[gaius]

Exactly, many critiques of C++ are well and truly obsolete. What are the practical advantages of Java's GC over C++'s smart pointers? Actually not much, so why carry that overhead.

Plus the elephant in the corner of the room is that actually, write-once-run-anywhere in the real world turns out to be, write on Linux on x64, run on Linux on x64. In the Java world, they like to run one VM (JVM) inside another (on Xen or whatever). Again why schlep all that around? The world is turning back to native code, for good reason.

[fauigerzigerk]

Smart pointers use a lot of memory and some implementations, like boost::shared_ptr are heap based. One thing I want from C++ is efficiency, so I don't subscribe to the recently popular idea of using smart pointers everywhere. A Java reference uses 32 bits even on a 64 bit machine (below 32GB). A shared_ptr uses 4 times that (the first one does).

[thoughtpolice]

What are the practical advantages of Java's GC over C++'s smart pointers? Actually not much, so why carry that overhead.

Smart pointers aren't free, memory or otherwise.

The syntactic overhead of declaring smart pointers is one. Say what you want, but the verbosity of these declarations is quickly tiring, and ergonomics is relevant. typedef's help, but that leads to the next bit, which is that smart pointers are leaky abstractions in the sense that they are never going to be directly interchangeable with a regular C++ pointer. So the abstraction breaks down (if only a little) anytime you need one.

Destruction and RAII have benefits that it's harder to leak memory, but they force you into awkward contortions where you may potentially need more copies than usual, and because copying is an O(n) process in some instances, this can result in the contortions changing the complexity characteristics of data structures or algorithms. Move semantics can alleviate this greatly, however, and they are a welcome addition, as are many other things in C++11 (moves, constexpr, for-each, better enumerations, lambdas, and auto are all very welcome.)

And finally: the massive, unadultered proliferation of reference types quickly becomes an incredible load intellectually. As a programmer, you likely don't care a ton about the sharing semantics of any individual object and whether it's smart or unique or whatever - that is something that can be done automatically with no intervention on your part.

If it turns out you do care about these things greatly, because they are important to your actual task at hand, it is likely C++ may be a good choice. I have written C++ on the job, and have gone from nightmarish code to much nicer code, and it is a suitable tool for many issues.

The counterargument, which is valid too I think, is that sharing semantics form an important part of an API - you can determine whether or not you own a object because it's unique, or whether you should be careful, as it's shared. But this of course a benefit that extends to any typed data, in any typed language (that's worth its salt.)

But people who throw around "just use a smart_ptr and you're like, just as good as Java, obviously" aren't really helping. There's lots of valid points for both sides.

Again why schlep all that around?

Because in a vast majority of cases, it's irrelevant and your time and money is probably just as well spent somewhere else.

The world is turning back to native code, for good reason.

What indications do you have for this? I suppose iOS is a good case example, for one. But the massive amount of web property and the needs of those institutions alone for example, shows that while the world does need native code, it's not turning back to it with reckless abandon.

Futhermore, native code and garbage collection have nothing to do with each other. And the JVM isn't anywhere near the best example: Java having a very heavy per-object overhead (something like 5-7 words per object for heap metadata) doesn't help memory benchmarks in the slightest, although the JVM does have impressive compilation facilities.

But if you want a much better baseline, compare to something like LuaJIT, that gets comparably close to even C++ with only a tiny memory footprint, or compare it to something like GHC where the collector is remarkably robust, and the per-object overhead can be significantly decreased (as all objects instead only have 1 word overhead, and unlike a language like Java, you can totally unpack structures of composite, non-primitive types, meaning new data types can come 'totally free.')

[chancho]

Totally agree, but I just have to pick one nit on your final point re GHC and unpacking: If you do this you forfeit all generic programming because a 'lifted' polymorphic type has to be represented by pointer to a heap object. This is where C++'s unique take on generic programming still wins big.

[zurn]

Aren't C++ smart pointers essentially reference counting? That's well known to be much slower than well implemented GC.

[beagle3]

[citation needed]

Depending on how many cores you use, what processor you use, and other use cases (e.g. copy-on-write, which is free with RC and super expensive with GC), one of them can be significantly faster than the other. But for a non-specific use case, it has been my experience that they are roughly equivalent.

The place where GC consistently excels is the "no random pauses" - most GCs will occasionally need to stop the world, even when they can mostly do incremental collections. Note that this does not mean they are slower - it is just that the overhead tends to be concentrated in bursts instead of uniformly spread out as in RC.

The place where RC consistently excels is reference loops, and less dependence on implementation robustness.

[beagle3]

Just reread, and I got the GC and RC mixed up there (thanks, chancho) too late to edit, so I'll repost a fixed version:

The place where RC consistently excels is the "no random pauses" - most GCs will occasionally need to stop the world, even when they can mostly do incremental collections. Note that this does not mean they are slower - it is just that the overhead tends to be concentrated in bursts instead of uniformly spread out as in RC.

The place where GC consistently excels is reference loops, and less dependence on implementation robustness.

[chancho]

You have a strange definition of "excel".

[beagle3]

Thanks! I managed to get the two lines confused, not sure how, and it is too late to edit; just posted a correction:

The place where RC consistently excels is the "no random pauses" - most GCs will occasionally need to stop the world, even when they can mostly do incremental collections. Note that this does not mean they are slower - it is just that the overhead tends to be concentrated in bursts instead of uniformly spread out as in RC.

The place where GC consistently excels is reference loops, and less dependence on implementation robustness.

[maximilianburke]

GC may be faster in terms of total amount of time spent managing memory but smart pointers are more predictable. Many domains in which C++ is used are more sensitive to the performance spikes that can arise in GC'd environments than they are to the more expensive but also more spread out cost of managing reference counted objects. Many video game technical directors would choose spending 1ms/30hz frame in memory bookkeeping rather than going 29 frames with no problems only to encounter a 15ms GC hiccup.

[tomjen3]

Well that depends on the type of pointer -- if you only need the reference till the end of the function, (or when you deconstruct a class) you can get a much more performant implementation.

As for speed, with benchmarking every thing is in the details and while GC can be made to run fast the actual speed depends on the access patterns, the cache and when memory is used (and for how long).

In addition in C++ you can put quite a lot of things on the stack (that is actually how you make a smart-pointer) which is always going to be much, much faster than anything on the heap.