C++ Frequently Questioned Answers

by kmavm • 17 years ago

60 73 17 years ago

spectre• 17 years ago

Wish there were more sites like this. We mostly just believe the answers we recieve.

timr6• 17 years ago

"Unfortunately, you can't tell the class of a C++ object given a pointer to it at run time....which is why you can't automatically collect the garbage of C++ programs."

What nonsense. Here's what I learned from this link: trolling isn't as effective when it requires the trollee to read 10 pages of text.

This is garbage. Please flag it for collection.

mrtron• 17 years ago

This is C++. Implement your own deconstructor, and manage your own damn memory.

I would probably use Boost's smart pointers to handle the reference counting though.

_cbsz• 17 years ago

No, GP's point is that there are garbage collectors for C++. I believe Stroustrup even mentions one in The C++ Programming Kanguage. Furthermore, you CAN tell the type of an object given a pointer to it, assuming it has at least one virtual function, using the typeid operator. As the GP said, the original quote is nonsense.

DarkShikari2• 17 years ago

Just because you disagree with something does not automatically make it a troll.

dave_au• 17 years ago

AFAIK it's reasonably well known as an unintentional troll - there was a stackoverflow question about the FQA (still looking for the link) that basically mooted almost every point.

I was reading it as a neutral party, however some very knowledgeable C++ appeared to convince all comers that the FQA is either based on ignorance or is disingenuous.

timr6• 17 years ago

My disagreement has nothing to do with it -- the quoted remark was completely without basis in fact.

kmavm3• 17 years ago

"Unfortunately, you can't tell the class of a C++ object given a pointer to it at run time....which is why you can't automatically collect the garbage of C++ programs."

That's a somewhat self-serving use of ellipses. Here's the actual text:

"Unfortunately, you can't tell the class of a C++ object given a pointer to it at run time. So if you debug a crashed C++ program and find a pointer somewhere in its guts, and you don't know its type, you'll have to guess that "0000000600000005" is a Point. Which is completely obvious, because that's the way a pair of adjacent integers looks like in hexadecimal memory listings of a little endian 32 bit machine. And two adjacent integers might be a Point. Or some other two-integer structure. Or a part of a three-integer-and-a-float structure. Or they might be two unrelated numbers which just happen to be adjacent.

Which is why you can't automatically collect the garbage of C++ programs."

Which part are you claiming is wrong? (Be careful.) The entire first paragraph even Bjarne Stroustrup would agree with once he was done hemming and hawing. Post-mortem inference of types of dynamic objects in C++ programs is undecidable. In higher-level OO languages, memory is organized as framed objects, not as a bag of bytes. The main thrust of this questioned answer had nothing to do with GC.

And he's right about this being what makes GC and C++ bad fits for one another, too, though he skipped a few steps. Getting out the sock puppets, without the ability to tell the type of an in-memory object, you can't compact the heap. If you want to move an object, you can't just assume anything in memory with the bit pattern of that object's address is a pointer, because it might be two shorts or a double or whatever that happens to have that bit pattern.

While conservative, non-compacting garbage collectors can be built in theory, they're too slow in practice. Check out the source to Hans Boehm's C/C++ collector; it stops all the threads in the process, and analyzes liveness starting from each thread's CPU registers, each thread's entire stack, and the entire static data segment. By the implementors' own admission, this will not scale to large numbers of threads, or large address spaces. Even if this approach is refined significantly, if your garbage-collected program lives long enough, you need compacting garbage collection, or fragmentation will kill you just as surely as exhaustion would have.

And if you are so convinced that performant, general purpose GC can be dropped into large C++ codebases, you have a straightforward way of both shutting me up, and having a very interesting ycombinator startup. A drop-in operator new overload that does compacting, generational, safe GC would have a readymade market, with very deep pockets, from companies stuck maintaining enormous piles of C++ code with intractable memory management bugs.

timr6• 17 years ago

I used ellipses because none of the quoted argument was correct, and there was no point making a long quote, when a short one would suffice.

First, the premise was false: you can tell the type of a pointer to a dynamic object -- you use RTTI. You don't have to "guess" that the object is a Point, because you can use dynamic_cast<>() or typeid() to tell for sure.

Second, the conclusion was false: it is possible to write a garbage collector for C++. You cited a very popular one in your response.

So what do we have? You can tell the type of an object at runtime, and you can write a garbage collector for C++. When both the premise and the conclusion of an argument are incorrect, well...let's just say that it's hard to be less correct than that. It's Not Even Wrong.

As for your defense, let's be clear: you're not suggesting that it's impossible to "automatically collect the garbage of C++ programs" -- you're arguing that it's impossible to do it efficiently. I dispute that argument, too, but it has absolutely nothing to do with the comment that I quoted. It's a straw-man.

The bottom line is that the quoted statement was absurd, and you're only setting up a straw-man (about the efficiency of C++ GCs, and about the ability to drop them into large codebases.) We could perhaps have an interesting discussion about those topics, but our discussion wouldn't make the original article any less of a flame-bait.

DarkShikari2• 17 years ago

On a quite related note, I like this page as well:

http://harmful.cat-v.org/software/c++/I_did_it_for_you_all

;)

joey• 17 years ago

It's probably more important to have a good set of coding guidelines when writing C++ code than with other languages, but there's no reason not to write new projects in C++ if it's the best language for the task and team.

Voxli is built on C++ and boost. We make liberal use of scoped_ptr and shared_ptr where appropriate and we have yet to experience resource leaks in our client.

dryicerx3• 17 years ago

This.

Actually to add to your comment, it's not just good, but a MUST to have a set of good coding guidelines. The language is complex for a reason, you tell the Compiler EXACTLY what you want it to do, and this takes away overhead from the compiler trying to guess. It's a trade off between the convenience to the programmer to the performance of the application.

C++ isn't perfect, it's not meant for everything. But for fast, large, and complex systems, C++ is one of the best ways to go about it.

kowsik• 17 years ago

C++ is so not perfect and is definitely not for large scale development. Read this: http://labs.mudynamics.com/2007/07/23/writing-c-within-ruby/

kmavm3• 17 years ago

The author, Yossi Kreinin, is also one of the most underrated bloggers about software implementation on the planet. Before dismissing him as a one-trick pony language bigot, please sample http://www.yosefk.com/blog/ a bit. http://www.yosefk.com/blog/extreme-programming-explained.htm.... is just waiting to be dusted off and resubmitted by some enterprising young news.yc karma junky.

mojuba7• 17 years ago

Oh, his Cardinal Programming Jokes have been here already, but thanks for reminding, this rare and awesome blog is somewhere between the worlds of technology and bed-time fiction.

gcv• 17 years ago

Fascinating that Yossi Kreinin hates Boost libraries so much. I have had my share of problems with them (slooooooow compilation and horrible template compiler error messages, mainly), but the Boost.Spirit parser library turned out to have been a lifesaver. I needed to parse JSON in a C++ system, had Boost available, but could not realistically install any other external library dependencies. Well --- the TinyJSON (http://blog.beef.de/projects/tinyjson/) looks a little funky thanks to all the templates and operator overloading magic, but it works well and I dropped it into the system as a header file. Problem solved.

I'd make heavier use of Boost.Filesystem, too, but I'm stuck deploying with an older version of Boost which lacks a few essentials. I never understood the messiness of concatenating path strings by hand until I was forced to stop using os.path (Python) and java.io.File (Java, Clojure).

kmavm3• 17 years ago

Templates taking too long to compile and producing ridiculous error messages is actually one of, if not the, major complaints Yossi has about C++, though. One of the rarely spoken of differences between C and C++ is that the latter has a ludicrously slower compile leg of the compile/edit/debug cycle, and once you start digging into it, the slowness of compilation is built into the language (basically, matching template specializations is computationally hard), and is not an artifact of so-called "poor implementations."

Confusion2• 17 years ago

If you're criticising something as vast as the C++ FAQ lite and find issue with every single claim, then you are bigoted and your review probably isn't worth it.

rmaccloy4• 17 years ago

This is both a bad argument and kind of lacking a sense of humor, since programmers do this (point by point rebuttals) all the time (see: every popular thread on the LKML, ever)

scott_s2• 17 years ago

I think it's a bad practice in general, as it encourages nit-picking the small details and missing the main point of what someone says.

Confusion2• 17 years ago

Even if programmers do something all the time, that doesn't mean it's right. The reason is simple: every opinion has arguments in favor of it. If someone offers five arguments in favor of an opinion, usually at least two are reasonable. If someone attempts to rebuke all five points, they're usually being unreasonable. It's also counterproductive, because bad counter-arguments call for counter-counter-arguments, which detracts from the actual points under discussoin. If you think an argument isn't persuasive and doesn't have much weight, then that is what you should say, instead of searching for some nitpicky way to counter the argument, usually ending up in discussions about semantics because you actually agree on the point.

As for this article: if this was intended to be humorous, then it contains far too much that seems to have been intended seriously, for my taste. After a few points he could have stopped beating the dead horse.

Wrapping criticism in humor is an annoying strategy similar to the 'poisoning the well' fallacy: as soon as someone tries to start a discussion, you can go "where's your sense of humor?" and your humorously wrapped argument remains unchallenged. That gets under my skin and may have impaired my humor detecting capabilities here.

rmaccloy4• 17 years ago

Also worth reading (edit: if you've got an hour to kill and care about the subject): http://groups.google.com/group/comp.lang.c++.moderated/brows...

The author defends his position pretty coherently and civilly; you can disagree with his conclusions but calling him a troll or an idiot is certainly not warranted.

DanielBMarkham2• 17 years ago

For those of you who are skimming: this is an attack job on C++

Instead of wading in to somebody else's religious war (and it's obviously personal for somebody to spend so much time finding fault with every item in the original FAQ), I'll just say: works great in some situations.

I was building a shell extension for Microsoft Windows several years ago. Booted up C++ and MVC, got up to speed on COM and shell extensions, and made some magic happen. Once I got up to speed (which took some time) I found I was just as productive in C++ as any other language. Same goes for a couple of other projects I've done in C++.

C++ is my favorite language, and it's a god-awful power tool of a beast. I don't use it regularly and don't plan to, but when you need it, it kicks ass. Most of the time, however, you need other stuff, like fast development times, ease-of-learning, common runtime, managed GC, and ease-of-understanding. Criticizing C++ for not having those things is completely missing the point of the language.

Hexstream2• 17 years ago

If x - (fast_dev_time + ease_of_learning + common_runtime + managed_GC + easy_of_understanding) = kicks_ass, then what's x?

In other words, what redeems C++? What's the point of the language?

kingkongrevenge2• 17 years ago

> What's the point of the language?

It's a drop-in replacement for C that is much more powerful and succinct.

mojuba7• 17 years ago

C and C++ (and a couple of other languages like Delphi) provide naked, unprotected pointers, which implies almost full control over memory and the CPU that no higher-level language can give. With pointers you are closer to hardware, in other words.

On top of that, C++ has damn powerful meta-language facilities no other language of any level has, which are, however, beneficial only when in good hands. It's too easy to misuse C++ unfortunately.

DanielBMarkham2• 17 years ago

I love advanced math.

But 99% of my life can be managed just fine with addition, subtraction, multiplication, and division.

It's that other 1%, though, where more advanced math kicks ass. If you've never seen it or watch it happen it's hard to describe how cool it is. With C++ you're controlling the CPU at a level just above machine language.

I love advanced math, but I rarely ever use it. Advanced math kicks ass.

frig• 17 years ago

The redeeming quality of C++ is that there's nothing better in its specific niches.

The question to ask is:

- do you just care about results: I need a program that produces these outputs when given these inputs

- or do you care about what actually happens: I want the computer to do this, then this, then this, in exactly this way please.

...?

It's usually a good idea only to care about the former -- the essence of functional programming, higher-order functions, etc., is to be able to focus on the former as much as possible -- and so long as you only care about the former C++ doesn't have much to recommend it except in special circumstances.

On the other hand, there are times when what happens, exactly is important (eg: device drivers, other low-level code, directly interacting with hardware, etc.). In that scenario what C++ offers is a language that standardizes a lot of the functionality that most sufficiently-large raw-C programs have implemented in an ad-hoc basis.

There are other ways of doing low-level code, but at the present time C++ is the best of bunch (or at least the safest choice if you have an eye to the long term), not so much b/c it's particularly awesome but b/c the others have either faded away for business or political reasons, or b/c the others made limiting choices (C++ makes very few decisions for you, which means it's equally suited -- or unsuited -- to every task in its particularly niche).

william-newman2• 17 years ago

It is seldom a good idea, or even sane, to choose to machine something out of titanium (or beryllium). But sometimes there are reasons.

I think C++ does a decent job of being as logically expressive as practical while still supporting C-level memory micromanagement. Typically exposing C-level memory micromanagement issues is a bug, but occasionally it's a feature. So to me, the point of C++ is to express abstractions like polymorphism and exceptions in the same few pages of code as low level memory operations. How often do I want to do that? Not very often, but it can happen.

Also, while C++ is very hairy and I wouldn't want to have to implement it, C++ seems to be implementable enough and standard enough that it may be usefully supported on a platform well before other more expressive languages. Perhaps it's because a lot of the C++ hair is frontend target-independent stuff like parsing and templates, so that once you've ported the C backend you're almost done porting C++ too? Dunno for sure why, but it seems to happen: e.g., it was my impression on my Atari ST many many years ago, and is my casual impression that something similar happened for the Arduino more recently.

C++ is admittedly a weird mess in various ways, but given the choice of source almost-compatibility with C (which I don't particularly value, but which I can at least understand) and C-level linkers (which I have mixed feelings about) a mess seems inevitable. Thus, I still consider it an impressive design.

Occsionally I've seen IMO-excessively broad claims that higher level languages are the best. It sometimes makes me think I might be able to cook up an example where C++ would be hard to beat: a small backtracking search problem where trailing operations (in the sense of "trailing" that you'll see if you Google "trailing backtracking") are in the inner loop. Take this thought of mine with a generous amount of salt, because I've never boiled it down to a really clean challenge. OTOH, I have at least implemented such search in several languages, and I even did a few casual benchmarks --- but 10+ years ago, when performance tradeoffs were rather different.

That's a particular case of a general theme where C-level programming could win: performance dominated by a very simple memory allocation regularity (LIFO, in this case) that the GC doesn't know about. (This theme also involves the precondition that you should care more about a factor of 2 in performance than a factor of 2 in programmer time, which often doesn't hold.) In many instances of this theme, the accreted featuritis bloat of C++ would be overkill, so you'd prefer C. In this instance, though, placement new() of polymorphic objects into LIFO-allocated raw memory is more expressive than anything I can see how to do in C.

Now, we should not forget that many programmers can go all year without running into a single problem where a factor of 2 in performance is worth a factor of 2 in programmer time. And we should not forget that even the ones who need to care deeply about performance often find it's not determined by a few low-level hotspots where low-level tweaking helps. But in some subfields, it's not too unusual to rationally choose to micromanage stuff at the level of C/C++. (I nominate the subfields of embedded programming and compute-heavy search from personal experience, and Quake/Unreal-like engine programming by reputation; I also explicitly do not nominate implementing web browsers, ow ow ow.)

It's harder to think of reasons I'd choose to write sizable programs in C++ (other than the compelling but unsatisfying accidental-seeming reason of early availability on a platform). One reason, though, is that there are many subproblems for which the choice of language doesn't matter all that much. E.g., Fortran is adequately expressive for a lot of the stuff in a book like _Numerical Recipes_, and you can famously write Fortran in (almost) any language. Because of this, I sometimes encounter a program where some key subproblem really cries for a particular style of language, possibly C++, and then I can be language-agnostic about solving the other subproblems, not minding writing it in whatever language I chose for the key part.

EliAndrewC• 17 years ago

I also end up disagreeing with the FQA's underlying conclusions about C++. Nevertheless, I highly recommend everyone who uses C++ to read through the FQA in its entirety. It's well written and highly entertaining and gives you insight into many of the problems you can run into when using C++ which are glossed over by the FAQ.

biohacker42• 17 years ago

C++ is the first language I used, the one that pays my bills, and probably also still my favorite one. And while the FQA is an attack job, it's not just any attack job. It's done by a C++ expert and reading it will make you a better C++ programmer.

sreque• 17 years ago

I feel like people miss one of the main points of this FQA. The author repeatedly asserts that most people who think they need C++ do so because they think they need the performance it can offer. Most of these people, however, don't actually need the extra performance and would be much better off with a higher level language. Those that actually do need the performance and know what they are doing would be better off just using C with assembler.

In many ways, I think he is right. C++ tries to be high-level, low-level and everything in between all at once, and the end result can bring a lot of potentially unnecessary frustration to programmers. There are several projects, such as LLVM, for example, that get by using a "tasteful subset" of C++, and I would be interested in learning what that subset is.

C++ Frequently Questioned Answers

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