I finally started using ELPA

The Lone C++ Coder's Blog from The Lone C++ Coder's Blog

My normal development workflow doesn’t use that many different Emacs packages. With a few exceptions I’ve mainly worked with a “stock” Emacs distribution and augmented that with a few select Emacs packages that I downloaded manually. It worked for me for a decade or so, and it made it reasonable easy to move configurations between machines - zip & copy was my friend for that, although I’ve since changed that to using dropbox.

Optional streaming

Phil Nash from level of indirection

Catch has a number of macros that allow values of arbitrary types to be streamed into an ostringstream. The canonical example is the INFO macro:

INFO( "There were " << bottles.size() << " green bottles, hanging on the wall" );

This macro builds up a string that will be passed to the next assertion to be included as an annotation. Note that, unlike with a naked ostringstream there is no leading <<. This makes it clean and uncluttered when you just want to log a single value (such as a string), for example:

INFO( "Weirdness" );
The obvious way to do this is for the macro to provide the leading << prior to its argument. Conceptually something like this:
#define INFO( log ) { \
	std::ostringstream oss; \
	oss << log; \
	useTheString( oss.str() ); 
}

This all works quite nicely. But there are a few other macros that use this idiom, too: WARN, SUCCEED and FAIL.

The last two are of interest because the logging behaviour is more of a secondary concern. The primary behaviour is to appear like a passing or failing assertion, respectively, without the need to actually assert on anything. SUCCEED can be useful if you otherwise have no assertions in a test and you don't want to see warnings about it. FAIL is useful if the situation that leads to the failure is not captured in an expression for some reason. It can also be useful to force a test to fail, perhaps as a placeholder. These are useful macros to have available, but they are not often needed in practice. So when they are it's nice to be able to annotate their useage inline - hence the streamed argument.

This is all well and good. But I've found there are still enough cases where I don't want to annotate that having to pass an empty string or make something up is a little annoying. I also use a similar idiom in other projects where it would be nice to be able to make the stream completely optional.

This is not as easy as it sounds, though. The first, and most obvious, issue is that this requires support for variadic macros. Catch has made use of variadic macros, where available, for some time now. In theory they are available to any C++11 compiler. In practice most, if not all, compilers that support any reasonable chunk of C++11 support variadic macros - and most supported them as an extension even before that. That's certainly true of Visual C++, GCC and Clang.

The technically more interesting problem, though, is dealing with that initial <<. Remember the first << is being supplied inside the macro. It will still be there even if the caller does not supply an argument to the macro. If we wrote FAIL the same way we presented INFO earlier (but with variadic macros) it might look something like this:

#define FAIL( ... ) { \
	std::ostringstream oss; \
	oss << __VA_ARGS__; \
	notifyFail( oss.str() ); \
}
... which, with no argument provided, would expand to...
{ 
	std::ostringstream oss; 
	oss << ; 
	notifyFail( oss.str() ); 
}

Do you see the problem? With nothing following the << this will not compile.

So do we need a different operator? What properties would we need? It seems we'd need an operator that comes in two forms: a binary operator that allows us to capture an argument, and a unary operator that allows us to omit the argument. Furthermore the binary form must not require its argument to be enclosed in any sort of brackets. Finally it must have higher precedence than << so we can switch over to normal stream insertion at that point.

That's a long list. Does such an operator exist? Fortunately there's not just one but two such operators to choose from! + and -. The only slight hitch is that the unary form is right-to-left associative, whereas the binary form is left-to-right. So how can we work these in?

Let's pick one of the operators. I've gone with +, but I don't think there is any advantage either way. Because unary + is right-to-left associative it needs to prefix something. So we can't use it at the start of our streaming expression. We can, however, use it at the end. Then we'll need an object to apply it to. The object doesn't actually need to do anything else. I've gone with this implementation of StreamEndStop in Catch:

struct StreamEndStop {
    std::string operator+() {
        return std::string();
    }
};
With this definition the expression, +StreamEndStop() now yields an empty string - which is idempotent with a stringstream. Which means we can write:
{
	std::ostringstream oss; 
	oss << +StreamEndStop();
	notifyFail( oss.str() ); 
}
And oss.str() evaluates to an empty string. Perfect. But what about when we do stream something? Well that would expand to:
{
	std::ostringstream oss; 
	oss << something +StreamEndStop();
	notifyFail( oss.str() ); 
}
... where something could be a string or variable or literal of any type. So we need some way for the expression:
something +StreamEndStop()
to yield the value of something. That's where the binary form of operator+ comes in:
template<typename T>
T const& operator + ( T const& value, StreamEndStop& ) {
	return value;
}
Now, whether we supply nothing, a single value or multiple values joined by <<s we'll end up with a stringstream containing what we expect. The relevant bit of code in Catch actually looks like this:
Catch::ExpressionResultBuilder( messageType ) \
	<< __VA_ARGS__ \
	+::Catch::StreamEndStop()
which yields an ExpressionResultBuilder that gets passed on elsewhere. This is all protected by CATCH_CONFIG_VARIADIC_MACROS. Otherwise it falls back to:
Catch::ExpressionResultBuilder( messageType ) << log
So a lot of work to save a few explicit empty strings, but sometimes it's the little things.

Exit code crimes

Frances Buontempo from BuontempoConsulting

I've seen a few exit code crimes recently, so I thought I'd list them.

1. Catch something specific

if __name__ == "__main__":
    try:
        run(None)
        sys.exit(0)
    except ValueError:
        sys.exit(1)

So, what does this do if another type of error is thrown? Why is it catching a specific type of error? The rest of the code is logging info - why don't we log the error? How does this make troubleshooting easy, or even possible?

2. Throw something specific

The pattern above is of course fine when the run function works like this:

def run(config):
    try:
    catch:
        raise ValueError

3. Tell no-one

Make the last line in you script

exit(0)

regardless of whatever just happened. There was a chance the script could return the exit code of whatever it called, so typing the extra line is first, effort, second achieves nothing, and finally means no-one will notice if something went wrong. Or at least not at the time. Perhaps whoever did that thought a shell script won't exit without the keyword exit at the end.



There are others - I'll add them as I find them.




Setting up the Ghost blogging system on FreeBSD

The Lone C++ Coder's Blog from The Lone C++ Coder&#039;s Blog

Ah, a meta blogging post. Sorry, I try to keep these to a minimum… For those who haven’t been caught up in the hype yet, Ghost is a new blogging system that is much more minimal than WordPress and the other more popular systems. It’s designed to be much smaller and faster (plus it uses a lot of cool tools like node.js, handlebars etc). I recently tried to set up the 0.

With Emacs on Windows, make sure you know where your $HOME is

The Lone C++ Coder's Blog from The Lone C++ Coder&#039;s Blog

The Gnu Emacs for Windows distribution appears to be pretty good at inferring where a reasonable place for $HOME is, straight out of the box. In my case, said reasonable place was %USERPROFILE%/AppData/Roaming which was an entirely acceptable default. That is, until several other tools entered the picture and disagreed with Emacs. We’ve recently switched to using git at work and the git ecosystem├é┬áneeded to have some ideas where its home was.

I know, never install an OS the day it was released

The Lone C++ Coder's Blog from The Lone C++ Coder&#039;s Blog

Throwing caution into the wind this morning, I’m having an updatefest only a few hours after the software was released: While I’ve grown accustomed to using Win8 - the Surface RT helped - from using the 8.1 prerelease it really looked like 8.1 would be an improvement, so I bit the bullet and installed the upgrades as soon as they were available. Well, as soon as I got up the day they were available, that is.

Writing: The Ethical Programmer

Pete Goodliffe from Pete Goodliffe

The latest C Vu magazine from ACCU is out now. It contains my latest Becoming a Better Programer column. This month it's called The Ethical Programmer; the first instalment of a two-part series on ethics and the modern programmer. Gripping stuff.

This month, I look at our attitudes, at legal issues, and discuss software licenses.

To make the world a better place, you can enjoy a picture of some old rope, and a chicken. I also throw in some bad puns.

Visual Studio 2012 C++ Variadic Template support in the runtime library

The Lone C++ Coder's Blog from The Lone C++ Coder&#039;s Blog

As VS2012’s C++ compiler doesn’t support “true” variadic templates, the new runtime library classes that use variadic templates are implemented using macro magic behind the scenes. In order to get the “variadic” templates to accept more than the default of five parameters, you’ll have to set _VARIADIC_MAX to the desired maximum number of parameters (between five and ten). For more information, see the “faux variadics” section of this blog post on MSDN.