there were commercial compilers supporting exceptions since at least '92 apparently. See: http://www.stroustrup.com/hopl2.pdf. The release history of GCC possibly isn't that relevant, a number of proprietary implementations of C++ had more significant usage than g++ back then iirc.
Anyway..
No thanks. Posted Nov 12, 2012 14:38 UTC (Mon) by paulj (subscriber, #341) [Link]
Oh, the '95 Schildt book has a section on exceptions.
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