Sunday, January 18, 2015

On C++2011, Quality of Implementation & Continuous Integration

Over the past years, I've done a few projects outside the main PowerDNS tree, and for all of them I've used C++2011. A wonderful mark of how big of an improvement C++2011 is, is how much pain you feel when you return to programming in 'regular' C++.

Recently, I've started to note that the new powers of C++ can either translate into better productivity (ie 'more functionality added/hour of work') or perhaps more importantly, in higher quality of implementation ('QoI').

And this drove me to ponder the concept of QoI a bit, as I think it is underrated compared to writing fast (by some measure) and bug free code.

I recently had to pick a reasonable value as an estimate while writing C++03 code, and I found that my fingers considered it too much work to actually scan three vectors of objects to make a decent estimate. As a result, the code ended up with a hardcoded number which (for now) is reasonable.

This is not quality of implementation. For example, a low QoI implementation of a generally useful memory allocator functions well for the amount of memory the author used it for - say, 1 gigabyte. Unbeknownst to you, lots of the inner workings are overkill when on an embedded platform, for example an O(N) algorithm that is actually pretty slow for small N. Meanwhile, other parts of the library might scale badly to 2GB of memory arena.

A high QoI implementation of a generic memory allocator would find ways to scale itself to multiple domains of scale. It would not surprise you over the years as your project evolves. It would adapt.

We often hear (correctly) 'make it work, make it right, make it fast'. QoI is the part of making it right. It might in turn also make your code fast!

In my example, C++2011 would've allowed me to scan three different vectors like this:

for(const auto& vec : {vecA, vecB, vecC}) {
  for(const auto& entry : vec)
     totSize += entry.length();

Whereas the equivalent in C++03 is something like:
unsigned int countVec(const vector& vec)
    int ret=0;
    for(vector::const_iterator iter = vec.begin(); iter!=vec.end(); ++iter)
       ret += iter->length();
    return ret;

... lots of code in between ... 
totSize = countVec(vecA);
totSize += countVec(vecB);
totSize += countVec(vecC);

You can see how the second variant might not happen ("100kb of entries is a good guess!").

If for this reason alone, I expect my C++2011 code to not only be more pleasing to read, but also to deliver higher quality of implementation.

It is therefore that it pains me to report that in 2015, I can't find a Continuous Integration provider taking C++2011 seriously (for free or for money).

Travis-CI which I otherwise love dearly uses an antiquated version of g++ that doesn't do C++2011 at all. If you modify the platform into installing g++-4.8, you find that the supplied version of Boost predates C++2011 and fails to compile. The deployed version of clang fares better, but can't do threads, and bails out the moment you #include anything thread related.

Meanwhile, Circle CI does ship with a slightly more recent gcc (but not recent enough), but for some reason uses a version of Ubuntu that can't install 'libboost-all-dev', or even 'libboost-serialization-dev'.

I spent some hours on it this morning, and I'm sure there are solutions that don't involved compiling Boost for every commit, but I haven't found them yet.

So pretty please, with sugar on top, could the CI platforms up their game a bit? If the goal of CI is to quickly find bugs and issues, they should surely feel motivated to support a language that offers ways to do this.