C++11 Features in Visual C++ 11 - VS2010对C++11特性支持的情况

原文: http://blogs.msdn.com/b/vcblog/archive/2011/09/12/10209291.aspx

UPDATE - March 2, 2012: the range-based for-loop and override/final v1.0 have been implemented in VC11 Beta.

 

There's a new C++ Standard and a new version of Visual C++, and it's time to reveal what features from the former we're implementing in the latter!

Terminology notes: During its development, the new C++ Standard was (optimistically) referred to as C++0x.  It's finally being published in 2011, and it's now referred to as C++11.  (Even International Standards slip their release dates.)  The Final Draft International Standard is no longer publicly available.  It was immediately preceded by Working PaperN3242, which is fairly close in content.  (Most of the people who care about the differences are compiler/Standard Library devs who already have access to the FDIS.)  Eventually, I expect that the C++11 Standard will be available from ANSI, like C++03 is.

As for Visual C++, it has three different version numbers, for maximum fun.  There's the branded version (printed on the box), the internal version (displayed in Help About), and the compiler version (displayed by cl.exe and the_MSC_VER macro - this one is different because our C++ compiler predates the "Visual" in Visual C++).  For example:

VS 2005 == VC8 == _MSC_VER 1400
VS 2008 == VC9 == _MSC_VER 1500
VS 2010 == VC10 == _MSC_VER 1600

The final branding for the new version hasn't been announced yet; for now, I'm supposed to say "Visual C++ in Visual Studio 11 Developer Preview".  Internally, it's just VC11, and its_MSC_VER macro is 1700.  (That macro is of interest to people who want to target different major versions of VC and emit different code for them.)  I say VC10 and VC11 because they're nice and simple - the 11 in VC11 does not refer to a year.  (VS 2010 == VC10 was a confusing coincidence.)

If you read C++0x Core Language Features In VC10: The Table last year, the following table will look familiar to you.  This time, I started with GCC's table again, but I reorganized it more extensively for increased accuracy and clarity (as many features went through significant revisions):

 

C++11 Core Language Features	VC10	VC11
Rvalue references v0.1, v1.0, v2.0, v2.1, v3.0	v2.0	v2.1*
ref-qualifiers	No	No
Non-static data member initializers	No	No
Variadic templates v0.9, v1.0	No	No
Initializer lists	No	No
static_assert	Yes	Yes
auto v0.9, v1.0	v1.0	v1.0
Trailing return types	Yes	Yes
Lambdas v0.9, v1.0, v1.1	v1.0	v1.1
decltype v1.0, v1.1	v1.0	v1.1**
Right angle brackets	Yes	Yes
Default template arguments for function templates	No	No
Expression SFINAE	No	No
Alias templates	No	No
Extern templates	Yes	Yes
nullptr	Yes	Yes
Strongly typed enums	Partial	Yes
Forward declared enums	No	Yes
Attributes	No	No
constexpr	No	No
Alignment	TR1	Partial
Delegating constructors	No	No
Inheriting constructors	No	No
Explicit conversion operators	No	No
char16_t and char32_t	No	No
Unicode string literals	No	No
Raw string literals	No	No
Universal character names in literals	No	No
User-defined literals	No	No
Standard-layout and trivial types	No	Yes
Defaulted and deleted functions	No	No
Extended friend declarations	Yes	Yes
Extendedsizeof	No	No
Inline namespaces	No	No
Unrestricted unions	No	No
Local and unnamed types as template arguments	Yes	Yes
Range-based for-loop	No	Yes
override and final v0.8, v0.9, v1.0	Partial	Yes
Minimal GC support	Yes	Yes
noexcept	No	No

 

C++11 Core Language Features: Concurrency	VC10	VC11
Reworded sequence points	N/A	N/A
Atomics	No	Yes
Strong compare and exchange	No	Yes
Bidirectional fences	No	Yes
Memory model	N/A	N/A
Data-dependency ordering	No	Yes
Data-dependency ordering: function annotation	No	No
exception_ptr	Yes	Yes
quick_exit andat_quick_exit	No	No
Atomics in signal handlers	No	No
Thread-local storage	Partial	Partial
Magic statics	No	No

 

C++11 Core Language Features: C99	VC10	VC11
__func__	Partial	Partial
C99 preprocessor	Partial	Partial
long long	Yes	Yes
Extended integer types	N/A	N/A

 

Here's a quick guide to this table, but note that I can't explain everything from scratch without writing a whole book, so this assumes moderate familiarity with what's in C++11:

Rvalue references: N1610 "Clarification of Initialization of Class Objects by rvalues" was an early attempt to enable move semantics without rvalue references.  I'm calling it "rvalue references v0.1", as it's of historical interest only.  It was superseded by rvalue referencesv1.0, the original wording.  Rvalue references v2.0, which is what we shipped in VC10 RTM/SP1, prohibits rvalue references from binding to lvalues, fixing a major safety problem.  Rvalue referencesv2.1 refines this rule.  Consider vector::push_back(), which has the overloadspush_back(const string&) and push_back(string&&), and the call v.push_back("meow").  The expression "meow" is a string literal, and it is an lvalue.  (All other literals like 1729 are rvalues, but string literals are special because they're arrays.)  The rvalue references v2.0 rules looked at this and said, string&& can't bind to "meow" because "meow" is an lvalue, so push_back(const string&) is the only viable overload.  This would create a temporarystd::string, copy it into the vector, then destroy the temporarystd::string.  Yuck!  The rvalue references v2.1 rules recognize that bindingstring&& to "meow" would create a temporary std::string, and that temporary is an rvalue.  Therefore, both push_back(const string&) and push_back(string&&) are viable, andpush_back(string&&) is preferred.  A temporary std::string is constructed, then moved into the vector.  This is more efficient, which is good!  (Yes, I'm ignoring the Small String Optimization here.)

The table says "v2.1*" because these new rules haven't been completely implemented in the VC11 Developer Preview.  This is being tracked by an active bug.  (Indeed, this is a Standard bugfix.)

Rvalue references v3.0 adds new rules to automatically generate move constructors and move assignment operators under certain conditions.  This will not be implemented in VC11, which will continue to follow VC10's behavior of never automatically generating move constructors/move assignment operators.  (As with all of the not-yet-implemented features here, this is due to time and resource constraints, and not due to dislike of the features themselves!)

(By the way, all of this v0.1, v1.0, v2.0, v2.1, v3.0 stuff is my own terminology, which I think adds clarity to C++11's evolution.)

Lambdas: After lambdas were voted into the Working Paper (v0.9) and mutable lambdas were added (v1.0), the Standardization Committee overhauled the wording, producing lambdas v1.1.  This happened too late for us to implement in VC10, but we've already implemented it in VC11.  The lambdas v1.1 wording clarifies what should happen in corner cases like referring to static members, or nested lambdas.  This fixes a bunch of bugs triggered by complicated lambdas.  Additionally, stateless lambdas are now convertible to function pointers in VC11.  This isn't in N2927's wording, but I count it as part of lambdas v1.1 anyways.  It's FDIS 5.1.2 [expr.prim.lambda]/6: "The closure type for a lambda-expression with no lambda-capture has a public non-virtual non-explicitconst conversion function to pointer to function having the same parameter and return types as the closure type’s function call operator. The value returned by this conversion function shall be the address of a function that, when invoked, has the same effect as invoking the closure type’s function call operator."  (It's even better than that, since we've made stateless lambdas convertible to function pointers with arbitrary calling conventions.  This is important when dealing with APIs that expect __stdcall function pointers and so forth.)

decltype: After decltype was voted into the Working Paper (v1.0), it received a small but important bugfix at the very last minute (v1.1).  This isn't interesting to most programmers, but it's of great interest to programmers who work on the STL and Boost.  The table says "v1.1**" because this isn't implemented in the VC11 Developer Preview, but the changes to implement it have already been checked in.

Strongly typed/forward declared enums: Strongly typed enums were partially supported in VC10 (specifically, the part about explicitly specified underlying types), and C++11's semantics forforward declared enums weren't supported at all in VC10.  Both have been completely implemented in VC11.

Alignment: Neither VC10 nor VC11 implement the Core Language keywordsalignas/alignof from thealignment proposal that was voted into the Working Paper.  VC10 had aligned_storage from TR1.  VC11 adds aligned_union andstd::align() to the Standard Library.

Standard-layout and trivial types: As far as I can tell, the user-visible changes fromN2342 "POD's Revisited; Resolving Core Issue 568 (Revision 5)" are the addition ofis_trivial and is_standard_layout to.  (N2342 performed a lot of surgery to Core Language wording, but it just makes stuff well-defined that users could have gotten away with anyways, hence no compiler changes are necessary.)  We had these type traits in VC10, but they just duplicated is_pod, so I'm calling that "No" support.  In VC11, they're powered by compiler hooks that should give accurate answers.

Extended friend declarations: Last year, I said that VC10 partially supported this.  Upon closer inspection ofN1791, I've determined that VC's support for this is essentially complete (it doesn't even emit "non-Standard extension" warnings, unlike some of the other Ascended Extensions in this table).  So I've marked both VC10 and VC11 as "Yes".

override and final: This went through a short but complicated evolution.   Originally (v0.8) there were [[override]], [[hiding]], and [[base_check]] attributes.  Then (v0.9) the attributes were eliminated and replaced with contextual keywords.  Finally (v1.0), they were reduced to "final" on classes, and "override" and "final" on functions.  This makes it an Ascended Extension, as VC already supports this "override" syntax on functions, with semantics reasonably close to C++11's.  "final" is also supported, but under the different spelling "sealed".  This qualifies for "Partial" support in my table.

Minimal GC support: As it turns out, N2670's only user-visible changes are a bunch of no-op Standard Library functions, which we already picked up in VC10.

Reworded sequence points: After staring at N2239's changes, replacing C++98/03's "sequence point" wording with C++11's "sequenced before" wording (which is more useful, and more friendly to multithreading), there appears to be nothing for a compiler or Standard Library implementation to do.  So I've marked this as N/A.

Atomics, etc.: Atomics, strong compare and exchange, bidirectional fences, and data-dependency ordering specify Standard Library machinery, which we're implementing in VC11.

Memory model: N2429 made the Core Language recognize the existence of multithreading, but there appears to be nothing for a compiler implementation to do (at least, one that already supported multithreading).  So it's N/A in the table.

Extended integer types: N1988 itself says: "A final point on implementation cost: this extension will probably cause no changes in most compilers. Any compiler that has no integer types other than those mandated by the standard (and some version of long long, which is mandated by the N1811 change) will likely conform already."  Another N/A feature!

That covers the Core Language.  As for the Standard Library, I don't have a pretty table of features, but I do have good news:

In VC11, we intend to completely support the C++11 Standard Library, modulo not-yet-implemented compiler features.  (Additionally, VC11 won't completely implement the C99 Standard Library, which has been incorporated by reference into the C++11 Standard Library.  Note that VC10 and VC11 already have .)  Here's a non-exhaustive list of the changes we're making:

New headers: , , , , , , , and .  (And I've removed the broken header that I accidentally left in VC10.)

Emplacement: As required by C++11, we've implemented emplace()/emplace_front()/emplace_back()/emplace_hint()/emplace_after() in all containers for "arbitrary" numbers of arguments (see below).  For example, vector has "template void emplace_back(Args&&... args)" which directly constructs an element of type T at the back of the vector from an arbitrary number of arbitrary arguments, perfectly forwarded.  This can be more efficient thanpush_back(T&&), which would involve an extra move construction and destruction.  (VC10 supported emplacement from 1 argument, which was not especially useful.)

Faux variadics: We've developed a new scheme for simulating variadic templates.  Previously in VC9 SP1 and VC10, we repeatedly included subheaders with macros defined differently each time, in order to stamp out overloads for 0, 1, 2, 3, etc. arguments.  (For example, included the internal subheader repeatedly, in order to stamp out make_shared(args, args, args).)  In VC11, the subheaders are gone.  Now we define variadic templates themselves as macros (with lots of backslash-continuations), then expand them with master macros.  This internal implementation change has some user-visible effects.  First, the code is more maintainable, easier to use (adding subheaders was a fair amount of work), and slightly less hideously unreadable.  This is what allowed us to easily implement variadic emplacement, and should make it easier to squash bugs in the future.  Second, it's harder to step into with the debugger (sorry!).  Third, pair's pair(piecewise_construct_t, tuple, tuple) constructor had "interesting" effects.  This requires N^2 overloads (if we support up to 10-tuples, that means 121 overloads, since empty tuples count here too).  We initially observed that this (spamming out so many pair-tuple overloads, plus all of the emplacement overloads) consumed a massive amount of memory during compilation, so as a workaround we reduced infinity.  In VC9 SP1 and VC10, infinity was 10 (i.e. "variadic" templates supported 0 to 10 arguments inclusive).  In the VC11 Developer Preview, infinity is 5 by default.  This got our compiler memory consumption back to what it was in VC10.  If you need more arguments (e.g. you had code compiling with VC9 SP1 or VC10 that used 6-tuples), there's an escape hatch.  You can define_VARIADIC_MAX project-wide between 5 and 10 inclusive (it defaults to 5).  Increasing it will make the compiler consume more memory, and may require you to use the/Zm option to reserve more space for PCHes.

This story has a happy ending, though!  Jonathan Caves, our compiler front-end lord, investigated this and found that something our tuple implementation was doing (specifically, lots of default template arguments), multiplied by pair's N^2 overloads, multiplied by how much pair tends to get used by STL programs (e.g. every map), was responsible for the increased memory consumption.  He fixed that, and the fix is making its way over to our STL branch.  At that point, we'll see if we can raise the_VARIADIC_MAX default to 10 again (as I would prefer not to break existing code unnecessarily).

Randomness: uniform_int_distribution is now perfectly unbiased, and we've implementedshuffle() in , which directly accepts Uniform Random Number Generators likemersenne_twister.

Resistance to overloaded address-of operators: C++98/03 prohibited elements of STL containers from overloading their address-of operator.  This is what classes likeCComPtr do, so helper classes like CAdapt were required to shield the STL from such overloads.  During VC10's development, while massively rewriting the STL (for rvalue references, among other things), our changes made the STL hate overloaded address-of operators even more in some situations.  (You might remember one of my VCBlog posts about this.)  Then C++11 changed its requirements, making overloaded address-of operators acceptable.  (C++11, and VC10, provide the helper functionstd::addressof(), which is capable of getting the true address of an object regardless of operator overloading.)  Before VC10 shipped, we attempted to audit all STL containers for occurrences of "&elem", replacing them with "std::addressof(elem)" which is appropriately resistant.  In VC11, we've gone further.  Now we've audited all containers and all iterators, so classes that overload their address-of operator should be usable throughout the STL.  Any remaining problems are bugs that should be reported to us through Microsoft Connect.  (As you might imagine, grepping for "&elem" is rather difficult!)  I haven't audited the algorithms yet, but a casual glance indicated to me that they aren't especially fond of taking the addresses of elements.

We're also going beyond C++11 in a couple of ways:

SCARY iterators: As permitted but not required by the C++11 Standard, SCARY iterators have been implemented, as described byN2911 "Minimizing Dependencies within Generic Classes for Faster and Smaller Programs" andN2980 "SCARY Iterator Assignment and Initialization, Revision 1".

Filesystem: We've added the header fromthe TR2 proposal, featuring super-cool machinery like recursive_directory_iterator.  Note that the 2006 proposal (before work on TR2 was frozen due to C++0x running extremely late and turning into C++11) was derived from Boost.Filesystem V2.  It later evolved into Boost.Filesystem V3, but that will not be implemented in VC11.

Finally, in addition to numerous bugfixes, we've performed a major optimization!  All of our containers (loosely speaking) are now optimally small given their current representations.  This is referring to the container objects themselves, not their pointed-to guts.  For example, vector contains three raw pointers.  In VC10, x86 release mode, vector was 16 bytes.  In VC11, it's 12 bytes, which is optimally small.  This is a big deal if you have 100,000 vectors in your program - VC11 will save you 400,000 bytes.  Decreased memory usage saves both space and time.

This was achieved by avoiding the storage of empty allocators and comparators, asstd::allocator and std::less are stateless.  (We'll activate these optimizations for custom allocators/comparators too, as long as they're stateless.  Obviously, we can't avoid storing stateful allocators/comparators, but those are quite rare.)

Here are all of the sizes for x86 and x64.  (32-bit ARM is equivalent to x86 for these purposes).  Naturally, these tables cover release mode, as debug mode contains checking machinery that consumes space and time.  I have separate columns for VC9 SP1, where_SECURE_SCL defaulted to 1, and for VC9 SP1 with _SECURE_SCL manually set to 0 for maximum speed.  VC10 and VC11 default _SECURE_SCL to 0 (now known as _ITERATOR_DEBUG_LEVEL).

 

x86 Container Sizes (Bytes)	VC9 SP1	VC9 SP1 SCL=0	VC10	VC11
vector	24	16	16	12
array	20	20	20	20
deque	32	32	24	20
forward_list	N/A	N/A	8	4
list	28	12	12	8
priority_queue	28	20	20	16
queue	32	32	24	20
stack	32	32	24	20
pair	8	8	8	8
tuple	16	16	16	12
map	32	12	16	8
multimap	32	12	16	8
set	32	12	16	8
multiset	32	12	16	8
hash_map	72	44	44	32
hash_multimap	72	44	44	32
hash_set	72	44	44	32
hash_multiset	72	44	44	32
unordered_map	72	44	44	32
unordered_multimap	72	44	44	32
unordered_set	72	44	44	32
unordered_multiset	72	44	44	32
string	28	28	28	24
wstring	28	28	28	24

 

x64 Container Sizes (Bytes)	VC9 SP1	VC9 SP1 SCL=0	VC10	VC11
vector	48	32	32	24
array	20	20	20	20
deque	64	64	48	40
forward_list	N/A	N/A	16	8
list	56	24	24	16
priority_queue	56	40	40	32
queue	64	64	48	40
stack	64	64	48	40
pair	8	8	8	8
tuple	16	16	16	12
map	64	24	32	16
multimap	64	24	32	16
set	64	24	32	16
multiset	64	24	32	16
hash_map	144	88	88	64
hash_multimap	144	88	88	64
hash_set	144	88	88	64
hash_multiset	144	88	88	64
unordered_map	144	88	88	64
unordered_multimap	144	88	88	64
unordered_set	144	88	88	64
unordered_multiset	144	88	88	64
string	40	40	40	32
wstring	40	40	40	32

 

Stephan T. Lavavej
Visual C++ Libraries Developer