Bogus Error Messages with Qt .ui Files

July 11, 2011

Bogus errors together with their messages can have a large number of reasons – full hard drives being one of the classics. When it comes to programming and especially C++, the possibilities for cryptic, meaningless and misleading error message are infinite.

A nice one bit us at one of our customers the other day. The message was something like

QLayout can only have instances of QWidget as parent

and it appeared as standard error output during program start-up. Needless to say that the whole thing crashed with a segmentation fault after that. The only change that was made was a header file that was added to the Qt files list in the CMakeLists.txt file.  The Qt class in this header file was just in its beginnings and had not yet any QLayouts, or QWidgets in it. Even the  C++ standard measure of cleaning and recompiling everything didn’t help.

So how is it possible that an additional Qt header file that has not references to QLayout and QWidget can cause such an error message?

As all of you experienced C/C++ developers know, for the compiler, a code file is not only the stuff that it contains directly but also what is #included! The offending header file included a generated ui description file which you get when you design your windows – or Forms in Qt terminology – with the Qt designer and use the Compile-Time-Form-Processing-approach to incorporate the form into the code base.

But how can that effect anything?

The Qt designer saves the forms into .ui files. From that, the so-called User Interface Compiler (uic) generates a header file containing a C++ class together with inlined code that creates the form. Form components like line edits, or push buttons are generated as instance attributes. The name of the class is generated from the name of the form. You can even use namespaces.  By naming it e.g. myproject::BestFormEverDesigned the generated class is named BestFormEverDesigned   is put into namespace myproject.

So far, so nice, handy and easy to use.

When you create a new form in qt designer, the default name is Form. Maybe you can guess already where this leads to…

Two forms for which the respective developers forgot to set a proper name, existed in the same sub project and had been compiled and linked into the same shared library. The compiler has no chance to detect this, because it sees only one

class Form
{

at a time. The linker happily links all of this together since it thinks that all Forms are created equal. And then at run-time … Boom!

I will have to look into a little Jenkins helper which breaks the build when a Form form is checked in…


How to accidentally kill your CI build time

June 6, 2011

At one of our customers I do C++ consulting in a mid-sized project which uses cmake as build system. A clean build on our Jenkins CI server takes about 40 minutes (including unit tests) which is way too long to be considered “fast feedback” in an agile kind of way.

Because of that, we do clean builds only 2 times a day – some time during the night and during lunch break. The rest of the day the CI server only does a “svn update” and a normal “make”, which takes about 3-10 minutes depending on what files have been changed.

With C++ there are lots of ways to unnecessarily lengthen your build time. The most important factor is, of course, #include dependencies. One has to be very (very) disciplined in adding #include directives in header files. Otherwise, the whole world suddenly gets rebuild when some small header file somewhere in a little corner of the code has been changed.

And I have to say, for the most part, this project is in pretty good shape with regard to #include dependencies.

So what the hell has suddenly increased our build time from 3-10 minutes to 20-25 minutes? was what I was thinking some time last week while waiting for the CI server to spit out new latest and greatest rpm packages. For some reason, our normal, rest-of-the-day build started to compile what felt like everything in our main package even on the slightest code change in a remote .cpp file.

What happened?

In order to have the build time available (e.g. to show in an “about” box), we use a preprocessor symbol like REVISION_DATE which gets filled in a CMakeLists.txt file. The whole thing looks like this:

...
EXEC_PROGRAM(date ARGS '+%F_%T' OUTPUT_VARIABLE REVISION_DATE)
...
ADD_DEFINITIONS(-DREVISION_DATE=\"${REVISION_DATE}\")
...

Since the beginning of the time these lines of CMake code lived in a small sub-sub-..-directory with little to no incomming dependencies. Then, at some point, it became necessary to have the REVISION_DATE symbol at some other place, too, which led to a move of the above code into the CMakeLists.txt file of the main package.

The value of command date +%F_%T changes every second which leads to a changed REVISION_DATE on every build – which is what we initially intended. What changes, too, of course, is the value of the ADD_DEFINITIONS directive. And as CMake is very strict with the slightest change in this value, every make target below that line gets rebuild – which in our case was everything in the main package.

So there! Build time killing creatures are lurking everywhere in our C/C++ projects. Always be aware of them!


Looping in C++

May 16, 2011

One recurring discussion point in one of our customers C++ project team is the following:

What is “the best” way to loop over collections?

In a typical scenario there is a standard container like std::list, or some equivalent collection, and the task is to do something with every element in the collection. The straight forward way would be like this:

std::list<std::string> mylist;
for (std::list<std::string>::iterator iter = mylist.begin(); iter != mylist.end(); iter++)
{
   ...
}

This code is correct and readable. But my guess is that most of you instantly see at least two possible improvements:

  1. the call to mylist.end() occurs in every loop an can be expensive e.g. in case of long std::lists
  2. iter++ creates one unnecessary intermediate object on the stack

So this

for (std::list<std::string>::iterator iter = mylist.begin(), end = mylist.end(); iter != end; ++iter)
{
   ...
}

would be much better but can already be seen as a little less readable.

Using BOOST_FOREACH can save you much of this still tedious code but has one nasty pitfall when it comes to std::maps.

In some places of the code base std::for_each is used together with a function, or function object.  The downside of this is that the function/function object code is not located where the loop occurs. However, this can be made “readable enough” when the function, or function object does only one thing and has a telling name.

Looping is sometimes done to create other collections of objects for each element. What to do there? Define the new collection use a for-loop of BOOST_FOREACH like above, or use std::transform with the same downside as std::for_each?

The other day one team member suggested to use boost::lambda expressions in loops. The initial usage examples where very promising but let me tell you – readability can drop dramatically very fast if you don’t be careful. It is very easy to get carried away with boost’s lambdas. I happened that we found ourselves having spent the last hour to carve out a super crisp lambda expression that takes anybody else another hour to read.

So the initial question remains undecided and will most likely stay like that. As for everything else in programming, there doesn’t seem to be a silver bullet for this task.

How do you go about looping in C++? Do you have some kind of coding style in place? Do you use std::for_each, BOOST_FOREACH, or some other means?

Looking forward to some feedback.


CMakeBuilder Version 1.9

April 18, 2011

Today, I want to announce version 1.9 of the CMakeBuilder plugin for Jenkins (formerly known as Hudson). Concluding from the user feedback, there are no major missing features – at least for the moment.

So for this version, I implemented only one visible enhancement: It is now possible to use environment variables in every configuration setting. Even settings like “Preload Script” “Make Command” or “Install Command” can now be configured with the support of environment variables.

The major invisible change I did was the migration to the Jenkins development infrastructure using this very helpful guide. Moving the whole thing to git will be next.

Check it out!


Podcasts

March 21, 2011

Podcasts are a very good means to shorten your commute, to keep you entertained during otherwise boring house-keeping activities, or, if you’re into sports, during your training sessions. Here is a list of some of my favourite shows.

This Developer’s Life

Rob Conery and Scott Hanselman interview developers and other IT professionals who share their stories. Very interesting, very well edited and flavoured with some nice pieces of music.

TechZing

Basically, TechZing are two guys, Jason Roberts and Justin Vincent, who discuss different topics concerning their lives as freelance web developers and startup bootstrappers. They enjoy themselves very much just talking to each other which is very entertaining already. The occasional interview and panel shows are then the icing on the cake.

It’s impossible to give a clear range of  topics since they consist of technical stuff like ‘how to store images in web applications’, SEO, NoSQL, JavaScript and iPhone development, but also non IT stuff like Pioneer One, geological challenges, and the Luck-Surface-Area. Edutainment at its best! Highly recommended!

Software Engineering Radio

This is purely an interview show which addresses all sorts of topics of interest for professional software developers: languages, platforms, technologies, methodologies, etc. Very informative, high profile guests and very competent hosts. Unfortunately, the output rate has gone down a lot in the last year.

Software ArchitekTOUR Podcast

This german (with little bits of swabian) speaking podcast is mostly concerned with topics around software architecture (as the name already suggests). DSLs, NoSQL databases and REST have been some of the latest topics.

FLOSS Weekly

Randal Schwartz (mostly) and other hosts are talking about Free Libre Open Source Software projects, ranging from whole OSes like CentOS to smaller niche projects like Ledger. Great show if you want to know what’s going on in the Open Source world.

Security Now

Steve Gibson and Leo Laporte talk about everything related to IT security. This will keep you informed about the latest browser vulnerabilities, Adobe Flash updates and Windows patches. But you will also learn e.g. how SSL works, the details of Stuxnet and everything about BitCoins. Don’t miss the all-time favourite episode 248: The Portable Dog Killer.

What are your favourite shows?


Old Code

February 21, 2011

There is a saying that if you don’t be embarrassed by code that you wrote six month ago, you haven’t learned anything. Recently, I stumbled upon a C/C++ project that dates back to the very early days of my programming career – this was many * six months ago – and I can tell you, I was very embarrassed.

I had just “learned” C++ and object-orientation at that time and, of course, wanted to program that way. The result was terrible. The only small piece of object-orientation was the use of the keyword class. There were public fields all over the place,  no interfaces or abstractions of any kind, switches over type-ids, and so on.

Another highlight was the vast amount of literals scattered all over the code. For example, as it was a curses-based application, I had to read and display user input using curses methods like

int mvwgetch(WINDOW *win, int y, int x);

and

 int mvwaddch(WINDOW *win, int y, int x, const chtype ch);

And what did I do? I hard-coded y and x positions on every call of those methods. So it would often be the case that I changed, say, the y position in one part and … well, you guessed it already.

Naming of variables was also big. Boolean values would often be called “flag”, a name length of more than 4 was considered way too long.

But there was also progress. In later parts of the software I started to use “advanced” things like auto_ptrs, std::list, and std::map. Hooray!

The only positive thing about this project was that since I made every possible mistake one can imagine, I learned quite lot about programming. And I remember that at the end of the project, I was already very embarrassed about the whole thing…

So if you like reading horror stories, try digging up your old code ;-) And share if you like.


SSL with POCO

February 7, 2011

Admittedly, the topic of this post is very specific but I hope it will still be of some value for some people.The task for today is to setup SSL server and client with POCO framework classes. I will leave out the whole certificate managing issues and just assume that the right files are at hand.

The SSL related part of  the POCO libraries essentially wraps the OpenSSL library into a nice object-oriented interface. When you know OpenSSL, you can instantly relate to classes like Poco::Net::Context, or the …Handler classes (if you replace “handler” with “callback”).

“SSL” stands for Secure Socket Layer, so the first thing to discover is class Poco::Net::SecureServerSocket. As you would expect, this class is derived from Poco::Net::ServerSocket, extending it only with SSL related stuff. And sure enough, some constructors of Poco::Net::ServerSocket take a Poco::Net::ContextPtr as argument.

But why only some constructors? Since there is no setContext method, there must be some other mechanism in place by which SecureServerSockets get their SSL context.

Introducing Poco::Net::SSLManager. From the API docs:

SSLManager is a singleton for holding the default server/client Context and handling callbacks for certificate verification errors and private key passphrases.

Proper initialization of SSLManager is critical.

Aha! So all the constructors of SecureServerSocket that do not take Context pointers simply get it from the SSLManager singleton.

But how to initialize SSLManager?

1. The POCO Way:

If you developed your application with POCO from the ground up there probably exists a sub-class of Poco::Application, and all the configuration is handled by the built-in configuration classes.

With this in place, all you have to do is to add the proper ssl configuration elements:

openSSL.server.privateKeyFile = /path/to/key/file
openSSL.server.certificateFile = /path/to/certificate/file
openSSL.server.verificationMode = none
openSSL.server.verificationDepth = 9
openSSL.server.loadDefaultCAFile = false
openSSL.server.cypherList = ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH
openSSL.server.privateKeyPassphraseHandler.name = KeyFileHandler
openSSL.server.privateKeyPassphraseHandler.options.password = securePassword
openSSL.server.invalidCertificateHandler = AcceptCertificateHandler

2. Manually:

Depending on which side you are – client or server – you have to call SSLManager::initializeClient or  SSLManager::initializeServer. Both methods take three arguments:

  1. PrivateKeyPassphraseHandler pointer
  2. InvalidCertificateHandler pointer
  3. Context pointer

This is where it becomes a little bit tricky: If you try to instantiate a Context with a privateKey file in order to provide it as argument to the initialize… method, a PrivateKeyPassphraseHandler might be needed. This handler is fetched from the SSLManager singleton – which you are just about to initialize!.

This circular dependency between Context and SSLManager can be overcome e.g. if you call SSLManager::initializeServer first only with a PrivateKeyPassphraseHandler, a InvalidCertificateHandler and null Context pointer. Then instantiate the Context and call SSLManager::initializeServer again.

Now that SSL Manager is initialized we can use Secure… prefixed classes as we would used their non-SSL counterparts. As with SecureServerSocket, other Secure… classes are derieved from corresponding non-secure base classes.

Conclusion: Once you got around the initialization of SSLManager singelton, using SSL POCO classes is very easy and straight forward. Check it out!


SSD? Don’t think! Just Buy!

January 17, 2011

My personal experience with SSDs began with an Intel X25M that I built into a Lenovo Thinkpad R61. It replaced a Seagate 160 GB 5400rpm which in combination with Windows Vista … well, let’s just say, it wasn’t that fast.

The SSD changed everything. It was not just faster, it was downright awesome! As if I had a completely new computer.

With that in mind I thought about my desktop PC. It’s a little more than 2 year old Windows XP box, Intel Core2Duo 2.7 GHz, 4GB RAM, with a not so slow Samsung HDD. I use it mainly for programming, which is most of the time Grails programming under IntelliJ IDEA.

And let me tell you, the Grails + IDEA combination can get dog slow at times. The start-up time of IDEA alone gives you time to skim over the first three pages of Hacker News and read the latest XKCD.

So the plan was to put an extra SSD into the Windows box and put only programming related stuff on it. This would save me the potential hassle of moving my whole system but would still give me development speed-up.

I had to be a little careful because the standard settings for IDEA’s so-called “system path” and “config path” is in the user’s home directory. (Btw, this settings can be changed in file “idea.properties” which resides in “IDEA_INSTALLATION_DIR\bin”, e.g.: c:\Progam Files\JetBrains\IntelliJ IDEA 9.0.4\bin)

I think you already guessed the result. Three words: fast, faster, SSD. It’s just amazing! IDEA start-up is so fast now, I barely have time for a quick look at the newest headlines on InfoQ.

The next step is of course to put the whole system on SSD but that will probably have to wait until we upgrade the whole company to Win7. Can’t wait… :-)


Active Object with POCO’s Active Methods

December 20, 2010

Active Object is a well known design pattern for synchronizing access to an object and/or resource. The basic idea is to separate method invocation from method execution which is done in a dedicated thread.

Instead of using the objects interface directly, a client of an Active Object uses some kind of  proxy which enqueues a so-called Method Request for later execution. The proxy finishes immediately and returns to the client some sort of callback, or variable, by which the client can receive the result. These intermeditate result variables are also known as Futures.

As always, there are lots of ways to implement this pattern. For example, if you had an interface like this

class MyObject
{
  public:
    int doStuff(const std::string& param) =0;
    std::string doSomeOtherThing(int i) =0;
};

applying a straight forward implementation, you would first transform this into a proxy and method request classes:

class MyObjectProxy
{
  public:
    MyObjectProxy(MyObject* theObject);
    // proxy methods
    Future<int> doStuff(const std::string& param);
    Future<std::string> doSomeOtherThing(int i);
  private:
    MyObject * _myObject;
};

class MethodRequest_DoStuff :
  public AbstractMethodRequest
{
  public:
    MethodRequest_DoStuff(const std::string& param);
    // all method request classes must implement execute()
    virtual void execute(MyObject* theObject);

  private:
    const std::string _param;
};

… and so on (for more details see this basic paper by Douglas C. Schmidt, or read chapter Concurrency Patterns in POSA2).

It’s easy to see that this implementation produces a lot of boilerplate code. To solve this, you could either cook up some code generation, or look for some language support to reduce the amount of characters you have to type. In C++, some sort of template solution can be the way to go, or…

Introducing Active Methods

With class ActiveMethod together with support classes ActiveDispatcher and ActiveResult the POCO C++ libraries provide very simple and elegant building blocks for implementing  the Active Object pattern.

ActiveMethod:  this is the core piece. When called, an ActiveMethod executes in its own thread.

ActiveResult: this is what I referred to earlier as a Future. Instances of ActiveResult are used to pass the result of an ActiveMethod call back to the client.

ActiveDispatcher: if you only use ActiveMethods, every ActiveMethod thread can execute in parallel.  With ActiveDispatcher as base class, ActiveMethod calls are serialized, thus implementing real™ Active Object behaviour.

Here my earlier example using ActiveMethods:

class MyObject
{
  public:
    // ActiveMethods are initialized in the ctors
    // initializer list
    MyObject()
      : doStuff(this, &MyObject::doStuffImpl),
        doSomeOtherThing(this, &MyObject::doSomeOtherThingImpl)
    {}

    ActiveMethod<int, std::string, MyObject> doStuff;
    ActiveMethod<std::string, int, MyObject> doSomeOtherThing;
  private:
    int doStuffImpl(const std::string& param);
    std::string doSomeOtherThingImpl(int i);
};

This is used as follows:

MyObject myObject;
ActiveResult<std::string> result = myObject.doSomeOtherThing(42);
...
result.wait();
std::cout << result.data() << std::endl;

This solution requires minimal amounts of additional code to transform your lame and boring normal object into a full-fledged Active Object. The only downside is that Active Methods currently can only have one parameter. If you need more, you have to use tuples or parameter objects.

Have fun!


Shrink your dependency list with POCO

November 22, 2010

When you write C++ applications of any sort you are very likely to need support libraries in addition to what comes with C++ (which is not much, btw). Of course, this holds true for any other language. But with Java and its rich JDK for example this need is not so imminent.

Starting at the very beginning, let’s see how fast the need for support arises.

int main(int argc, char** argv)
{
// parsing command line arguments
...

How to parse those command line arguments in a simple and easy way? How about a little help output when the program is called with -h or –help? Ok, we got boost::program_options for this.

Going further in your program you may want to have some sort of logging capability. Unfortunately, as of boost version 1.45 there is nothing to be found there. So you add a nice logging library.

And so on.

But wait! You don’t want to depend on too many 3rd party libraries because, among other things, they add deployment complexity.

Not even Qt, as one of the major players in the C++ framework world, provides solutions to both previous examples. As of version 4.7, no logging and not much support with command line arguments. And you end-up having to use QString, one of many non-std::string classes in C++ frameworks, which can get annoying at times (of course there are reasons why those exist).

I could go on with the list of smaller or larger concerns for which you either roll your own implementation or include yet another library in your project.

Instead I would like to point you to POCO, a nice set of C++ libraries which provide easy solutions for many basic and/or advanced day-to-day tasks. From their website:

Modern, powerful open source C++ class libraries and frameworks for building network- and internet-based applications that run on desktop, server and embedded systems

Besides very basic stuff like logging, date/time handling, threads, memory management, UTF-8, etc. they also provide lots of higher level classes for things like SMTP, POP3, SQL database access and HTTP. They even have a so called C++ Server Page Compiler which is basically something like JSP or Active Server Pages.

And they have no own string class! Yay! Instead they provide lots of functions classes and streams to do string manuipulation on good old std::string.

One thing I like most about POCO, though, is its clean, well-documented and apparently very high quality code. Although it is not overly functional or template-heavy, like you see it in in boost very often, it still provides elegant solutions.

Check it out and shrink your dependency list.


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