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New Concurrency Features in C++14

Wednesday, 08 July 2015

It might have been out for 7 months already, but the C++14 standard is still pretty fresh. The changes include a couple of enhancements to the thread library, so I thought it was about time I wrote about them here.

Shared locking

The biggest of the changes is the addition of std::shared_timed_mutex. This is a multiple-reader, single-writer mutex. This means that in addition to the single-ownership mode supported by the other standard mutexes, you can also lock it in shared ownership mode, in which case multiple threads may hold a shared ownership lock at the same time.

This is commonly used for data structures that are read frequently but modified only rarely. When the data structure is stable, all threads that want to read the data structure are free to do so concurrently, but when the data structure is being modified then only the thread doing the modification is permitted to access the data structure.

The timed part of the name is analagous to std::timed_mutex and std::recursive_timed_mutex: timeouts can be specified for any attempt to acquire a lock, whether a shared-ownership lock or a single-ownership lock. There is a proposal to add a plain std::shared_mutex to the next standard, since this can have lower overhead on some platforms.

To manage the new shared ownership mode there are a new set of member functions: lock_shared(), try_lock_shared(), unlock_shared(), try_lock_shared_for() and try_lock_shared_until(). Obtaining and releasing the single-ownership lock is performed with the same set of operations as for std::timed_mutex.

However, it is generally considered bad practise to use these functions directly in C++, since that leaves the possibility of dangling locks if a code path doesn't release the lock correctly. It is better to use std::lock_guard and std::unique_lock to perform lock management in the single-ownership case, and the shared-ownership case is no different: the C++14 standard also provides a new std::shared_lock class template for managing shared ownership locks. It works just the same as std::unique_lock; for common use cases it acquires the lock in the constructor, and releases it in the destructor, but there are member functions to allow alternative access patterns.

Typical uses will thus look like the following:

std::shared_timed_mutex m;
my_data_structure data;

void reader(){
    std::shared_lock<std::shared_timed_mutex> lk(m);

void writer(){
    std::lock_guard<std::shared_timed_mutex> lk(m);

Performance warning

The implementation of a mutex that supports shared ownership is inherently more complex than a mutex that only supports exclusive ownership, and all the shared-ownership locks still need to modify the mutex internals. This means that the mutex itself can become a point of contention, and sap performance. Whether using a std::shared_timed_mutex instead of a std::mutex provides better or worse performance overall is therefore strongly dependent on the work load and access patterns.

As ever, I therefore strongly recommend profiling your application with std::mutex and std::shared_timed_mutex in order to ascertain which performs best for your use case.

std::chrono enhancements

The other concurrency enhancements in the C++14 standard are all in the <chrono> header. Though this isn't strictly about concurrency, it is used for all the timing-related functions in the concurrency library, and is therefore important for any threaded code that has timeouts.

constexpr all round

The first change is that the library has been given a hefty dose of constexpr goodness. Instances of std::chrono::duration and std::chrono::time_point now have constexpr constructors and simple arithmetic operations are also constexpr. This means you can now create durations and time points which are compile-time constants. It also means they are literal types, which is important for the other enhancement to <chrono>: user-defined literals for durations.

User-defined literals

C++11 introduced the idea of user-defined literals, so you could provide a suffix to numeric and string literals in your code to construct a user-defined type of object, much as 3.2f creates a float rather than the default double, however there were no new types of literals provided by the standard library.

C++14 changes that. We now have user-defined literals for std::chrono::duration, so you can write 30s instead of std::chrono::seconds(30). To get this user-defined literal goodness you need to explicitly enable it in your code with a using directive — you might have other code that wants to use these suffixes for a different set of types, so the standard let's you choose.

That using directive is:

using namespace std::literals::chrono_literals;

The supported suffixes are:

  • hstd::chrono::hours
  • minstd::chrono::minutes
  • sstd::chrono::seconds
  • msstd::chrono::milliseconds
  • usstd::chrono::microseconds
  • nsstd::chrono::nanoseconds

You can therefore wait for a shared ownership lock with a 50 millisecond timeout like this:

void foo(){
    using namespace std::literals::chrono_literals;
    std::shared_lock<std::shared_timed_mutex> lk(m,50ms);
    else {

Just::Thread support

As you might expect, Just::Thread provides an implementation of all of this. std::shared_timed_mutex is available on all supported platforms, but the constexpr and user-defined literal enhancements are only available for those compilers that support the new language features: gcc 4.6 or later for constexpr and gcc 4.7 or later for user-defined literals, with the -std=c++11 or std=c++14 switch enabled in either case.

Get your copy of Just::Thread while our 10th anniversary sale is on for a 50% discount.

Posted by Anthony Williams
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10th Anniversary Sale

Wednesday, 01 July 2015

We started Just Software Solutions Ltd in June 2005, so we've now been in business for 10 years. We're continuing to thrive, with both our consulting, training and development services, and sales of Just::Thread doing well.

To let our customers join in with our celebrations, we're running a month long sale: for the whole of July 2015, Just::Thread and Just::Thread Pro will be available for 50% off the normal price.

Just::Thread is our implementation of the C++11 and C++14 thread libraries, for Windows, Linux and MacOSX. It also includes some of the extensions from the upcoming C++ Concurrency TS, with more to come shortly.

Just::Thread Pro is our add-on library which provides an Actor framework for easier concurrency, along with concurrent data structures: a thread-safe queue, and concurrent hash map, and a wrapper for ensuring synchronized access to single objects.

All licences include a free upgrade to point releases, so if you purchase now you'll get a free upgrade to all 2.x releases.

Posted by Anthony Williams
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Standardizing Variant: Difficult Decisions

Wednesday, 17 June 2015

One of the papers proposed for the next version of the C++ Standard is N4542: Variant: a type safe union (v4). As you might guess from the (v4) in the title, this paper has been discussed several times by the committee, and revised in the light of discussions.

Boost has had a variant type for a long time, so it only seems natural to standardize it. However, there are a couple of design decisions made for boost::variant which members of the committee were uncomfortable with, so the current paper has a couple of differences from boost::variant. The most notable of these is that boost::variant has a "never empty" guarantee, whereas N4542 proposes a variant that can be empty.

Why do we need empty variants?

Let's assume for a second that our variant is never empty, as per boost::variant, and consider the following code with two classes A and B:

    variant<A,B> v1{A()};
    variant<A,B> v2{B()};

Before the assignment, v1 holds a value of type A. After the assignment v1=v2, v1 has to hold a value of type B, which is a copy of the value held in v2. The assignment therefore has to destroy the old value of type A and copy-construct a new value of type B into the internal storage of v1.

If the copy-construction of B does not throw, then all is well. However, if the copy construction of B does throw then we have a problem: we just destroyed our old value (of type A), so we're in a bit of a predicament — the variant isn't allowed to be empty, but we don't have a value!

Can we fix it? Double buffering

In 2003 I wrote an article about this, proposing a solution involving double-buffering: the variant type could contain a buffer big enough to hold both A and B. Then, the assignment operator could copy-construct the new value into the empty space, and only destroy the old value if this succeeded. If an exception was thrown then the old value is still there, so we avoid the previous predicament.

This technique isn't without downsides though. Firstly, this can double the size of the variant, as we need enough storage for the two largest types in the variant. Secondly, it changes the order of operations: the old value is not destroyed until after the new one has been constructed, which can have surprising consequences if you are not expecting it.

The current implementation of boost::variant avoids the first problem by constructing the secondary buffer on the fly. This means that assignment of variants now involves dynamic memory allocation, but does avoid the double-space requirement. However, there is no solution for the second problem: avoiding destroying the old value until after the new one has been constructed cannot be avoided while maintaining the never-empty guarantee in the face of throwing copy constructors.

Can we fix it? Require no-throw copy construction

Given that the problem only arises due to throwing copy constructors, we could easily avoid the problem by requiring that all types in the variant have a no-throw copy constructor. The assignment is then perfectly safe, as we can destroy the old value, and copy-construct the new one, without fear of an exception throwing a spanner in the works.

Unfortunately, this has a big downside: lots of useful types that people want to put in variants like std::string, or std::vector, have throwing copy constructors, since they must allocate memory, and people would now be unable to store them directly. Instead, people would have to use std::shared_ptr<std::string> or create a wrapper that stored the exception in the case that the copy constructor threw an exception.

    template<typename T>
    class value_or_exception{
        std::optional<T> value;
        std::exception_ptr exception;
        value_or_exception(T const& v){
            } catch(...) {
        value_or_exception(value_or_exception const& v){
            } catch(...) {
            return *this;
        value_or_exception& operator=(T const& v){
            } catch(...) {
            return *this;
        // more constructors and assignment operators
        T& get(){
            return *value;

Given such a template you could have variant<int,value_or_exception<std::string>>, since the copy constructor would not throw. However, this would make using the std::string value that little bit harder due to the wrapper — access to it would require calling get() on the value, in addition to the code required to retrieve it from the variant.

    variant<int,value_or_exception<std::string>> v=get_variant_from_somewhere();
    std::string& s=std::get<value_or_exception<std::string>>(v).get();

The code that retrieves the value then also needs to handle the case that the variant might be holding an exception, so get() might throw.

Can we fix it? Tag types

One proposed solution is to add a special case if one of the variant types is a special tag type like empty_variant_t. e.g. variant<int,std::string,empty_variant_t. In this case, if the copy constructor throws then the special empty_variant_t type is stored in the variant instead of what used to be there, or what we tried to assign. This allows people who are OK with the variant being empty to use this special tag type as a marker — the variant is never strictly "empty", it just holds an instance of the special type in the case of an exception, which avoids the problems with out-of-order construction and additional storage. However, it leaves the problems for those that don't want to use the special tag type, and feels like a bit of a kludge.

Do we need to fix it?

Given the downsides, we have to ask: is any of this really any better than allowing an empty state?

If we allow our variant to be empty then the code is simpler: we just write for the happy path in the main code. If the assignment throws then we will get an exception at that point, which we can handle, and potentially store a new value in the variant there. Also, when we try and retrieve the value then we might get an exception there if the variant is empty. However, if the expected scenario is that the exception will never actually get thrown, and if it does then we have a catastrophic failure anyway, then this can greatly simplify the code.

For example, in the case of variant<int,std::string>, the only reason you'd get an exception from the std::string copy constructor was due to insufficient memory. In many applications, running out of dynamic memory is exceedingly unlikely (the OS will just allocate swap space), and indicates an unrecoverable scenario, so we can get away with assuming it won't happen. If our application isn't one of these, we probably know it, and will already be writing code to carefully handle out-of-memory conditions.

Other exceptions might not be so easily ignorable, but in those cases you probably also have code designed to handle the scenario gracefully.

A variant with an "empty" state is a bit like a pointer in the sense that you have to check for NULL before you use it, whereas a variant without an empty state is more like a reference in that you can rely on it having a value. I can see that any code that handles variants will therefore get filled with asserts and preconditions to check the non-emptiness of the variant.

Given the existence of an empty variant, I would rather that the various accessors such as get<T>() and get<Index>() threw an exception on the empty state, rather than just being ill-formed.

Default Construction

Another potentially contentious area is that of default construction: should a variant type be default constructible? The current proposal has variant<A,B> being default-constructible if and only if A (the first listed type) is default-constructible, in which case the default constructor default-constructs an instance of A in the variant. This mimics the behaviour of the core language facility union.

This means that variant<A,B> and variant<B,A> behave differently with respect to default construction. For starters, the default-constructed type is different, but also one may be default-constructible while the other is not. For some people this is a surprising result, and undesirable.

One alternative options is that default construction picks the first default-constructible type from the list, if there are any, but this still has the problem of different orderings behaving differently.

Given that variants can be empty, another alternative is to have the default constructed variant be empty. This avoids the problem of different orderings behaving differently, and will pick up many instances of people forgetting to initialize their variants, since they will now be empty rather than holding a default-constructed value.

My preference is for the third option: default constructed variants are empty.

Duplicate types

Should we allow variant<T,T>? The current proposal allows it, and makes the values distinct. However, it comes with a price: you cannot simply construct a variant<T,T> from a T: instead you must use the special constructors that take an emplaced_index_t<I> as the first parameter, to indicate which entry you wish to construct. Similarly, you can now no longer retrieve the value merely by specifying the type to retrieve: you must specify the index, as this is now significant.

I think this is unnecessary overhead for a seriously niche feature. If people want to have two entries of the same type, but with different meanings, in their variant then they should use the type system to make them different. It's trivial to write a tagged_type template, so you can have tagged_type<T,struct SomeTag>and tagged_type<T,struct OtherTag> which are distinct types, and thus easily discriminated in the variant. Many people would argue that even this is not going far enough: you should embrace the Whole Value Idiom, and write a proper class for each distinct meaning.

Given that, I think it thus makes sense for variant<T,T> to be ill-formed. I'm tempted to make it valid, and the same as variant<T>, but too much of the interface depends on the index of the type in the type list. If I have variant<T,U,T,T,U,int>, what is the type index of the int, or the T for that matter? I'd rather not have to answer such questions, so it seems better to make it ill-formed.

What do you think?

What do you think about the proposed variant template? Do you agree with the design decisions? Do you have a strong opinion on the issues above, or some other aspect of the design?

Have your say in the comments below.

Posted by Anthony Williams
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Slides and code and for my ACCU 2015 presentation

Wednesday, 17 June 2015

It's now two months since the ACCU 2015 conference in Bristol, UK, so I thought it was about time I posted my slides.

This year my presentation was titled "Safety: off - How not to shoot yourself in the foot with C++ atomics". I gave a brief introduction to the C++ atomics facilities, some worked examples of usage, and guidelines for how to use atomics safely in your code.

The slides are available here, and the code examples here.

Posted by Anthony Williams
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Cryptography and Society

Tuesday, 05 May 2015

Politicians in both the UK and USA have been making moves towards banning secure encryption over the last few months. With the UK general election coming on Thursday I wanted to express why I think this is a seriously bad idea.


Back in January there were some terrorist attacks in Paris. These attacks were and are a serious matter, and stopping such attacks in future should be something that governments concern themselves with.

However, one aspect of the response by politicians has been to call for securely encrypted communication outlawed. In particular, the British Prime Minister, David Cameron, asked

"In our country, do we want to allow a means of communication between people which, even in extemis with a signed warrant from the Home Secretary personally, that we cannot read?"

It was clear from the context that he thinks the answer is a resounding "NO", and from the further actions of politicians both here and in the USA it appears that others in government agree with that point of view. They clearly believe that the government should be able to read all communications.

I think in a fair, open, democratic society, the answer must be "YES": private individuals must be able to communicate without risk of eavesdropping by government officials.

Secure Encryption is Not New

Firstly, there have ALWAYS been means of communication between people that the government cannot read. You might be able to intercept a letter, and read the words written on the piece of paper, but if the message is not in the words as they appear, then you cannot read it.

Ciphers which could not be cracked by contemporary eavesdroppers have been used since at least the time of the Roman Empire. New technology merely provides a new set of such ciphers.

For example, the proper use of a one-time pad provides completely secure encryption. This technique has been in use since 1882, if not earlier.

Other technology in widespread use today "merely" makes it exceedingly difficult to break the cipher, requiring hundreds, thousands or even millions of years to crack with a brute-force method. These time periods are enough that these ciphers can be considered uncrackable for all intents and purposes.

Consequently, governments are powerless to actually prevent communication that cannot be read by the security services. All that can be done is to make it hard for the average citizen to use such communication.

Terrorists are Criminals

By their very nature, terrorists are criminals: terrorist acts themselves are illegal, and even the possession of the weapons used for the terrorist acts is often also illegal.

Therefore, terrorists will not be put off from using secure communication just because that too is illegal.

In particular, criminal organisations will not think twice about using whatever means is available to ensure that their communications are private: if something gives them an edge of the police or anyone else who would seek to stop them, they will use it.

Society relies on secure encryption

Secure encrypted communication doesn't just prevent government agencies reading the communications of criminals, it also prevents criminals reading the communications of ordinary citizens.

This website, in common with an increasingly large number of websites, uses HTTPS for all traffic. When properly configured this means that someone intercepting the website traffic cannot identify which pages on the website you visited, or extract any of the data sent by you as a visitor to the website, or by the website back to you.

This is crucial for facilities such as online banking: it prevents computer criminals from obtaining passwords and account data by intercepting the communications between you and your bank. If such communications could not be relied upon to be secure then online banking would not be viable, as the potential for fraud due to stolen passwords would be too great.

Likewise, many businesses use secure Virtual Private Networks (VPNs) which rely on secure encryption to transfer data between computers that are only connected to each other via the internet. This allows them to securely transfer data between sites, or between remote workers, without worrying about the communications being intercepted by criminals. Without secure encryption, many large multi-national businesses would be hugely impacted, as they wouldn't be able to rely on transferring data across the internet safely, and would instead have to rely on physical transfer via courier.

A "back door" or "government secret key" destroys the security of encryption

Some of the proposals from politicians have been to require that companies that provide encryption services must also provide a means whereby government security services can also decrypt the communications if required.

This requires that either (a) the company in question keeps a database of all the encryption/decryption keys used for all communications, or (b) the encryption algorithm used allows for decryption via a "back door" or "secret key" in addition to the standard decryption key, so that the government security services can gain access if required, without needing to know the customer's decryption key.

Keeping a database of the decryption keys just provides a direct target for attack by computer criminals. Once such a database is breached, none of the communications provided by that company can be considered secure. This is clearly not a good state of affairs, given the number of times that password databases get compromised.

That leaves option (b): providing a "back door" or "secret key", or other means whereby an otherwise-encrypted communication can be read by the security services. However, this fundamentally compromises that encryption.

Knowing that the back door exists, criminal computer crackers will work to ensure that they too can gain access to the communication, and they won't wait for a warrant from the Home Secretary or whatever government department is responsible for issuing such warrants! Any such group that does manage to obtain access would probably not make it public knowledge, they would merely use it to ensure that they could access communications that were relevant to them, whether that was because they had a direct use for the information, or because it could be sold to other criminal organisations.

If there is a single key that can decrypt all communication using a given system then that dramatically reduces the computation effort required to break the key: the larger the number of messages that are transmitted with a given key, the easier it is to identify the key, especially if you have access to the raw unencrypted message. The huge volume of electronic communications in use today would mean that the secret back door key would be much more readily compromised than any individual encryption key.

Privacy is a human right

The Universal Declaration of Human Rights was adopted by the UN in 1948. Article 12 states:

No one shall be subjected to arbitrary interference with his privacy, family, home or correspondence, nor to attacks upon his honour and reputation. Everyone has the right to the protection of the law against such interference or attacks.

Secure encrypted communication protects our correspondence from interference, including interference by the government.

Restricting the use of encryption is also a violation of the right to freedom of expression, guaranteed to us by article 19 of the Universal Declaration of Human Rights:

Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers.

The restriction on our freedom of expression is easy to see: if I have true freedom of expression then I can impart any series of letters or numbers to anyone without interference. If that series of letters or numbers happens to be an encrypted message then that is of no consequence. Any attempt to limit the use of particular encryption algorithms therefore limits my ability to send whatever message I like, since particular sequences of letters and numbers are outlawed purely because of their meaning.

Human rights organisations such as Amnesty International use secure encrypted communications to communicate with their workers. If those communications could not be secured against interference then this would have a detrimental impact on their ability to do their humanitarian work, and could endanger their workers.

Encryption is mathematics

Computer encryption is just a mathematical algorithm applied to a series of numbers. It is ridiculous to consider that performing mathematical operations on a sequence of numbers could be outlawed merely because that sequence of numbers has meaning to someone.

End note

I strongly object to any move to restrict the use of encryption technology. It is technologically and morally unsound, with little or no upside and considerable downsides.

I urge politicians to likewise oppose any moves to restrict the use of encryption technology, and I urge those standing in the elections in the UK this week to make it known to their potential constituents that they will oppose such measures.

Finally, I think we should be encouraging the use of strong encryption rather than discouraging it, to protect us from those who would intercept our digital communication and use that for their gain and our detriment.

Posted by Anthony Williams
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Numbers in Javascript

Wednesday, 25 March 2015

I've been playing around with Javascript (strictly, ECMAScript) in my spare time recently, and one thing that I've noticed is that numbers are handled slightly strangely. I'm sure that many experienced Javascript programmers will just nod sagely and say "everyone knows that", but I've been using Javascript for a while and not encountered this strangeness before as I've not done extensive numerical processing, so I figured it was worth writing down.

Numbers are floating point

For the most part, Javascript numbers are floating point numbers. In particular, they are standard IEEE 754 64-bit double-precision numbers. Even though the IEEE spec allows for multiple NaN (Not-a-number) values, Javascript has exactly one NaN value, which can be referenced in code as NaN.

This has immediate consequences: there are upper and lower limits to the stored value, and numbers can only have a certain precision.

For example, 10000000000000001 cannot be represented in Javascript. It is the same value as 10000000000000000.

var x=10000000000000000;

This itself isn't particularly strange: one of the first things you learn about Javascript is that it has floating-point numbers. However, it's something that you need to bear in mind when trying to do any calculations involving very big numbers (larger than 9007199254740992 in magnitude) or where more than 53 bits of precision is needed (since IEEE 754 numbers have binary exponents and mantissas).

You might think that you don't need the precision, but you quickly hit problems when using decimal fractions:

var x=0.2*0.3-0.01;

The rounding errors in the representations of the decimal fractions here mean that the value of x in this example is 0.049999999999999996, not 0.05 as you would hope.

Again, this isn't particularly strange, it's just an inherent property of the numbers being represented as floating point. However, what I found strange is that sometimes the numbers aren't treated as floating point.

Numbers aren't always floating point

Yes, that's right: Javascript numbers are sometimes not floating point numbers. Sometimes they are 32-bit signed integers, and very occasionally 32-bit unsigned integers.

The first place this happens is with the bitwise operators (&, |, ^): if you use one of these then both operands are first converted to a 32-bit signed integer. This can have surprising consequences.

Look at the following snippet of code:

var x=0x100000000; // 2^32

What do you expect it to do? Surely x|0 is x? You might be excused for thinking so, but no. Now, x is too large for a 32-bit integer, so x|0 forces it to be taken modulo 2^32 before converting to a signed integer. The low 32-bits are all zero, so now x|0 is just 0.

OK, what about this case:

var x=0x80000000; // 2^31

What do you expect now? We're under 2^32, so there's no dropping of higher order bits, so surely x|0 is x now? Again, no. x|0 in this case is -x, because x is first converted to a signed 32-bit integer with 2s complement representation, which means the most-significant bit is the sign bit, so the number is negative.

I have to confess, that even with the truncation to 32-bits, the use of signed integers for bitwise operations just seems odd. Doing bitwise operations on a signed number is a very unusual case, and is just asking for trouble, especially when the result is just a "number", so you can't rely on doing further operations and having them give you the result you would expect on a 32-bit integer value.

For example, you might want to mask off some bits from a value. With normal 2s complement integers, x-(x&mask) is the same as x&~mask: in both cases, you're left with the bits set in x that were not set in mask. With Javascript, this doesn't work if x has bit 31 set.

var x=0xabcdef12;
var mask=0xff;

If you truncate back to 32-bits with x|0 then the values are indeed the same, but it's easy to forget.

Shifting bits

In languages such as C and C++, x<<y is exactly the same as x*(1<<y) if x is an integer. Not so in Javascript. If you do a bitshift operation (<<, >>, or >>>) then Javascript again converts your value to a signed integer before and after the operation. This can have surprising results.

var x=0xaa;

x<<24 converts x to a signed 32-bit integer, bit-shifts the value as a signed 32-bit integer, and then converts that result back to a Number. In this case, x<<24 has the bit pattern 0xaa000000, which has the highest bit set when treated as 32-bit, so is now a negative number with value -1442840576. On the other hand, 1<<24 does not have the high bit set, so is still positive, so x*(1<<24) is a positive number, with the same value as 0xaa000000.

Of course, if the result of shifting would have more than 32 bits then the top bits are lost: 0xaa<<25 would be truncated to 0x54000000, so has the value 1409286144, rather than the 5704253440 that you get from 0xaa*(1<<25).

Going right

For right-shifts, there are two operators: >> and >>>. Why two? Because the operands are converted to signed numbers, and the two operators have different semantics for negative operands.

What is 0x80000000 shifted right one bit? That depends. As an unsigned number, right shift is just a divide-by-two operation, so the answer is 0x40000000, and that's what you get with the >>> operator. The >>> operator shifts in zeroes. On the other hand, if you think of this as a negative number (since it has bit 31 set), then you might want the answer to stay negative. This is what the >> operator does: it shifts in a 1 into the new bit 31, so negative numbers remain negative.

As ever, this can have odd consequences if the initial number is larger than 32 bits.

var x=0x280000000;

0x280000000 is a large positive number, but it's greater than 32-bits long, so is first truncated to 32-bits, and converted to a signed number. 0x280000000>>1 is thus not 0x140000000 as you might naively expect, but -1073741824, since the high bits are dropped, giving 0x80000000, which is a negative number, and >> preserves the sign bit, so we have 0xc0000000, which is -1073741824.

Using >>> just does the truncation, so it essentially treats the operand as an unsigned 32-bit number. 0x280000000>>>1 is thus 0x40000000.

If right shifts are so odd, why not just use division?

Divide and conquer?

If you need to preserve all the bits, then you might think that doing a division instead of a shift is the answer: after all, right shifting is simply dividing by 2^n. The problem here is that Javascript doesn't have integer division. 3/2 is 1.5, not 1. You're therefore looking at two floating-point operations instead of one integer operation, as you have to discard the fractional part either by removing the remainder beforehand, or by truncating it afterwards.

var x=3;


For the most part, Javascript numbers are double-precision floating point, so need to be treated the same as you would floating point numbers in any other language.

However, Javascript also provides bitwise and shift operations, which first convert the operands to 32-bit signed 2s-complement values. This can have surprising consequences when either the input or result has a magnitude of more than 2^31.

This strikes me as a really strange choice for the language designers to make: doing bitwise operations on signed values is a really niche feature, whereas many people will want to do bitwise operations on unsigned values.

As browser Javascript processors get faster, and with the rise of things like Node.js for running Javascript outside a browser, Javascript is getting used for far more than just simple web-page effects. If you're planning on using it for anything involving numerical work or bitwise operations, then you need to be aware of this behaviour.

Posted by Anthony Williams
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Firefox is losing market share to Chrome

Monday, 09 March 2015

I read with interest an article on Computerworld about Firefox losing market share, wondering what people were using instead. Unsurprisingly, the answer seems to be Chrome: apparently Chrome now has a 27.6% share compared to Firefox's 11.8%. That's quite a big difference.

I checkout out the stats for this site for February 2015, and the figures bear it out: 30.7% of visitors use Chrome vs 14.9% Firefox and 12.8% Safari. Amusingly, 3.1% of visitors still use IE6!

What I did find interesting is the version numbers people are using: there were visitors using every version of Chrome from version 2 to version 43, and the same for Firefox — someone was even using Firefox 0.10! I'm a bit surprised by this, as I'd have thought that users of these browsers were probably amongst the most likely to upgrade.

Why the drop?

The big question of course is why the shift? I switched to Firefox because Internet Explorer was poor, and I've stuck with it, mainly through inertia, but I've used other browsers over the years, and still prefer Firefox. I've got Chrome installed on my desktop, but I don't particularly like it, and only really use it for cross-browser testing. I only really use it on my tablets, where it is the only browser I have installed — I tried Firefox for Android and was really disappointed.

Maybe that's the cause of the shift: everyone is using mobile devices for browsing, and Chrome/Safari are better than the others for mobile.

Which browser(s) do you use, and why?

Posted by Anthony Williams
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just::thread C++11 and C++14 Thread Library V2.1 released

Tuesday, 03 March 2015

I am pleased to announce that version 2.1 of just::thread, our C++11 and C++14 Thread Library has just been released with support for new compilers.

This release adds the long-awaited support for gcc 4.8 on MacOSX, as well as bringing linux support right up to date with support for gcc 4.9 on Ubuntu and Fedora.

Just::Thread is now supported for the following compilers:

  • Microsoft Windows XP and later:
    • Microsoft Visual Studio 2005, 2008, 2010, 2012 and 2013
    • TDM gcc 4.5.2, 4.6.1 and 4.8.1
  • Debian and Ubuntu linux (Ubuntu Jaunty and later)
    • g++ 4.3, 4.4, 4.5, 4.6, 4.7, 4.8 and 4.9
  • Fedora linux
    • Fedora 13: g++ 4.4
    • Fedora 14: g++ 4.5
    • Fedora 15: g++ 4.6
    • Fedora 16: g++ 4.6
    • Fedora 17: g++ 4.7.2 or later
    • Fedora 18: g++ 4.7.2 or later
    • Fedora 19: g++ 4.8
    • Fedora 20: g++ 4.8
    • Fedora 21: g++ 4.9
  • Intel x86 MacOSX Snow Leopard or later
    • MacPorts g++ 4.3, 4.4, 4.5, 4.6, 4.7 and 4.8

Get your copy of Just::Thread

Purchase your copy and get started with the C++11 and C++14 thread library now.

Posted by Anthony Williams
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Using Enum Classes as Bitfields

Thursday, 29 January 2015

C++11 introduced a new feature in the form of scoped enumerations, also referred to as enum classes, since they are introduced with the double keyword enum class (though enum struct is also permissible, to identical effect). To a large extent, these are like standard enumerated types: you can declare a list of enumerators, which you may assign explicit values to, or which you may let the compiler assign values to. You can then assign these values to variables of that type. However, they have additional properties which make them ideal for use as bitfields. I recently answered a question on the accu-general mailing list about such a use, so I thought it might be worth writing a blog post about it.

Key features of scoped enumerations

The key features provided by scoped enumerations are:

  • The enumerators must always be prefixed with the type name when referred to outside the scope of the enumeration definition. e.g. for a scoped enumeration colour which has an enumerator green, this must be referred to as colour::green in the rest of the code. This avoids the problem of name clashes which can be common with plain enumerations.
  • The underlying type of the enumeration can be specified, to allow forward declaration, and avoid surprising consequences of the compiler's choice. This is also allowed for plain enum in C++11. If no underlying type is specified for a scoped enumeration, the underlying type is fixed as int. The underlying type of a given enumeration can be found using the std::underlying_type template from the <type_traits> header.
  • There is no implicit conversion to and from the underlying type, though such a conversion can be done explicitly with a cast.

This means that they are ideal for cases where there is a limited set of values, and there are several such cases in the C++ Standard itself: std::errc, std::pointer_safety, and std::launch for example. The lack of implicit conversions are particularly useful here, as it means that you cannot pass raw integers such as 3 to a function expecting a scoped enumeration: you have to pass a value of the enumeration, though this is of course true for unscoped enumerations as well. The lack of implicit conversions to integers does mean that you can overload a function taking a numeric type without having to worry about any potential ambiguity due to numeric conversion orderings.

Bitmask types

Whereas the implicit conversions of plain enumerations mean that expressions such as red | green and red & green are valid if red and green are enumerators, the downside is that red * green or red / green are equally valid, if nonsensical. With scoped enumerations, none of these expressions are valid unless the relevant operators are defined, which means you can explicitly define what you want to permit.

std::launch is a scoped enumeration that is also a bitmask type. This means that expressions such as std::launch::async | std::launch::deferred and std::launch::any & std::launch::async are valid, but you cannot multiply or divide launch policies. The requirements on such a type are defined in section [bitmask.types] of the C++ Standard, but they amount to providing definitions for the operators |, &, ^, ~, |=, &= and ^= with the expected semantics.

The implementation of these operators is trivial, so it is easy to create your own bitmask types, but having to actually define the operators for each bitmask type is undesirable.

Bitmask operator templates

These operators can be templates, so you could define a template for each operator, e.g.

    template<typename E>
    E operator|(E lhs,E rhs){
        typedef typename std::underlying_type<E>::type underlying;
        return static_cast<E>(
            static_cast<underlying>(lhs) | static_cast<underlying>(rhs));

Then you could write mask::x | mask::y for some enumeration mask with enumerators x and y. The downside here is that it is too greedy: every type will match this template. Not only would you would be able to write std::errc::bad_message | std::errc::broken_pipe, which is clearly nonsensical, but you would also be able to write "some string" | "some other string", though this would give a compile error on the use of std::underlying_type, since it is only defined for enumerations. There would also be potential clashes with other overloads of operator|, such as the one for std::launch.

What is needed is a constrained template, so only those types which you want to support the operator will match.

SFINAE to the rescue

SFINAE is a term coined by David Vandevoorde and Nicolai Josuttis in their book C++ Templates: The Complete Guide. It stands for "Substitution Failure is Not an Error", and highlights a feature of expanding function templates during overload resolution: if substituting the template parameters into the function declaration fails to produce a valid declaration then the template is removed from the overload set without causing a compilation error.

This is a key feature used to constrain templates, both within the C++ Standard Library, and in many other libraries and application code. It is such a key feature that the C++ Standard Library even provides a library facility to assist with its use: std::enable_if.

We can therefore use it to constain our template to just those scoped enumerations that we want to act as bitmasks.

    template<typename E>
    struct enable_bitmask_operators{
        static constexpr bool enable=false;

    template<typename E>
    typename std::enable_if<enable_bitmask_operators<E>::enable,E>::type
    operator|(E lhs,E rhs){
        typedef typename std::underlying_type<E>::type underlying;
        return static_cast<E>(
            static_cast<underlying>(lhs) | static_cast<underlying>(rhs));

If enable_bitmask_operators<E>::enable is false (which it is unless specialized) then std::enable_if<enable_bitmask_operators<E>::enable,E>::type will not exist, and so this operator| will be discarded without error. It will thus not compete with other overloads of operator|, and the compilation will fail if and only if there are no other matching overloads. std::errc::bad_message | std::errc::broken_pipe will thus fail to compile, whilst std::launch::async | std::launch::deferred will continue to work.

For those types that we do want to work as bitmasks, we can then just specialize enable_bitmask_opoerators:

    enum class my_bitmask{
    struct enable_bitmask_operators<my_bitmask>{
        static constexpr bool enable=true;

Now, std::enable_if<enable_bitmask_operators<E>::enable,E>::type will exist when E is my_bitmask, so this operator| will be considered by overload resolution, and my_bitmask::first | my_bitmask::second will now compile.

Final code

The final code is available as a header file along with a simple example demonstrating its use. It has been tested with g++ 4.7, 4.8 and 4.9 in C++11 mode, and with MSVC 2012 and 2013, and is released under the Boost Software License.

Posted by Anthony Williams
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Gotchas Upgrading Apache from 2.2 to 2.4

Wednesday, 10 December 2014

I finally got round to upgrading one of my servers from Ubuntu 12.04 (the previous LTS release) to Ubuntu 14.04 (the latest LTS release). One of the consequences of this is that Apache gets upgraded from 2.2 to 2.4. Sadly, the upgrade wasn't as smooth as I'd hoped, so I'm posting this here in case anyone else has the same problem that I did.

Oh no! It's all broken!

It's nice to upgrade for various reasons — not least of which being the support benefits of being on the most recent LTS release — except after the upgrade, several of the websites hosted on the server stopped working. Rather than getting the usual web page, they just returned an "access denied" error page, and in the case of the https pages they just returned an SSL error. This was not good, and led to much frantic checking of config files.

After verifying that all the config files for all the sites were indeed correct, I finally resorted to googling the problem. It turns out that the default apache2.conf file was being used, as all the "important" config was in the module config files, or the site config files, so the upgrade had just replaced it with the new one.

Whereas the old v2.2 default config file has the line

Include sites-enabled/

The new v2.4 default config file has the line

IncludeOptional sites-enabled/*.conf

A Simple Fix

This caused problems with my server because many of the config files were named after the website (e.g. example.com) and did not have a .conf suffix. Renaming the files to e.g example.com.conf fixed the problem, as would have changing the line in apache2.conf so it didn't force the suffix.

Access Control

The other major change is to the access control directives. Old Allow and Deny directives are replaced with new Require directives. The access_compat module is intended to allow the old directives to work as before, but it's probably worth checking if you use any in your website configurations.

Exit Stage Left

Thankfully, all this was happening on the staging server, so the websites weren't down while I investigated. Testing is important — what was supposed to be just a simple upgrade turned out not to be, and without a staging server the websites would have been down for the duration.

Posted by Anthony Williams
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