A synchronous observer of asynchronous events

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In the Observer design pattern, a subject holds a list of interested parties – the observers – which it will notify about changes in status. Simply put, it’s a form of subscription, and this design comes up in all sorts of places (which is one of the definitions of the term ‘design pattern‘). It’s well suited for handling asynchronous events, like user interaction in a GUI, sensor information, and so on.

There is, however, often a need to re-synchronise asynchronous events. For instance, you might keep the latest status update until it’s actually needed for display, storage or some calculation. By doing this, you disregard the asynchronous nature of its source, and treat it as just another variable, as if it had been read from the subject right then. In other words, you synchronise a status from the past with the present. Sometimes, though, you don’t want the last value, but the next, which is a bit more complex, as it requires you to wait for the future to happen before we can say it’s the present.

In this article, we will write a simple multi-threaded example implementation of the Observer pattern, and show how to re-synchronise a past event to look current. Then we’ll demonstrate a technique to treat future events like they’re current, too.


Splitting strings again – strtok redeemed

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The C++ source files for the string tokenisers discussed in this post and the Splitting strings post, plus the code for Removing whitespace and Static assert in C++, can be found here:

One of the more curious omissions from the C++ standard library is a string splitter, e.g. a function that can take a string and split it up into its constituent parts, or tokens, based on some delimiter. There is one in other popular languages ((C# – String.Split, Java – String.split, Python – string.split etc), but C++ programmers are left to roll their own, or use one from a third-party library like the boost::tokenizer (or the one I presented in Splitting strings).

There are many ways of going this; the Stack Overflow question How do I tokenize a string in C++? has 23 answers at the time of writing, and those contain 20 different solutions (boost::tokenizer and strtok are suggested multiple times).

The strtok recommendations, however, all have comments pointing out the problems with this function – it’s destructive, and not reentrant (it can’t be nested or run in parallell on multiple strings). As functions go, strtok has a rather poor reputation – there’s even a popular reentrant version, strtok_r, available in many C library implementations, though it’s not a standard function.

All your base64 are different to us

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The C++ source files for the stand-alone base64 encoder and decoder discussed in this post, plus a separate implementation of quoted-printable (RFC 2045, section 6.7), and the hex string converter I presented last year, can be found here:

There is a quote that goes “Standards are great! Everyone should have one.” or something along those lines. (Somewhat ironically, this quote, too, has many different variations, and has many attributions. The earliest I’ve found attributes it to George Morrow in InfoWorld 21 Oct 1985).

A case in point is the base64 encoding. Put simply, it’s a method of encoding an array of 8-bit bytes using an alphabet consisting of 64 different printable characters from the ASCII character set. This is done by taking three 8-bit bytes of source data, arranging them into a 24-bit word, and converting that into four 6-bit characters that maps onto the 64-character alphabet (since 6 bits is 0-63).

The original implementation was for privacy-enhanced e-mail (RFC 1421), then altered slightly for MIME (RFC 2045), and again in its own standard (RFC 4648).

When I was looking at base64, I was interested in three different varieties or flavours, namely the MIME version, the (per RFC 4648) standard base64, and base64url. These differ in how they handle line breaks and other illegal characters, what characters are used in the 64-character alphabet, and the use of padding at the end to make up an even triplet of bytes.

Removing whitespace

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Here’s a std::string, please remove all whitespace from it. How would you do it? Despite its seeming simplicity, it’s an interesting question, because it can be done in so many ways.

To start with, how do you identify whitespace? Let’s have a look at some different approaches (all of which I’ve seen in the wild):

// Simple
bool iswhitespace1(char c)
  // Is it  space   or    tab      or    return   or    newline?
  return (c == ' ') || (c == '\t') || (c == '\r') || (c == '\n');
// Cute attempt at cleverness
bool iswhitespace2(char c)
  // Is it one of the whitespace characters?
  static const std::string spaces(" \t\r\n");
  return (std::string::npos != spaces.find(c));
// Probably ok, for English at least
bool iswhitespace3(char c)
  // Using C function, from <cctype>
  return ::isspace(c);
// As above, but standard C++ instead of standard C
bool iswhitespace4(char c)
  // Using current locale, and std function from <locale>
  static const std::locale loc;
  return std::isspace(c, loc);

If we were to run through these four functions with values of c from 0 to 255, the first two would produce the same result, and the latter two would (probably) produce the same result, but those wouldn’t be the same as for the first two.

Splitting strings

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Back in the dawn of time, when men were real men, bytes were real bytes, and floating point numbers were real, um, reals, the journeyman test of every aspiring programmer was to write their own text editor. (This was way before the concept of “life” had been invented, so no-one knew they were supposed to have one.)

Nowadays, we know better, and don’t write new code to solve problems that have already been solved. Well, unless we need an XML parser – everybody (including myself, but that’s a post for another time) has written one of those – or at least a string tokeniser (aka splitter).

Other languages get tokenisers for free (C# – String.Split, Java – String.split, Python – string.split, and so on, and even C has strtok), but not C++. Which is why it’s something almost every C++ programmer writes, at some point or other.

Of course, you can use the rather nifty boost::tokenizer, if the place where you work is okay with using Boost (a surprising number of places aren’t, for various reasons), or find one of the numerous example implementations out there. Like this one, for instance:

Redux: Hex strings to raw data and back

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During the writing of my last post, I did the due dilligence thing and considered alternative implementations and algorithms to solve the problem at hand (converting a string representation of an 8-bit hexadecimal value to an unsigned 8-bit integer value). Because I was, in effect, documenting code written some years ago, I can’t recall exactly what other options, if any, I tried at the time.

I think I first tried using a std::stringstream, but gave up on that as being too slow, and went with strtoul instead. I might also have played around with using a std::map lookup table, with all the headaches that brought in terms of storage and initialisation, and decided against it.

What I didn’t try was a straight, non-clever switch-based lookup table to find the integer value of a hexadecimal character digit:

inline unsigned char hex_digit_to_nybble(char ch)
  switch (ch)
    case '0': return 0x0;
    case '1': return 0x1;
    case '2': return 0x2;
    case 'f': return 0xf;
    case 'F': return 0xf;
    default: throw std::invalid_argument();

Hex strings to raw data and back

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Here’s a problem that tends to crop up in a lot of communication domains: how do you transfer binary data in a protocol which limits what characters are permitted? The answer is to encode it into permissible characters (for historical reasons often 7-bit printable ASCII), and because there are few things this wonderful industry likes more than re-inventing the wheel, there’s a plethora of binary-to-text encoding schemes around. Each has its own trade-offs in terms of speed and space efficiency, and almost every one has a more or less glorious history of being the favoured scheme on some platform, or in some protocol or application.

The simplest encoding is (in my opinion) the “hexadecimal text” encoding. It’s so simple, it doesn’t even have a fancy or clever name. You simply take each byte and type its value as a hexadecimal number. Working on the assumption that a byte is 8 bits, its value can be expressed in two characters – 0x00-0xff. Assuming that a character occupies one byte, we see that the size of the data will double by writing it as hexadeximal text, so it’s not very efficient space-wise. But it is simple to understand and implement, and quite useful, so I wrote a pair of encoding/decoding functions.

Home on the range

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Continuing on the train of thought started in bounds class I presented a few days ago in Bounds, and staying within them.

As so often happens, just having bounds available made me think of what variants of it could be useful. For instance, it would be handy to have it work for floating point or non-POD types, which isn’t possible as it is written. Since the bounds class uses ‘non-type template parameters‘ for its limits, only integer types and enums are accepted.[1]

Even disregarding this restriction, I found that I had use for a dynamic range class, as opposed to the static bounds which has its boundaries set at compile time. Just a simple one, and like std::pair only having two values, but with both of the same type, and with them guaranteed to be ordered.

The last part there would make it a bit more complex than the simple std::pair struct, as I’d need to validate the values given in order to ensure that the minimum was lower than or equal to the maximum, but still, a simple enough little class.

Static assert in C++

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Just a quick little note today, to clarify something I mentioned in passing the other day in Bounds, and staying within them. I said I “I added a static assert to validate the template parameters at compile time”, and it’s probably worthwhile to spell out how that works for those who haven’t seen it before.

As a rule, the earlier you find an error, the easier it is to identify and fix. The errors spotted by your compiler are, naturally, easier to fix than the errors exhibited by your program as it is running. A static assert helps by letting you sanity-check the code you write, and generating a compiler error if you write code that is syntactically correct, but logically incorrect.

For instance, the bounds class takes a lower and upper boundary as template parameters, and assumes them to be ordered. Say we didn’t have a static assert, and used it to find out if a randomly generated world in a space game is suitable for colonisation, and what animals can be introduced.

typedef bounds<int, -5, -30> polarbear_temp;
// generating a randomworld and it's temperature
// Adding various animals
if (polarbear_temp::in_bounds(randomworld_temp))

This would lead to a universe completely devoid of polar bears (which I’m sure we all can agree would be a bad thing), because there is no temperature that can be both greater than -5 and less than -30.

Because the limits are known at compile time, it makes sense to check them at compile time, too.

Bounds, and staying within them

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How often have you written a line of code that looks something like this?

if (3 <= var && 14 >= var)

There might (read “should”) be named constant variables instead of the magic numbers there, but in essence it’s a very common piece of code for a very common type of test – is this value within pre-defined, constant bounds?

Some years ago, I was working on a project that had lots of tests like that, and I came across a surprisingly large number of errors one can commit with this simple code. For instance:

// non-paired constants
if (minTempUK <= var && maxTempUS >= var)

// wrong comparison 
if (minTempUK < var && maxTempUK >= var)

// test wrong way around  
if (maxTempUK <= var && minTempUK >= var)

// maxTempUK is compared to bool  
if (minTempUK <= var <= maxTempUK)

// bitwise rather than logical AND    
if (minTempUK <= var & maxTempUK >= var)  

All of these are legal C++, and only the last two or three might generate compiler warnings. The last would still work properly, but is a bit iffy. If it isn’t a typo, someone needs to read up on operators. In most cases, these errors were typos (except the fourth, which was written by someone more used to other languages), but since they compiled, and sort of worked, they only showed up as bugs every now and then, at the edge cases. And because the code looks sort of okay, it was hard to spot the typos right away.