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.

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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.

Redux: RAII adapter for Xerces

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In my previous entry, C++ RAII adapter for Xerces, I presented a simple memory management wrapper for Xerces types. Because of the way Xerces manages memory, I said, the quite handy boost::shared_ptr couldn’t be used, so I wrote the memory management code myself to produce a safe wrapper in the style of std::auto_ptr.

However, as Alf P. Steinbach pointed out, I was wrong, in that boost::shared_ptr could be used by taking advantage of the custom deleter facility offered by that class. One benefit is that I can get rid of my hand-rolled memory management, but on the other hand, I’ll have to adjust the public interface to reflect different semantics.

C++ RAII adapter for Xerces

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Xerces is a powerful validating XML parser that supports both DOM and SAX. It’s written in a simple subset of C++, and designed to be portable across the greatest possible number of platforms. For a number of reasons, the strings used in Xerces are zero-terminated 16-bit integer arrays, and data tends to be passed around by pointers. The responsibility for managing the lifetime of the DOM data passed around is usually Xerces’, but not always. Some types must always be released explicitly, while for others, this is optional.

In other words, this is a job for the RAII idiom. Alas, we can’t reach for our boost::shared_ptr[1] or std::auto_ptr, since Xerces has its own memory manager, and when Xerces creates an object for you, it is not guaranteed to be safe to simply call delete. Instead, you must call the object’s release() function.